Publikationen von
Prof. Dr. Malte C. Kaluza
Alle Publikationen des HI Jena
2022
Abstract: We report on experimental results on laser-driven proton acceleration using high-intensity laser pulses. We present power law scalings of the maximum proton energy with laser pulse energy and show that the scaling exponent 4 strongly depends on the scale length of the preplasma, which is affected by the temporal intensity contrast. At lower laser intensities, a shortening of the scale length leads to a transition from a square root toward a linear scaling. Above a certain threshold, however, a significant deviation from this scaling is observed. Two-dimensional particle-in-cell simulations show that, in this case, the electric field accelerating the ions is generated earlier and has a higher amplitude. However, since the acceleration process starts earlier as well, the fastest protons outrun the region of highest field strength, ultimately rendering the acceleration less effective. Our investigations thus point to a principle limitation of the proton energy in the target normal sheath acceleration regime, which would explain why a significant increase of the maximum proton energy above the limit of 100 MeV has not yet been achieved.
2021
Abstract: We present the setup of a compact, q-switched, cryogenically cooled Yb:YAG laser, which is capable of producing over 1 J output energy in a 10 ns pulse at 10 Hz. The system’s design is based on the recently published unstable cavity layout with gain shaping of the spatial intra-cavity intensity distribution. Using a hexagonal homogenized pump beam, the laser generated an according hexagonal output beam profile. The suitability of such laser properties for the intended use in a laser shock peening process is demonstrated. In the experiment an aluminum plate was treated and the generated residual stresses in the sample subsequently measured. Other applications of this laser system like laser pumping or surface cleaning are conceivable.
Abstract: We present a diode-pumped, electro-optically Q-switched Tm:YAG laser with a cryogenically cooled laser crystal at 120 K. Output pulses of up to 2.55 mJ and 650 ns duration were demonstrated in an actively Q-switched configuration with a repetition rate of 1 Hz. By using cavity dumping the pulse duration was shortened to 18 ns with only a slightly lower output energy of 2.22 mJ. Furthermore, using a simplified rate equation model, we discuss design constraints on the pump fluence in a pulse pump approach for Tm:YAG to maximize the energy storage capability at a given pump power.
2020
Abstract: We present a study on temperature dependent spectroscopic data for Yb:KGW, Yb:KYW and Yb:YLF between 80K and 280K and Yb:YAP between 100K and 300 K. Absorption and emission cross sections are determined. The latter ones are obtained by using a combination of the McCumber relation and the Füchtbauer-Ladenburg equation. Fluorescence lifetimes are measured within a setup optimized for the suppression of re-absorption and compared to the radiative lifetimes calculated from the previously determined cross sections to cross check the validity of the measurements. The cross sections are evaluated with regard to the materials' potential for supporting the generation of ultra-short laser pulses, low quantum defect lasing and requirements for suitable diode laser pump sources.
Abstract: A compact, femtosecond-pumped noncollinear optical parametric amplifier (NOPA) is presented with a passive spectral shaping technique, employed to produce a flat-top-like ultrabroadband output spectrum. The NOPA is pumped by a dedicated 2 mJ, 120 fs Yb3+- based CPA system, which generates both the second harmonic pump pulse and white light supercontinuum as the signal pulse. A chirped mirror pair pre-compensates the material GVD within the optical path of the signal pulse to produce a near-FTL pulse duration at the OPA crystal output. By optimizing both the pump/signal cross angle and the pump/signal delay, the 40 cm × 40 cm footprint, single-pass, fs-pumped, direct NOPA (non-NOPCPA) system generates a record 20 μJ, 11 fs pulses at 820 nm central wavelength with a bandwidth of 230 nm FWHM, to be used as an ultrashort optical probe pulse for relativistic laser-plasma interactions at the petawatt-class POLARIS laser system.
Abstract: This report presents the conceptual design of a new European research infrastructure EuPRAXIA. The concept has been established over the last four years in a unique collaboration of 41 laboratories within a Horizon 2020 design study funded by the European Union. EuPRAXIA is the first European project that develops a dedicated particle accelerator research infrastructure based on novel plasma acceleration concepts and laser technology. It focuses on the development of electron accelerators and underlying technologies, their user communities, and the exploitation of existing accelerator infrastructures in Europe. EuPRAXIA has involved, amongst others, the international laser community and industry to build links and bridges with accelerator science — through realising synergies, identifying disruptive ideas, innovating, and fostering knowledge exchange. The Eu-PRAXIA project aims at the construction of an innovative electron accelerator using laser- and electron-beam-driven plasma wakefield acceleration that offers a significant reduction in size and possible savings in cost over current state-of-the-art radiofrequency-based accelerators. The foreseen electron energy range of one to five gigaelectronvolts (GeV) and its performance goals will enable versatile applications in various domains, e.g. as a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, table-top test beams for particle detectors, as well as deeply penetrating X-ray and gamma-ray sources for material testing. EuPRAXIA is designed to be the required stepping stone to possible future plasma-based facilities, such as linear colliders at the high-energy physics (HEP) energy frontier. Consistent with a high-confidence approach, the project includes measures to retire risk by establishing scaled technology demonstrators. This report includes preliminary models for project implementation, cost and schedule that would allow operation of the full Eu-PRAXIA facility within 8—10 years.
Abstract: We demonstrate the three-fold post-chirped-pulse-amplification (post-CPA) pulse compression of a high peak power laser pulse using ally) diglycol carbonate (CR39), which was selected as the optimal material for near-field self-phase modulation out of a set of various nonlinear plastic materials, each characterized with respect to its nonlinear refractive index and optical transmission. The investigated materials could be applied for further pulse compression at high peak powers, as well as for gain narrowing compensation within millijoule-class amplifiers. The post-CPA pulse compression technique was tested directly after the first CPA stage within the POLARIS laser system, with the compact setup containing a single 1 mm thick plastic sample and a chirped mirror pair, which enabled a substantial shortening of the compressed pulse duration and, hence, a significant increase in the laser peak power without any additional modifications to the existing CPA chain.
Abstract: We present Emission and absorption cross sections of thulium doped calcium fluoride (Tm:CaF2) in the visible to short wave infrared (SWIR) wavelength range for temperatures between 80 K and 300 K. For spectral regions of high and low absorption the McCumber relation and the Fuchtbauer–Ladenburg equation have been used to give reliable results. Furthermore, an estimation for the cross relaxation efficiency is derived from the emission spectra as a function of doping concentration and temperature. In addition, nearly re-absorption-free fluorescence lifetimes for various doping concentrations were studied. It was found that a double exponential fit model is better suited than a migration model to represent the fluorescence decay curves. The measurement results are interpreted in the light of the application of Tm:CaF2 as an efficient active medium in high-energy class diode-pumped solid state lasers.
Abstract: We present experimental evidence of relativistic electron-cyclotron resonances (RECRs) in the vicinity of the relativistically intense pump laser of a laser wakefield accelerator (LWFA). The effects of the RECRs are visualized by imaging the driven plasma wave with a few-cycle, optical probe in transverse geometry. The probe experiences strong, spectrally dependent and relativistically modified birefringence in the vicinity of the pump that arises due to the plasma electrons’ relativistic motion in the pump’s electromagnetic fields. The spectral birefringence is strongly dependent on the local magnetic field distribution of the pump laser. Analysis and comparison to both 2D and 3D particle-in-cell simulations confirm the origin of the RECR effect and its appearance in experimental and simulated shadowgrams of the laser-plasma interaction. The RECR effect is relevant for any relativistic, magnetized plasma and in the case of LWFA could provide a nondestructive, in situ diagnostic for tracking the evolution of the pump’s intensity distribution with propagation through tenuous plasma.
2019
Abstract: The Horizon 2020 project EuPRAXIA (European Plasma Research Accelerator with eXcellence In Applications) is producing a conceptual design report for a highly compact and cost-effective European facility with multi-GeV electron beams accelerated using plasmas. EuPRAXIA will be set up as a distributed Open Innovation platform with two construction sites, one with a focus on beam-driven plasma acceleration (PWFA) and another site with a focus on laser-driven plasma acceleration (LWFA). User areas at both sites will provide access to free-electron laser pilot experiments, positron generation and acceleration, compact radiation sources, and test beams for high-energy physics detector development. Support centres in four different countries will complement the pan-European implementation of this infrastructure.
Abstract: We report on a proton acceleration experiment in which high-intensity laser pulses with a wavelength of 0.4 mm and with varying temporal intensity contrast have been used to irradiate water droplets of 20 mm diameter. Such droplets are a reliable and easy-to-implement type of target for proton acceleration experiments with the potential to be used at very high repetition rates. We have investigated the influence of the laser's angle of incidence by moving the droplet along the laser polarization axis. This position, which is coupled with the angle of incidence, has a crucial impact on the maximum proton energy. Central irradiation leads to an inefficient coupling of the laser energy into hot electrons, resulting in a low maximum proton energy. The introduction of a controlled pre-pulse produces an enhancement of hot electron generation in this geometry and therefore higher proton energies. However, two-dimensional particle-in-cell simulations support our experimental results confirming, that even slightly higher proton energies are achieved under grazing laser incidence when no additional pre-plasma is present. Illuminating a droplet under grazing incidence generates a stream of hot electrons that flows along the droplet's surface due to self-generated electric and magnetic fields and ultimately generates a strong electric field responsible for proton acceleration. The interaction conditions were monitored with the help of an ultra-short optical probe laser, with which the plasma expansion could be observed.
Abstract: We report on the successful implementation and characterization of a cryogenic solid hydrogen target in experiments on high-power laser-driven proton acceleration. When irradiating a solid hydrogen filament of 10 mm diameter with 10-Terawatt laser pulses of 2.5 J energy, protons with kinetic energies in excess of 20?MeV exhibiting non-thermal features in their spectrum were observed. The protons were emitted into a large solid angle reaching a total conversion efficiency of several percent. Two-dimensional particle-in-cell simulations confirm our results indicating that the spectral modulations are caused by collisionless shocks launched from the surface of the the high-density filament into a low-density corona surrounding the target. The use of solid hydrogen targets may significantly improve the prospects of laser-accelerated proton pulses for future applications.
Abstract: Thermal profile modification of an active material in a laser amplifier via optical pumping results in a change in the material’s refractive index, and causes thermal expansion and stress, eventually leading to spatial phase aberrations, or even permanent material damage. For this purpose, knowledge of the 3D spatio-temporal thermal profile, which can currently only be retrieved via numerical simulations, is critical for joule-class laser amplifiers to reveal potentially dangerous thermal features within the pumped active materials. In this investigation, a detailed, spatio-temporal numerical simulation was constructed and tested for accuracy against surface thermal measurements of various end-pumped Yb³⁺-doped laser-active materials. The measurements and simulations show an excellent agreement and the model was successfully applied to a joule-class Yb³⁺-based amplifier currently operating in the POLARIS laser system at the Friedrich-Schiller-University and Helmholtz-Institute Jena in Germany.
Abstract: We present the absorption spectroscopy and continuous -wave laser operation of Tm:YLF at cryogenic temperatures. At 100 K, a maximum output power of 2.55 W corresponding to a maximum slope efficiency of 22.8% is obtained using 15% output coupling transmission. The output laser wavelength is centered at 1877 nm for Ellc.
Abstract: We present a novel approach to combine diode-pumped, moderately low-gain media with the advantages of an unstable cavity. To this end, we propose to utilize a spatially tailored gain profile in the active medium instead of using a graded reflectivity mirror to provide an effective shaping mechanism for the intra-cavity intensity distribution. The required gain profile can be easily generated with a state-of-the-art homogenized laser diode pump beam in an end-pumped configuration.
2018
Abstract: Free Electron Lasers (FEL) are commonly regarded as the potential key application of laser wakefield accelerators (LWFA). It has been found that electron bunches exiting from state-of-the-art LWFAs exhibit a normalized 6-dimensional beam brightness comparable to those in conventional linear accelerators. Effectively exploiting this beneficial beam property for LWFA-based FELs is challenging due to the extreme initial conditions particularly in terms of beam divergence and energy spread. Several different approaches for capturing, reshaping and matching LWFA beams to suited undulators, such as bunch decompression or transverse-gradient undulator schemes, are currently being explored. In this article the transverse gradient undulator concept will be discussed with a focus on recent experimental achievements.
Abstract: Controlling the parameters of a laser plasma accelerated electron beam is a topic of intense research with a particular focus placed on controlling the injection phase of electrons into the accelerating structure from the background plasma. An essential prerequisite for high-quality beams is dark-current free acceleration (i.e., no electrons accelerated beyond those deliberately injected). We show that small-scale density ripples in the background plasma are sufficient to cause the uncontrolled (self-)injection of electrons. Such ripples can be as short as ∼50 μm and can therefore not be resolved by standard interferometry. Background free injection with substantially improved beam characteristics (divergence and pointing) is demonstrated in a gas cell designed for a controlled gas flow. The results are supported by an analytical theory as well as 3D particle in cell simulations.
Abstract: High-intensity femtosecond laser–plasma interaction experiments were performed to investigate laser–plasma wakefield acceleration in the “bubble” regime. Using a 15 TW laser pulse, the emission of side-scattered radiation was spectrally and spatially resolved and was consequently used to diagnose the evolution of the laser pulse during the acceleration process. Side-scattered emission was observed immediately before wavebreaking at a frequency of ωL + 1.7ωp (where ωL is the laser frequency and ωp is the background plasma frequency). This emission may result from scattering of laser light by large amplitude plasma oscillations generated in the shell of the wakefield “bubble” and which occurs immediately prior to the wavebreaking/injection process. The observed variation of the frequency of scattered light with electron density agrees with theoretical estimates.
Abstract: We present experimental evidence of ultra-high energy density plasma states with the keV bulk electron temperatures and near-solid electron densities generated during the interaction of high contrast, relativistically intense laser pulses with planar metallic foils. Experiments were carried out with the Ti:Sapphire laser system where a picosecond pre-pulse was strongly reduced by the conversion of the fundamental laser frequency into 2ω. A complex diagnostics setup was used for evaluation of the electron energy distribution in a wide energy range. The bulk electron temperature and density have been measured using x-ray spectroscopy tools; the temperature of supra-thermal electrons traversing the target was determined from measured bremsstrahlung spectra; run-away electrons were detected using magnet spectrometers. Analysis of the bremsstrahlung spectra and results on measurements of the run-away electrons showed a suppression of the hot electron production in the case of the high laser contrast. Characteristic x-ray radiation has been used for evaluation of the bulk electron temperature and density. The measured Ti line radiation was simulated both in steady-state and transient approaches using the code FLYCHK that accounts for the atomic multi-level population kinetics. The best agreement between the measured and the synthetic spectrum of Ti was achieved at 1.8 keV electron temperature and 2 10^23 cm^{−3} electron density. By application of Ti-foils covered with nm-thin Fe-layers, we have demonstrated that the thickness of the created keV hot dense plasma does not exceed 150 nm. Results of the pilot hydro-dynamic simulations that are based on a wide-range two-temperature Equation of States, wide-range description of all transport and optical properties, ionization, electron, and radiative heating, plasma expansion, and Maxwell equations (with a wide-range permittivity) for description of the laser absorption are in excellent agreement with experimental results. According to these simulations, the generation of keV-hot bulk electrons is caused by the collisional mechanism of the laser pulse absorption in plasmas with a near solid step-like electron density profile. The laser energy, first deposited into the nm-thin skin-layer, is then transported into 150 nm depth by the electron heat conductivity. This scenario is opposite to the volumetric character of the energy deposition produced by supra-thermal electrons.
Abstract: Plasma-based accelerators that impart energy gain as high as several GeV to electrons or positrons within a few centimeters have engendered a new class of diagnostic techniques very different from those used in connection with conventional radio-frequency (rf) accelerators. The need for new diagnostics stems from the micrometer scale and transient, dynamic structure of plasma accelerators, which contrasts with the meter scale and static structure of conventional accelerators. Because of this micrometer source size, plasma-accelerated electron bunches can emerge with smaller normalized transverse emittance (εn<0.1 mm mrad) and shorter duration (τb∼1 fs) than bunches from rf linacs. Single-shot diagnostics are reviewed that determine such small εn and τb noninvasively and with high resolution from wide-bandwidth spectral measurement of electromagnetic radiation the electrons emit: εn from x rays emitted as electrons interact with transverse internal fields of the plasma accelerator or with external optical fields or undulators; τb from THz to optical coherent transition radiation emitted upon traversing interfaces. The duration of ∼1 fs bunches can also be measured by sampling individual cycles of a copropagating optical pulse or by measuring the associated magnetic field using a transverse probe pulse. Because of their luminal velocity and micrometer size, the evolving structure of plasma accelerators, the key determinant of accelerator performance, is exceptionally challenging to visualize in the laboratory. Here a new generation of laboratory diagnostics is reviewed that yield snapshots, or even movies, of laser- and particle-beam-generated plasma accelerator structures based on their phase modulation or deflection of femtosecond electromagnetic or electron probe pulses. Spatiotemporal resolution limits of these imaging techniques are discussed, along with insight into plasma-based acceleration physics that has emerged from analyzing the images and comparing them to simulated plasma structures.
Abstract: We present detailed measurements of laser relevant cross sections of thulium doped yttrium-aluminum-garnet (Tm:YAG) and yttrium-aluminum-perovskite (Tm:YAP), including the absorption cross sections for the H63 to H43 transition near 800nm, and the absorption and emission cross sections for the transitions between the H63 and F43 manifolds in the short-wavelength infrared region. For Tm:YAP we present data for all polarization axes. The measurements were carried out at temperatures ranging from 80 K to 300 K. Furthermore, re-absorption free fluorescence lifetimes of the F43 to H63 transition at 77 K, 200 K and 29 5K were obtained using the pinhole method. We observed a significant enhancement of the fluorescence lifetime when cooling from room temperature to 77 K. The lifetime was increased from 9.42 ms to 15.22 ms in Tm:YAG and from 3.81 ms to 4.93 ms in Tm:YAP. This indicates that lifetime quenching is present at room temperature, which can be overcome, at least partially, by cryogenic cooling. These data are presented with the scope to qualify these materials for their use in a new generation of cryogenically cooled, short-wavelength infrared, high-energy class diode pumped solid state lasers utilizing the cross relaxation mechanism for pumping.
Abstract: The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target’s normal direction has been observed. The ring’s opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here—in addition to the longitudinal electric fields responsible for the forward acceleration of the protons—a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.
Abstract: A tomographic gas-density diagnostic using a Single-Beam Wollaston Interferometer able to characterize non-symmetric density distributions in gas jets is presented. A real-time tomographic algorithm is able to reconstruct three-dimensional density distributions. A Maximum Likelihood-Expectation Maximization algorithm, an iterative method with good convergence properties compared to simple back projection, is used. With the use of graphical processing units, real-time computation and high resolution are achieved. Two different gas jets are characterized: a kHz, piezo-driven jet for lower densities and a solenoid valve-based jet producing higher densities. While the first jet is used for free electron laser photon beam characterization, the second jet is used in laser wake field acceleration experiments. In this latter application, well-tailored and non-symmetric density distributions produced by a supersonic shock front generated by a razor blade inserted laterally to the gas flow, which breaks cylindrical symmetry, need to be characterized.
Abstract: Non-symmetric side-scattering has been observed during the interaction between a high-intensity laser pulse and under-dense argon plasma. The angle between the laser's forward direction and the scattered radiation is found to decrease for increasing electron densities ranging from 0.01 to 0.25n c , where n c is the critical density for the laser wavelength. We show that the observed features of the scattering cannot be described by Raman side-scattering but can be explained to be a consequence of the non-uniform density distribution of the plasma with the scattering angle being oriented along the direction of the resulting electron density gradient.
Abstract: Abstract A comprehensive spatio-temporal characterization is presented describing the pump-induced wavefront aberrations in Yb3 + -doped YAG, CaF2, and fluorophosphate glass. Time-resolved interferometric measurements were performed to reveal the profiles of the total optical path differences (OPDs), which are described by the spatio-temporal superposition of thermal as well as electronic contributions, across the free aperture of the considered diode-pumped active materials. These contributions were individually determined by a COMSOL-based thermal profile model along with a detailed characterization of the electronic changes by measuring the single-pass gain and the spatial fluorescence profile. Due to the low quantum defect, the amplitude of the electronic component becomes comparable for all three materials and, in the case of Yb:CaF2, almost completely compensates the thermal component resulting from a pump pulse during the time frame of laser pulse amplification. Finally, all relevant material constants – such as the photoelastic constant and the polarizability difference – could be determined during this investigation, allowing the accurate modeling of the total pump-induced wavefront aberrations and subsequent optimization for laser systems worldwide employing these Yb3 + -doped materials.
2017
Abstract: The detailed temporal evolution of the laser-wakefield acceleration process with controlled injection, producing reproducible high-quality electron bunches, has been investigated. The localized injection of electrons into the wakefield has been realized in a simple way - called shock-front injection - utilizing a sharp drop in plasma density. Both experimental and numerical results reveal the electron injection and acceleration process as well as the electron bunch's temporal properties. The possibility to visualize the plasma wave gives invaluable spatially resolved information about the local background electron density, which in turn allows for an efficient suppression of electron self-injection before the controlled process of injection at the sharp density jump. Upper limits for the electron bunch duration of 6.6 fs FWHM, or 2.8 fs (r.m.s.) were found. These results indicate that shock-front injection not only provides stable and tunable, but also few-femtosecond short electron pulses for applications such as ultrashort radiation sources, time-resolved electron diffraction or for the seeding of further acceleration stages.
Abstract: We present the single-shot carrier-envelope phase (CEP) determination of a 1 Hz, multi-terawatt (TW) laser system with a setup based on spectral broadening in a hollow-core fiber and a stereographic measurement of the energy-dependent above-threshold ionization plateau. The latter is extremely sensitive to variations in CEP. As compared to the f-2f interferometers, this technique reduces the uncertainties due to the shot-to-shot intensity fluctuations, which are prevalent in the TW laser systems. The experimental results pave the way towards the investigation and control over CEP-sensitive processes at ultra-high intensities.
Abstract: Shadowgraphy is a technique widely used to diagnose objects or systems in various fields in physics and engineering. In shadowgraphy, an optical beam is deflected by the object and then the intensity modulation is captured on a screen placed some distance away. However, retrieving quantitative information from the shadowgrams themselves is a challenging task because of the nonlinear nature of the process. Here, we present a method to retrieve quantitative information from shadowgrams, based on computational geometry. This process can also be applied to proton radiography for electric and magnetic field diagnosis in high-energy-density plasmas and has been benchmarked using a toroidal magnetic field as the object, among others. It is shown that the method can accurately retrieve quantitative parameters with error bars less than 10%, even when caustics are present. The method is also shown to be robust enough to process real experimental results with simple pre- and postprocessing techniques. This adds a powerful tool for research in various fields in engineering and physics for both techniques.
Abstract: The Horizon 2020 Project EuPRAXIA (“European Plasma Research Accelerator with eXcellence In Applications”) is preparing a conceptual design report of a highly compact and cost-effective European facility with multi-GeV electron beams using plasma as the acceleration medium. The accelerator facility will be based on a laser and/or a beam driven plasma acceleration approach and will be used for photon science, high-energy physics (HEP) detector tests, and other applications such as compact X-ray sources for medical imaging or material processing. EuPRAXIA started in November 2015 and will deliver the design report in October 2019. EuPRAXIA aims to be included on the ESFRI roadmap in 2020.
Abstract: Abstract A low quantum defect is the fundamental key to a high efficiency of any laser. To study the anticipated performance boost for a 980 nm-diode pumped cryogenically cooled Yb:CaF2 disk laser we compared its operation at output wavelengths of 991 nm, 996 nm, and 1032 nm. Despite the higher quantum defect a maximum efficiency of 74% (output versus incident power) with an output power of 15.8 W was achieved at the 1032 nm output wavelength. This observation led to a detailed analysis of remaining loss mechanisms we are reporting on in this paper.
Abstract: We present pulse stretching with an intracavity Offner-type pulse stretcher applied to a high-energy, short-pulse laser system. The compact intracavity design, offering a tunable stretching factor, allows the pulses to be stretched to several nanoseconds and, at the same time, to be amplified to 100 μJ. The stretched pulses have been further amplified with the high-power laser system Polaris and have been recompressed to durations as short as 102 fs, reaching peak powers of 100 TW. Furthermore, the temporal intensity contrast is investigated and compared to the formerly used stretcher setup.
Abstract: Similar to ytterbium doped laser materials laser operation with thulium doped media is possible within a quasi-three level scheme, which especially for pulse pumped lasers is a drawback for efficient laser operation, as a significant amount of energy is required to bleach out the laser medium. Since this energy cannot be extracted, it is lost for the amplification process. Hence, operation of such lasers at cryogenic temperatures seems to be an appropriate solution. For further modeling and derivation of design rules for future laser systems based on such a scheme reliable spectral data is needed. We will present absorption and emission measurements on Tm:YAG as a function of temperature in the range from 80 K to 300 K covering both the absorption bands around 800 nm and the emission bands up to 2.1 μm. The spectral measurements were carried out on two samples of Tm:YAG with doping levels of 2 at.% and 8 at.%. Precautions for reabsorption effects were taken to allow for accurate results over the whole measurement range. From these measurements we have derived absorption and emission cross sections and radiative lifetimes. By comparing the latter values to values obtained by highly accurate measurements of the lifetime using the pinhole method we could also estimate the quantum efficiency.
2016
Abstract: We present the results from a theoretical investigation of laser beam propagation in relay imaging multi-pass layouts, which recently found application in high-energy laser amplifiers. Using a method based on the well-known ABCD-matrix formalism and proven by ray tracing, it was possible to derive a categorization of such systems. Furthermore, basic rules for the setup of such systems and the compensation for low order aberrations are derived. Due to the introduced generalization and parametrization, the presented results can immediately be applied to any system of the investigated kinds for a wide range of parameters, such as number of round-trips, focal lengths and optics sizes. It is shown that appropriate setups allow a close-to-perfect compensation of defocus caused by a thermal lens and astigmatism caused by non-normal incidence on the imaging optics, as well. Both are important to avoid intensity spikes leading to damages of optics in multi-pass laser amplifiers.
Abstract: We report on results from the fully diode-pumped chirped-pulse amplification laser system Polaris. Pulses were amplified to a maximum energy of 54.2 J before compression. These pulses have a full width at half-maximum spectral bandwidth of 18 nm centered at 1033 nm and are generated at a repetition rate of 0.02 Hz. To the best of our knowledge, these are the most energetic broadband laser pulses generated by a diode-pumped laser system so far. Due to the limited size of our vacuum compressor, only attenuated pulses could be compressed to a duration of 98 fs containing an energy of 16.7 J, which leads to a peak power of 170 TW. These pulses could be focused to a peak intensity of 1.3×1021 W/cm2. Having an ultra-high temporal contrast of 1012 with respect to amplified spontaneous emission these laser pulses are well suited for high-intensity laser–matter experiments.
Abstract: We present tunable spectral filters (TSFs) as a variable and precisely adjustable method to control the spectral gain of short-pulse laser systems. The TSFs provide a small residual spectral phase and a high damage threshold, and generate no pre- or post-pulses. The method is demonstrated for two different laser materials and can be applied as an intracavity compensation in regenerative amplifiers as well as a method for pre-compensation in high-energy multipass amplifiers. With this method, a full width at half-maximum bandwidth of 23.9 nm could be demonstrated in a diode-pumped, 50 J Yb:CaF2 amplifier.
Abstract: A series of glasses in the system Li2O/MgO/La2O3/Al2O3/B2O3/ Yb2O3/SiO2 was prepared and doped with 6·1020 Yb3+ ions per cm3 which corresponds to about 1.3 mol% Yb2O3 in the glass composition. In this series, Al2O3 was partially replaced by B2O3 up to a concentration of 15 mol%. Furthermore, Li2O and MgO were partially substituted for LiF and MgF2. All melts were bubbled with argon gas to ensure low OH- concentrations. B2O3 and fluoride additions strongly decrease the glass transition temperatures and viscosities of the melts enabling lower melting temperatures, lower Pt solubility and better homogenization of the melt. Furthermore, both additions slightly decrease the density and refractive indices of the glasses. The relatively low coefficients of thermal expansion remain largely unaffected. Increasing B2O3 concentrations do not negatively affect the luminescence lifetimes of Yb3+ despite their relatively high phonon energy. However, the fluoride addition and the OH- concentration strongly influence the luminescence lifetimes.
Abstract: We demonstrate the generation of 86 fs, 35 mJ, high-contrast laser pulses at 1030 nm with a repetition rate of 1 Hz from a diode-pumped double chirped-pulse amplification setup. The pulses exhibit a spectral bandwidth exceeding 27 nm full width at half-maximum. This could be achieved by using a laser architecture comprising two stages of chirped pulse amplification with a cross-polarized wave generation filter in between, by applying spectral shaping and by increasing the spectral hard-clip of the second stretcher. These are, to the best of our knowledge, the shortest pulses at the mJ level with ultra-high contrast generated with a diode-pumped front end at 1030 nm.
Abstract: We report on the first demonstration of passive all-optical plasma lensing using a two-stage setup. An intense femtosecond laser accelerates electrons in a laser wakefield accelerator (LWFA) to 100 MeV over millimeter length scales. By adding a second gas target behind the initial LWFA stage we introduce a robust and independently tunable plasma lens. We observe a density dependent reduction of the LWFA electron beam divergence from an initial value of 2.3 mrad, down to 1.4 mrad (rms), when the plasma lens is in operation. Such a plasma lens provides a simple and compact approach for divergence reduction well matched to the mm-scale length of the LWFA accelerator. The focusing forces are provided solely by the plasma and driven by the bunch itself only, making this a highly useful and conceptually new approach to electron beam focusing. Possible applications of this lens are not limited to laser plasma accelerators. Since no active driver is needed the passive plasma lens is also suited for high repetition rate focusing of electron bunches. Its understanding is also required for modeling the evolution of the driving particle bunch in particle driven wake field acceleration.
Abstract: We demonstrate how to tune the main ion acceleration mechanism in laser-plasma interactions to collisionless shock acceleration, thus achieving control over the final ion beam properties (e. g. maximum energy, divergence, number of accelerated ions). We investigate this technique with three-dimensional particle-in-cell simulations and illustrate a possible experimental realisation. The setup consists of an isolated solid density target, which is preheated by a first laser pulse to initiate target expansion, and a second one to trigger acceleration. The timing between the two laser pulses allows to access all ion acceleration regimes, ranging from target normal sheath acceleration, to hole boring and collisionless shock acceleration. We further demonstrate that the most energetic ions are produced by collisionless shock acceleration, if the target density is near-critical, ne ≈ 0.5 ncr. A scaling of the laser power shows that 100 MeV protons may be achieved in the PW range.
Abstract: A technique to measure the spatially resolved temperature distribution in a laser medium is presented. It is based on the temperature dependence of the absorption cross section close to the zero-phonon line of the active medium. Since other materials in the beam path exhibit a high (and constant) transmission at this wavelength, the method can easily be applied in realistic amplifier setups. The method was successfully tested on three different samples, which were pumped by a pulsed laser diode with up to 150 W average power: side-cooled Yb:YAG and Yb:fluoride-phosphate glass at room temperature and face-cooled Yb:CaF2 at 120 K.
Abstract: Ultrafast shadowgraphy is a new experimental technique that uses few-cycle laser pulses to image density gradients in a rapidly evolving plasma. It enables structures that move at speeds close to the speed of light, such as laser driven wakes, to be visualized. Here we study the process of shadowgraphic image formation during the propagation of a few cycle probe pulse transversely through a laser-driven wake using three-dimensional particle-in-cell simulations. In order to construct synthetic shadowgrams a near-field snapshot of the ultrashort probe pulse is analyzed by means of Fourier optics, taking into account the effect of a typical imaging setup. By comparing synthetic and experimental shadowgrams we show that the generation of synthetic data is crucial for the correct interpretation of experiments. Moreover, we study the dependence of synthetic shadowgrams on various parameters such as the imaging system aperture, the position of the object plane and the probe pulse delay, duration and wavelength. Finally, we show that time-dependent information from the interaction can be recovered from a single shot by using a broadband, chirped probe pulse and subsequent spectral filtering.
Abstract: An analytical model is presented describing the temporal intensity contrast determined by amplified spontaneous emission in high-intensity laser systems which are based on the principle of chirped pulse amplification. The model describes both the generation and the amplification of the amplified spontaneous emission for each type of laser amplifier. This model is applied to different solid state laser materials which can support the amplification of pulse durations ≤350 fs . The results are compared to intensity and fluence thresholds, e.g. determined by damage thresholds of a certain target material to be used in high-intensity applications. This allows determining if additional means for contrast improvement, e.g. plasma mirrors, are required for a certain type of laser system and application. Using this model, the requirements for an optimized high-contrast front-end design are derived regarding the necessary contrast improvement and the amplified “clean” output energy for a desired focussed peak intensity. Finally, the model is compared to measurements at three different high-intensity laser systems based on Ti:Sapphire and Yb:glass. These measurements show an excellent agreement with the model.
Abstract: ELI-NP facility will enable for the first time the use of two 10 PW laser beams for quantum electrodynamics (QED) experiments. The first beam will accelerate electrons to relativistic energies. The second beam will subject relativistic electrons to the strong electromagnetic field generating QED processes: intense gamma ray radiation and electron-positron pair formation. The laser beams will be focused to intensities above 10^21 W/cm^2 and reaching 10^22–10^23 W/cm^2 for the first time. We propose to use this capability to investigate new physical phenomena at the interfaces of plasma, nuclear and particle physics at ELI-NP. This High Power Laser System Technical Design Report (HPLS-TDR2) presents the experimental area E6 at ELI-NP for investigating high field physics and quantum electrodynamics and the production of electron-positron-pairs and of energetic gamma-rays. The scientific community submitted 12 commissioning runs for E6 interaction chamber with two 10 PW laser beams and one proposal for the CETAL interaction chamber with 1 PW laser. The proposals are representative of the international high field physics community being written by 48 authors from 14 European and US organizations. The proposals are classified according to the science area investigated into: Radiation Reaction Physics: Classical and Quantum; Compton and Thomson Scattering Physics: Linear and Non Linear Regimes; QED in Vacuum; Atoms in Extreme Fields. Two pump-probe colliding 10 PW laser beams are proposed for the E6 interaction chamber. The focused pump laser beam accelerates the electrons to relativistic energies. The accelerated electron bunches interact with the very high electro-magnetic field of the focused probe laser beam. We propose two main types of experiments with: (a) gas targets in which the pump laser-beam is focused by a long focal length mirror and drives a wakefield in which the electron bunch is accelerated to multi-GeV energies and then exposed to the EM field of the probe laser which is tightly focused; (b) solid targets in which both the pump and probe laser beams are focused on the solid target, one accelerating the electrons in the solid and the other, delayed, providing the high electric field to which the relativistic electrons are subjected. We propose four main focusing configurations for the pump and probe laser beams, two for each type of target: counter-propagating 10 PW focused laser beams and the two 10 PW laser beams focused in the same direction. For solid targets we propose an additional configuration with plasma-mirror on the pump laser beam: the plasma mirror placed between the focusing mirror and target. It is proposed that the 10 PW laser beams will have polarization control and focus control by means of adaptive optics. Initially only one 10 PW may have polarization control and adaptive optics. In order to accommodate the two laser beams and diagnostics the proposed interaction chamber is quasi-octagonal with a diameter of 4.5 m. A large electron-spectrometer is proposed for multi-GeV electrons. Other diagnostics are requested for: gamma-rays, electrons and positrons, protons and ions, plasma characterization, transmitted and reflected laser beam. Targets will be provided by the ELI-NP Target Laboratory or purchased. The E6 experiments and diagnostics will benefit from the ELI-NP Electronics Laboratory, the Workshop and the Optics Laboratory. In order to ensure radiation-protection, a large beam-dump is planned for both multi-GeV electrons and multi-100 MeV protons.
2015
Abstract: We report on the frequency doubling of femtosecond laser pulses at 1030 nm center wavelength generated from the fully diode-pumped laser system POLARIS. The newly generated pulses at a center wavelength of 515 nm have a pulse energy of 3 J with a pulse duration of 120 fs. On the basis of initially ultra-high contrast seed pulses we expect a temporal intensity contrast better 10^17 200 ps before the peak of the main pulse. We analyzed the temporal intensity contrast from milliseconds to femtoseconds with a dynamic range covering more than 20 orders of magnitude. The pulses were focussed with a f/2-focussing parabola resulting in a peak intensity exceeding 10^20 W/cm2. The peak power and intensity are to the best of our knowledge the highest values for 515 nm-laser-pulses achieved so far.
Abstract: It has been shown in the past that pulsed laser systems operating in the so-called “burst mode” are a beneficial approach to generate high peak power laser pulses at high repetition rates suitable for various applications. So far, most high-energy burst-mode laser systems put great effort into generating a homogeneous energy distribution across the burst duration, e.g., by shaping the pump pulse. In this work, we present a new shaping technique, which is able to produce arbitrary energy distributions within the burst by pre-shaping the seed pulse burst with a Pockels cell. Furthermore, this technique allows for the precompensation of any static modulations across the burst, which may be introduced during the subsequent amplification process. Therefore, a pulse burst with a uniform energy distribution can also be generated. The method is tested with an ultra-short pulse burst mode laser amplifier system producing bursts of a 1 ms duration with a pulse repetition rate of 1 MHz and a maximum output power of 800 W during the burst. Furthermore, a method to predict the influence of the amplifier on a non-uniformly shaped burst is presented and successfully tested to produce a pre-defined pulse shape after amplification.
Abstract: In this contribution, we analyze the effect of several preparation methods of Yb3+ doped alumino silicate glasses on their quantum efficiency by using photo-acoustic measurements in comparison to standard measurement methods including the determination via the fluorescence lifetime and an integrating sphere setup. The preparation methods focused on decreasing the OH concentration by means of fluorine-substitution and/or applying dry melting atmospheres, which led to an increase in the measured fluorescence lifetime. However, it was found that the influence of these methods on radiative properties such as the measured fluorescence lifetime alone does not per se give exact information about the actual quantum efficiency of the sample. The determination of the quantum efficiency by means of fluorescence lifetime shows inaccuracies when refractive index changing elements such as fluorine are incorporated into the glass. Since fluorine not only eliminates OH from the glass but also increases the “intrinsic” radiative fluorescence lifetime, which is needed to calculate the quantum efficiency, it is difficult to separate lifetime quenching from purely radiative effects. The approach used in this contribution offers a possibility to disentangle radiative from non-radiative properties which is not possible by using fluorescence lifetime measurements alone and allows an accurate determination of the quantum efficiency of a given sample. The comparative determination by an integrating sphere setup leads to the well-known problem of reabsorption which embodies itself in the measurement of too low quantum efficiencies, especially for samples with small quantum efficiencies.
Abstract: The necessity for compact table-top x-ray sources with higher brightness, shorter wavelength and shorter pulse duration has led to the development of complementary sources based on laser-plasma accelerators, in contrast to conventional accelerators. Relativistic interaction of short-pulse lasers with underdense plasmas results in acceleration of electrons and in consequence in the emission of spatially coherent radiation, which is known in the literature as betatron radiation. In this article, we report on our recent results in the rapidly developing field of secondary x-ray radiation generated by high-energy electron pulses. The betatron radiation is characterized with a novel setup allowing to measure the energy, the spatial energy distribution in the far-field of the beam and the source size in a single laser shot. Furthermore, the polarization state is measured for each laser shot. In this way, the emitted betatron x-rays can be used as a non-invasive diagnostic tool to retrieve very subtle information of the electron dynamics within the plasma wave. Parallel to the experimental work, 3D particle-in-cell simulations were performed, proved to be in good agreement with the experimental results.
Abstract: We present few-femtosecond shadowgraphic snapshots taken during the nonlinear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma-wave period is induced by a lengthening of the first plasma period. Three-dimensional particle-in-cell simulations support our observations.
Abstract: The long-term goal to integrate laser-based particle accelerators into radiotherapy clinics not only requires technological development of high-intensity lasers and new techniques for beam detection and dose delivery, but also characterization of the biological consequences of this new particle beam quality, i.e. ultra-short, ultra-intense pulses. In the present work, we describe successful in vivo experiments with laser-driven electron pulses by utilization of a small tumour model on the mouse ear for the human squamous cell carcinoma model FaDu. The already established in vitro irradiation technology at the laser system JETI was further enhanced for 3D tumour irradiation in vivo in terms of beam transport, beam monitoring, dose delivery and dosimetry in order to precisely apply a prescribed dose to each tumour in full-scale radiobiological experiments. Tumour growth delay was determined after irradiation with doses of 3 and 6 Gy by laser-accelerated electrons. Reference irradiation was performed with continuous electron beams at a clinical linear accelerator in order to both validate the dedicated dosimetry employed for laser-accelerated JETI electrons and above all review the biological results. No significant difference in radiation-induced tumour growth delay was revealed for the two investigated electron beams. These data provide evidence that the ultra-high dose rate generated by laser acceleration does not impact the biological effectiveness of the particles.
Abstract: In this paper, we report on measurements of bremsstrahlung in laser ion acceleration experiments from ultra-thin, polymer-based target foils. The influence of laser polarization on the generated γ radiation, the maximum achievable proton energy and the total proton number is investigated. A clear benefit in terms of γ radiation reduction by the use of circular polarized light can be observed. At the same time, the total number of accelerated protons was increased.
Abstract: Ytterbium doped lithium-alumino-silicate glasses suitable for diode-pumped laser applications were investigated concerning the hydroxyl quenching of the Yb^(3+) fluorescence. Glasses of the nominal composition 18 mol% Li2O, 22 mol% Al2O3 and 60 mol% SiO2 with variable OH concentrations NOH (between 0.04 and 6.01 ∙ 1019 cm−3) and Yb3+ concentrations NYb (between 0.1 and 9 ∙ 1020 cm−3) were produced and a direct correlation between spontaneous emission decay rate and the product NYb ∙ NOH was observed. The radiative spontaneous emission rate in the glass host is around 1,000 s−1 (radiative lifetime 1.0 ms) and the microparameter for Yb-Yb energy migration, CYb-Yb, was found to be 1.358∙10^−38 cm^6 s−1. It was calculated that on average 17% of the OH groups in the glass contribute to the quenching of the Yb3+ fluorescence. By analysis of the UV edge of the glass it was concluded that melting under inert conditions leads to reduction of iron impurities to Fe2+, which can act as quenching sites for the Yb3+ ions and therefore may additionally reduce the energy storage capability of the laser material.
Abstract: Copper activation was used to characterize high-energy proton beam acceleration from near-critical density plasma targets. An enhancement was observed when decreasing the target density, which is indicative for an increased laser-accelerated hot electron density at the rear target-vacuum boundary. This is due to channel formation and collimation of the hot electrons inside the target. Particle-in-cell simulations support the experimental observations and show the correlation between channel depth and longitudinal electric field strength is directly correlated with the proton acceleration.
2014
Abstract: Hochleistungslaser ermöglichen es inzwischen, relativistische Elektronenpulse mit bemerkenswerten Eigenschaften zu erzeugen. Neben der äußerst kurzen Beschleunigungslänge sind vor allem die kleine Quellgröße und auch die kurze Pulsdauer interessant. Mit diesen Elektronenpulsen lässt sich zudem elektromagnetische Sekundärstrahlung im Kiloelektronenvolt-Bereich erzeugen. Mit dieser Röntgenstrahlung würden auch Universitäten relativ kompakte Röntgenquellen zur Verfügung stehen, mit denen sich Effekte beobachten lassen, die auf äußerst kurzen räumlichen und zeitlichen Skalen ablaufen. Momentan ist diese Art von Forschung nur an großen, konventionellen Synchrotron-Beschleunigern möglich.
Abstract: We present the results from a new frontend within a double-chirped pulse amplification architecture (DCPA) utilizing crossed-polarized wave generation (XPW) for generating ultra-high contrast, 150 μJ-level, femtosecond seed pulses at 1030 nm. These pulses are used in the high energy class diode-pumped laser system Polaris at the Helmholtz Institute in Jena. Within this frontend, laser pulses from a 75 MHz oscillator-pulse train are extracted at a repetition rate of 1 Hz, temporally stretched, amplified and then recompressed reaching a pulse energy of 2 mJ, a bandwidth of 12 nm and 112 fs pulse duration at a center wavelength of 1030 nm. These pulses are temporally filtered via XPW in a holographic-cut BaF2 crystal, resulting in 150 μJ pulse energy with an efficiency of 13 %. Due to this non-linear filtering, the relative intensity of the amplified spontaneous emission preceding the main pulse is suppressed to 2×10^−13. This is, to the best of our knowledge, the lowest value achieved in a high peak power laser system operating at 1030 nm center wavelength.
Abstract: Laser-accelerated electron pulses have been used to irradiate human tumors grown on mice’s ears during radiobiological experiments. These experiments have been carried out with the JETI laser system at the Institute of Optics and Quantum Electronics in Jena, Germany. To treat a total of more than 50 mice, a stable and reliable operation of the laser-electron accelerator with a dose rate exceeding 1 Gy/min was necessary. To achieve this, a sufficient number of electrons at energies in excess of 5 MeV had to be generated. The irradiation time for a single mouse was a few minutes. Furthermore, the particle pulses’ parameters needed to remain achievable for a time period of several weeks. Due to the online detection of the radiation dose, the unavoidable shot-to-shot fluctuations, currently still typical for laser-based particle accelerators, could be compensated. The results demonstrate that particle pulses generated with laser-based accelerators have the potential to be a future alternative for conventional particle accelerators used for the irradiation of tumors.
Abstract: Various alumino silicate glasses (network modifier ions: Li^+ , Mg^2+ , Zn^2+ and/or La^^3+ ) doped with 1 × 10^20 Yb^3+ cm^−3 (about 0.2 mol% Yb_2 O_3 ) were prepared. The glasses were studied with respect to their thermo-mechanical and fluorescence properties. Huge differences are found for the coefficients of thermal expansion which determine the thermal shock resistance of the material and hence are required for ultra-high power laser applications. Here, zinc and magnesium alumino silicate glasses show the lowest values. The fluorescence lifetimes of the glasses increase with decreasing average atomic weight of the glass composition (685–1020 µ s). All glasses show broad and smooth emission spectra with little variations due to compositional changes. Mixed lithium zinc or lithium magnesium alumino silicate glasses could be promising new laser materials especially with respect to ultra-high peak power systems or applications with high repetition rates.
Abstract: We present measurements of the absorption and emission cross-sections for Yb:YAG, Yb:LuAG and Yb:CaF_{2} as a function of temperature between 80 and 340 K. The cross-sections are determined by the combination of the McCumber relation and the Fuchtbauer–Ladenburg (FL) equation to achieve reliable results in spectral regions of high and low absorption. The experimental setup used for the fluorescence measurements minimizes re-absorption effects due to the measurement from small sample volume, providing nearly undisturbed raw data for the FL approach. The retrieved cross-sections together with the spectral characteristics of the tested materials provide important information for the design of energy efficient, high-power laser amplifiers.
Abstract: We present a novel and highly sensitive method to determine the residual angular dispersion of high-power laser pulses after stretching, amplification, and re-compression of the pulses in a chirped-pulse amplification laser system. This method is based on the intentional deflection of a part of the the spectrum within the compressor and aligning the centers of gravity of the two resulting and separated foci with largest possible spectral separation in the far field. Using this technique, we were able to reduce the residual angular dispersion on pulses to less than 0.05 μrad/nm in the vertical plane and less than 0.03 μrad/nm in the horizontal plane, respectively. With this method, it is possible to minimize the deviation of the actual peak intensity for the focused laser pulses to less than 2% of its theoretical limit.
Abstract: The development, the underlying technology and the current status of the fully diode-pumped solid-state laser system POLARIS is reviewed. Currently, the POLARIS system delivers 4 J energy, 144 fs long laser pulses with an ultra-high temporal contrast of 5×10^12 for the ASE, which is achieved using a so-called double chirped-pulse amplification scheme and cross-polarized wave generation pulse cleaning. By tightly focusing, the peak intensity exceeds 3.5×10^20 W cm^{−2}. These parameters predestine POLARIS as a scientific tool well suited for sophisticated experiments, as exemplified by presenting measurements of accelerated proton energies. Recently, an additional amplifier has been added to the laser chain. In the ramp-up phase, pulses from this amplifier are not yet compressed and have not yet reached the anticipated energy. Nevertheless, an output energy of 16.6 J has been achieved so far.
Abstract: In this paper the development and implementation of a novel amplifier setup as an additional stage for the CPA pump laser of the Petawatt Field Synthesizer, currently developed at the Max-Planck-Institute of Quantum Optics, is presented. This amplifier design comprises 20 relay-imaged passes through the active medium which are arranged in rotational symmetry. As the gain material, an in-house-developed Yb:YAG active-mirror is used. With this setup, stretched 4 ns seed pulses are amplified to output energies exceeding 1 J with repetition rates of up to 2 Hz. Furthermore, a spectral bandwidth of 3.5 nm (FWHM) is maintained during amplification and the compression of the pulses down to their Fourier-limit of 740 fs is achieved. To the best of our knowledge, this is the first demonstration of 1 TW pulses generated via CPA in diode-pumped Yb:YAG.
Abstract: We present the first complete temporal and spatial characterization of the amplified spontaneous emission (ASE) of laser radiation generated by a diode-pumped high-power laser system. The ASE of the different amplifiers was measured independently from the main pulse and was characterized within a time window of \minus10ms \leq t \leq 10ms and an accuracy of up to 15fs around the main pulse. Furthermore, the focusability and the energy of the ASE from each amplifier was measured after recompression. Using our analysis method, the laser components, which need to be optimized for a further improvement of the laser contrast, can be identified. This will be essential for laser-matter interaction experiments requiring a minimized ASE intensity or fluence.
Abstract: Various ternary aluminosilicate glasses with the molar compositions 20 Al2O3-60 SiO2-20 R2O (R = Li or Na), 20 Al2O3-60 SiO2-20 RO (R = Mg, Ca or Zn) and 23.1 Al2O3-69.2 SiO2-7.7 R2O3 (R = Y or La) doped with 1 [times] 1020 Sm3+ cm-3 or 1 [times] 1020 Eu3+ cm-3 (about 0.2 mol% Sm2O3 or Eu2O3) were prepared. The glasses were studied with respect to their molecular structure, and their thermo-mechanical and fluorescence properties. All glasses show relatively broad fluorescence excitation and only a weak effect of the glass composition on the emission spectra is observed. Although the glasses should be structurally very similar, huge differences are found in the coefficients of thermal expansion and the glass transition temperatures. The fluorescence lifetime increases steadily with decreasing mean atomic weight and decreasing refractive index of the glasses, which may be explained by local field effects. The only exception from this rule is the zinc aluminosilicate glass, which shows a relatively high fluorescence lifetime. The highest fluorescence lifetime is found for the lithium aluminosilicate glass. The lowest coefficients of thermal expansion are found for zinc- and magnesium aluminosilicate glasses. A low coefficient of thermal expansion is a prerequisite for a high thermal shock resistance of the material and hence favorable for high-power laser applications.
Abstract: Within the last decade, laser-ion acceleration has become a field of broad interest. The possibility to generate short proton- or heavy ion bunches with an energy of a few tens of MeV by table-top laser systems could open new opportunities for medical or technical applications. Nevertheless, today's laser-acceleration schemes lead mainly to a temperature-like energy distribution of the accelerated ions, a big disadvantage compared to mono-energetic beams from conventional accelerators. Recent results [1] of laser-ion acceleration using radiation-pressure appear promising to overcome this drawback. In this paper, we demonstrate the influence of a second counter-propagating laser pulse interacting with a nm-thick target, creating a well defined pre-plasma.
Abstract: Abstract The generation of intense ion beams from high-intensity laser-generated plasmas has been the focus of research for the last decade. In the LIGHT collaboration the expertise of heavy ion accelerator scientists and laser and plasma physicists has been combined to investigate the prospect of merging these ion beams with conventional accelerator technology and exploring the possibilities of future applications. We report about the goals and first results of the LIGHT collaboration to generate, handle and transport laser driven ion beams. This effort constitutes an important step in research for next generation accelerator technologies.
Abstract: We report the amplification of laser pulses at a center wavelength of 1034 nm to an energy of 16.6 J from a fully diode-pumped amplifier using Yb:CaF2 as the active medium. Pumped by a total optical power of 300 kW from high-power laser diodes, a gain factor of g=6.1 was achieved in a nine-pass amplifier configuration agreeing with numerical simulations. A measured spectral bandwidth of 10 nm full width at half-maximum promises a bandwidth-limited compression of the pulses down to a duration of 150 fs. These are, to our knowledge, the most energetic laser pulses achieved from a diode-pumped chirped-pulse amplifier so far.
2013
Abstract: The development of a few-cycle optical probe-pulse for the investigation of laser-plasma interactions driven by a Ti:sapphire, 30 Terawatt (TW) laser system is described. The probe is seeded by a fraction of the driving laser's energy and is spectrally broadened via self-phase modulation in a hollow core fiber filled with a rare gas, then temporally compressed to a few optical cycles via chirped mirrors. Shadowgrams of the laser-driven plasma wave created in relativistic electron acceleration experiments are presented with few-fs temporal resolution, which is shown to be independent of post-interaction spectral filtering of the probe-beam.
Abstract: This paper reports on the effect of the chemical composition on the glass structure, the coefficients of thermal expansion and the fluorescence properties of Sm3+-doped La2O3–Al2O3–SiO2-glasses. The silica concentration was varied between 50 and 70 mol% and the La2O3:Al2O3 ratio between 50:50 and 30:70. The glass formation and the densities are evaluated and FTIR reflectance spectra, coefficients of thermal expansion and fluorescence lifetimes are determined. It is shown that high SiO2 concentrations and low La2O3:Al2O3 ratios result in relatively high fluorescence lifetime (2.19 ms, 4G5/2) and low coefficients of thermal expansion (4.6 × 10^{−6} / K). The coefficients of thermal expansion and the fluorescence lifetimes show a linear dependency on the ratio LaO3/2/(AlO3/2 + SiO2).
Abstract: We present a novel approach for the amplification of high peak power femtosecond laser pulses at a high repetition rate. This approach is based on an all-diode pumped burst mode laser scheme. In this scheme, pulse bursts with a total duration between 1 and 2 ms are be generated and amplified. They contain 50 to 2000 individual pulses equally spaced in time. The individual pulses have an initial duration of 350 fs and are stretched to 50 ps prior to amplification. The amplifier stage is based on Yb3+:CaF2 cooled to 100 K. In this amplifier, a total output energy in excess of 600 mJ per burst at a repetition rate of 10 Hz is demonstrated. For lower repetition rates the total output energy per burst can be scaled up to 915 mJ using a longer pump duration. This corresponds to an efficiency as high as 25% of extracted energy from absorbed pump energy. This is the highest efficiency, which has so far been demonstrated for a pulsed Yb3+:CaF2 amplifier.
Abstract: Laser-plasma particle accelerators could provide more compact sources of high-energy radiation than conventional accelerators. Moreover, because they deliver radiation in femtosecond pulses, they could improve the time resolution of X-ray absorption techniques. Here we show that we can measure and control the polarization of ultra-short, broad-band keV photon pulses emitted from a laser-plasma-based betatron source. The electron trajectories and hence the polarization of the emitted X-rays are experimentally controlled by the pulse-front tilt of the driving laser pulses. Particle-in-cell simulations show that an asymmetric plasma wave can be driven by a tilted pulse front and a non-symmetric intensity distribution of the focal spot. Both lead to a notable off-axis electron injection followed by collective electron–betatron oscillations. We expect that our method for an all-optical steering is not only useful for plasma-based X-ray sources but also has significance for future laser-based particle accelerators.
Abstract: Magnesium aluminosilicate glasses doped with 0.2 mol% Sm2O3 (1 x 10^{20} Sm3+ cm^{-3}) have been prepared in a very broad compositional range. The effect of the MgO, Al2O3 and SiO2 concentrations as well as the effect of partial substitution of MgO by CaO, SrO, BaO, ZnO or MgF2 have been studied. Increasing the network modifier concentration results in decreasing the glass transformation temperature and increasing the coefficient of thermal expansion due to the formation of non-bridging oxygen sites and decreasing glass network connectivity. Although the network connectivity is changed substantially by the addition of network modifier oxides, the maximum phonon energy and the fluorescence lifetime of Sm3+ are not affected. Equimolar replacement of up to 9 mol% MgO by MgF2 results in increasing Sm3+ fluorescence lifetimes without increasing the coefficient of thermal expansion or decreasing the glass forming ability. Glasses with fairly small thermal expansion coefficients (≤ 3.2 x 10^{-6} K^{-1}), low thermal stress values (≤ 0.5 MPa K^{-1}), broad fluorescence emission peaks and fluorescence lifetimes in the range from 2.4 to 2.8 ms are obtained. Such glasses are interesting candidates for laser host materials in ultrahigh peak power laser systems.
Abstract: Temperature dependent absorption and emission cross-sections of 5 at-% Yb3+ doped yttrium lanthanum oxide (Yb:YLO) ceramic between 80K and 300K are presented. In addition, we report on the first demonstration of ns pulse amplification in Yb:YLO ceramic. A pulse energy of 102mJ was extracted from a multi-pass amplifier setup. The amplification bandwidth at room temperature confirms the potential of Yb:YLO ceramic for broad bandwidth amplification at cryogenic temperatures.
Abstract: Relativistic electrons with small energy spread propagating through undulators produce monochromatic radiation with high spectral intensity. The working principle of undulators requires a small energy spread of the electron beam in the order of ΔE/E ~ 0.1%. Laser-wakefield accelerators can produce electron bunches with an energy of several 100 MeV within a few millimeters acceleration length, but with a relatively large energy spread (ΔE/E ~ 1 - 10%). In order to produce monochromatic undulator radiation with these electrons, a novel scheme involving transverse-gradient superconducting undulators was proposed in an earlier work. This paper reports on the design-optimization and construction of an iron-free cylindrical superconducting undulator tailored to the particular beam properties of the laser-wakefield electron accelerator at the University of Jena, Germany.
Abstract: We present a novel approach for the construction of a high energy, high power burst mode laser system, based on diode pumped cryogenically cooled Yb:CaF2. The system consists of a frontend producing pulses of 300 fs duration with 1 MHz. Bursts of 1000 subsequent pulses are cut from the continuous train by an electro optical modulator. Afterwards the duration of the individual pulses is stretched to 50 ps.
The amplifier system consists of two amplifiers. Both amplifiers utilize mirror based relay imaging schemes to allow for a sufficient number of extraction passes for achieving efficient energy extraction. The goal parameters of the system are to achieve a total energy of 5 J per burst with a repetition rate of 10Hz.
Amplification results for the first of two amplifiers are demonstrated. A total output energy of 480 mJ was achieved corresponding to an optical to optical efficiency from absorbed pump energy to extracted energy of more than 17%. Single pulse energies of up to 7.5mJ are generated when changing to less pulses per burst.
To achieve a constant energy from pulse to pulse during the burst we present a technique based on the modulation of the laser diode current during one pulse. With this technique the gain variation during the burst was than 5% peak to peak.
Abstract: Advanced high intensity laser matter interaction experiments always call for optimized laser performance. In order to further enhance the POLARIS laser system, operational at the University of Jena and the Helmholtz-Institute Jena, in particular its energy, bandwidth and focusability, new amplifier technologies have been developed and are reported here. Additionally, existing sections were considerably improved. A new multi-pass amplification stage, which is able to replace two currently used ones, was developed in close collaboration with the MPQ (Garching). The new basic elements of this amplifier are well homogenized pump modules and the application of a successive imaging principle. By operating the amplifier under vacuum conditions a top hat beam profile with an output energy of up to 1.5 J per pulse is foreseen. The already implemented POLARIS amplifier A4 was further improved by adapting an advanced method for the homogenization of the multi-spot composed pump profile. The new method comprises a computer-based evolutionary algorithm which optimizes the position of the different spots regarding its individual size, shape and intensity. The latter allowed a better homogenization of the POLARIS near field profile.
Abstract: The expansion of a semi-infinite plasma slab into vacuum is analyzed with a hydrodynamic model implying a steplike electron energy distribution function. Analytic expressions for the maximum ion energy and the related ion distribution function are derived and compared with one-dimensional numerical simulations. The choice of the specific non-Maxwellian initial electron energy distribution automatically ensures the conservation of the total energy of the system. The estimated ion energies may differ by an order of magnitude from the values obtained with an adiabatic expansion model supposing a Maxwellian electron distribution. Furthermore, good agreement with data from experiments using laser pulses of ultrashort durations τ_L ≤ 80 fs is found, while this is not the case when a hot Maxwellian electron distribution is assumed.
Abstract: Experimental results on the acceleration of protons and carbon ions from ultra-thin polymer foils at intensities of up to 6 × 10^(19) W cm^(−2) are presented revealing quasi-monoenergetic spectral characteristics for different ion species at the same time. For carbon ions and protons, a linear correlation between the cutoff energy and the peak energy is observed when the laser intensity is increased. Particle-in-cell simulations supporting the experimental results imply an ion acceleration mechanism driven by the radiation pressure as predicted for multi-component foils at these intensities.
Abstract: We report on the first generation of high-contrast, 164 fs duration pulses from the laser system POLARIS reaching focused peak intensities in excess of 2×10^20 W/cm2. To our knowledge, this is the highest peak intensity reported so far that has been achieved with a diode-pumped, solid-state laser. Several passive contrast enhancement techniques have been specially developed and implemented, achieving a relative prepulse intensity smaller than 10^−8 at t=−30 ps before the main pulse. Furthermore a closed-loop adaptive-optics system has been installed. Together with angular chirp compensation, this method has led to a significant reduction of the focal spot size and an increase of the peak intensity.
2012
Abstract: We report on an experiment irradiating individual argon droplets of 20 μm diameter with laser pulses of several Joule energy at intensities of 10^{19} W/cm^{2}. K-shell emission spectroscopy was employed to determine the hot electron energy fraction and the time-integrated charge-state distribution. Spectral fitting indicates that bulk temperatures up to 160 eV are reached. Modelling of the hot-electron relaxation and generation of K-shell emission with collisional hot-electron stopping only is incompatible with the experimental results, and the data suggest an additional ultra-fast (sub-ps) heating contribution. For example, including resistive heating in the modelling yields a much better agreement with the observed final bulk temperature and qualitatively reproduces the observed charge state distribution.
Abstract: We developed a detection scheme, capable of measuring X-ray line shape of tracer ions in μm thick layers at the rear side of a target foil irradiated by ultra intense laser pulses. We performed simulations of the effect of strong electric fields on the K-shell emission of silicon and developed a spectrometer dedicated to record this emission. The combination of a cylindrically bent crystal in von Hámos geometry and a CCD camera with its single photon counting capability allows for a high dynamic range of the instrument and background free spectra. This approach will be used in future experiments to study electric fields of the order of TV/m at high density plasmas close to solid density.
Abstract: Accurate values of the emission and absorption cross sections of Yb:YAG, Yb:LuAG, and Yb:CaF_2 as a function of temperature between room temperature and 200 °C are presented. For this purpose, absorption and fluorescence spectra were measured using a setup optimized to reduce the effect of radiation trapping. From these data, emission cross sections were retrieved by combining the Fuchtbauer–Ladenburg equation and the reciprocity method. Based on our measurements, simple estimations illustrate the effect of temperature shifts that are likely to occur in typical laser setups. Our results show that even minor temperature variations can have significant impact on the laser performance using Yb:YAG and Yb:LuAG as an active medium, while Yb:CaF_2 appears to be rather insensitive.
Abstract: We show experimentally that extreme ultraviolet radiation is produced when a laser pulse is Thomson backscattered from sheets of relativistic electrons that are formed at the rear surface of a foil irradiated on its front side with a high-intensity laser. An all-optical setup is realized using the Jena titanium:sapphire ten-terawatt laser system with an enhanced amplified spontaneous emission background of 10^{−12}. The main pulse is split into two: one of them accelerates electrons from thin aluminium foil targets to energies of the order of some MeV and the other, counterpropagating probe pulse Thomson-backscatters off these electrons when they exit the target rear side. The process produced photons within a wide spectral range of some tens of eV as a result of the broad electron energy distribution. The highest scattering intensity is observed when the probe pulse arrives at the target rear surface 100 fs after irradiation of the target front side by the pump pulse, corresponding to the maximum flux of hot electrons at the interaction region. These results can provide time-resolved information about the evolution of the rear-surface electron sheath and hence about the dynamics of the electric fields responsible for the acceleration of ions from the rear surface of thin, laser-irradiated foils.
Abstract: Harmonic generation in the limit of ultrasteep density gradients is studied experimentally. Observations reveal that, while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale lengths (L_p/λ < 1), the absolute efficiency of the harmonics declines for the steepest plasma density scale length L_p → 0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the relativistic oscillating mirror was estimated to be in the range of 10^{-4} – 10^{-6} of the laser pulse energy for photon energies ranging from 20 – 40 eV, with the best results being obtained for an intermediate density scale length.
Abstract: We report the first experimental observation of terahertz (THz) radiation from the rear surface of a solid target while interacting with an intense laser pulse. Experimental and two-dimensional particle-in-cell simulations show that the observed THz radiation is mostly emitted at large angles to the target normal. Numerical results point out that a large part of the emission originates from a micron-scale plasma sheath at the rear surface of the target, which is also responsible for the ion acceleration. This opens a perspective for the application of THz radiation detection for on-site diagnostics of particle acceleration in laser-produced plasmas.
Abstract: We present the setup of a polarization rotating device and its adaption for high-power short-pulse laser systems. Compared to conventional halfwave plates, the all-reflective principle using three zero-phase shift mirrors provides a higher accuracy and a higher damage threshold. Since plan-parallel plates, e.g. these halfwave plates, generate postpulses, which could lead to the generation of prepulses during the subsequent laser chain, the presented device avoids parasitic pulses and is therefore the preferable alternative for high-contrast applications. Moreover the device is easily scalable for large beam diameters and its spectral reflectivity can be adjusted by an appropriate mirror coating to be well suited for ultra-short laser pulses.
Abstract: Experimental results on relativistic surface HHG at a repetition rate of 10 Hz are presented. Average powers in the 10 μW range are generated in the spectral range of 51 to 26 nm (24 - 48 eV). The surface harmonic radiation is produced by focusing the second-harmonic of a high-power laser onto a rotating glass surface to moderately relativistic intensities of 3 × 10^{19} W cm^{−2}. The harmonic emission exhibits a divergence of 26 mrad. Together with absolute photon numbers recorded by a calibrated spectrometer, this allows for the determination of the extreme ultraviolet (XUV) yield. The pulse energies of individual harmonics are reaching up to the μJ level, equivalent to an efficiency of 10^{−5}. The capability of producing stable and intense high-harmonic radiation from relativistic surface plasmas may facilitate experiments on nonlinear ionization or the seeding of free-electron lasers.
Abstract: We report on a four-mirror reflective wave-plate system based on a phase-shifting mirror (PSM) for a continuous variation of elliptical polarization without changing the beam position and direction. The system presented and characterized here can replace a conventional retardation plate providing all advantages of a PSM, such as high damage-threshold, large scalability, and low dispersion. This makes reflective wave-plates an ideal tool for ultra-high power laser applications.
Abstract: We investigate the properties of a laser-plasma electron accelerator as a bright source of keV x-ray radiation. During the interaction, the electrons undergo betatron oscillations and from the carefully measured x-ray spectrum the oscillation amplitude of the electrons can be deduced which decreases with increasing electron energies. From the oscillation amplitude and the independently measured x-ray source size of (1.8 ± 0.3) μm we are able to estimate the electron bunch diameter to be (1.6 ± 0.3) μm.
Abstract: We present a cryogenic source of periodic streams of micrometer-sized hydrogen and argon droplets as ideal mass-limited target systems for fundamental intense laser-driven plasma applications. The highly compact design combined with a high temporal and spatial droplet stability makes our injector ideally suited for experiments using state-of-the-art high-power lasers in which a precise synchronization between the laser pulses and the droplets is mandatory. We show this by irradiating argon droplets with multi-terawatt pulses.
2011
Abstract: A laser generated proton beam was used to measure the megagauss strength self-generated magnetic fields from a nanosecond laser interaction with an aluminum target. At intensities of 10^{15} W cm^{−2} , the significant hot electron production and strong heat fluxes result in non-local transport becoming important to describe the magnetic field dynamics. Two-dimensional implicit Vlasov–Fokker–Planck modeling shows that fast advection of the magnetic field from the focal region occurs via the Nernst effect at significantly higher velocities than the sound speed, v_{N} / c_{s} ≈ 10.
Abstract: We report on the performance of a system employing a multi-layer coated mirror creating circularly polarized light in a fully reflective setup. With one specially designed mirror we are able to create laser pulses with an ellipticity of more than ε = 98% over the entire spectral bandwidth from initially linearly polarized Titanium:Sapphire femtosecond laser pulses. We tested the homogeneity of the polarization with beam sizes of the order of approximately 10 cm. The damage threshold was determined to be nearly 400 times higher than for a transmissive quartz-wave plate which suggests applications in high intensity laser experiments. Another advantage of the reflective scheme is the absence of nonlinear effects changing the spectrum or the pulse-form and the scalability of coating fabrication to large aperture mirrors.
Abstract: We present a tomographic characterization of gas jets employed for high-intensity laser-plasma interaction experiments where the shape can be non-symmetrically. With a Mach-Zehnder interferometer we measured the phase shift for different directions through the neutral density distribution of the gas jet. From the recorded interferograms it is possible to retrieve 3-dimensional neutral density distributions by tomographic reconstruction based on the filtered back projections. We report on criteria for the smallest number of recorded interferograms as well as a comparison with the widely used phase retrieval based on an Abel inversion. As an example for the performance of our approach, we present the characterization of nozzles with rectangular openings or gas jets with shock waves. With our setup we obtained a spatial resolution of less than 60 μm for an Argon density as low as 2 × 10^{17} cm^{−3}.
Abstract: We describe a novel method to improve the temporal intensity contrast (TIC) between the main pulse and prepulses in a high-power chirped-pulse amplification (CPA) laser system. Pre- and post-pulses originating from the limited extinction ratio of the polarization gating equipment are suppressed by carefully adjusting the round-trip times of the regenerative amplifiers (RAs) with respect to the oscillator. As a result, leaking pulses from earlier or later round-trips in the RAs are hidden below the temporal shape of the main pulse. The synchronization can easily be controlled by a contrast measurement on a picosecond time scale using a third-order cross-correlator that enables a sub-mm precise adjustment of the cavity lengths. Finally, a method based on spectral interference is introduced that can be used for a fine-adjustment of the cavity lengths for the daily operation, making this new method easy to implement into existing laser systems.
Abstract: Electron acceleration by laser-driven plasma waves is capableof producing ultra-relativistic, quasi-monoenergetic electron bunches with orders of magnitude higher accelerating gradients and much shorter electron pulses than state-of-the-art radio-frequency accelerators. Recent developments have shown peak energies reaching into the GeV range and improved stability and control over the energy spectrum and charge. Future applications, such as the development of laboratory X-ray sources with unprecedented peak brilliance or ultrafast time-resolved measurements critically rely on a temporal characterization of the acceleration process and the electron bunch. Here, we report the first real-time observation of the accelerated electron pulse and the accelerating plasma wave. Our time-resolved study allows a single-shot measurement of the (5.8_(−2.1))^(+1.9) fs electron bunch full-width at half-maximum ((2.5_(−0.9))^(+0.8) fs root mean square) as well as the plasma wave with a density-dependent period of 12–22 fs and reveals the evolution of the bunch, its position in the surrounding plasma wave and the wake dynamics. The results afford promise for brilliant, sub-ångström-wavelength ultrafast electron and photon sources for diffraction imaging with atomic resolution in space and time.
Abstract: The absorption of an ultra-intense circularly-polarized laser pulse by a near-critical (0.1 n_c < n_e < a_0 n_c) plasma is studied. Previously two regimes of absorption have been suggested: a 'leading edge depletion' (LED) regime and a 'transverse ponderomotive acceleration' regime. Here we seek to describe these concepts more thoroughly, and determine if two distinct regimes actually exist. New analytic models to describe each regime are derived. These are compared with 1D and 2D particle-in-cell simulations, and good quantitative agreement is found, showing the existence of two separate regimes. The LED regime exhibits very efficient absorption of laser light, which is promising for applications.
Abstract: We present a plasma mirror configuration that improves the temporal pulse contrast of femtosecond terawatt laser pulses by a factor of thousand using a single antireflection coated glass target. The device provides ultra-high contrast for experiments with a maximum repetition rate of 10 Hz. A third-order cross-correlator has been used to measure the temporal pulse contrast for several different plasma mirror targets. It is shown that the ASE can be suppressed to a level of 10^(−11.) A comparison between a triggered and an untriggered plasma mirror reveals differences in the intensity distribution of the focused beam. The triggered plasma mirror produces a slightly larger focus due to the expansion of the triggered plasma mirror at -3 ps before the main pulse. We propose a cost-effective AR-coated and a blank glass target to reduce the costs of the consumable target material. High-harmonic radiation on solid surfaces has been generated with different plasma mirror targets to demonstrate the high laser contrast.
Abstract: We report on the measurement of the highest purity of polarization of X-rays to date. The measurements are performed by combining a brilliant undulator source with an X-ray polarimeter. The polarimeter is composed of a polarizer and an analyzer, each based on four reflections at channel-cut crystals with a Bragg angle very close to 45°. Experiments were performed at three different X-ray energies, using different Bragg reflections: Si(400) at 6457.0 eV, Si(444) at 11,183.8 eV, and Si(800) at 12,914.0 eV. At 6 keV a polarization purity of 1.5 × 10^{-9} is achieved. This is an improvement by more than two orders of magnitude as compared to previously reported values. The polarization purity decreases slightly for shorter X-ray wavelengths. The sensitivity of the polarimeter is discussed with respect to a proposed experiment that aims at the detection of the birefringence of vacuum induced by super-strong laser fields.
Abstract: We introduce a method to suppress prepulses of pulse picking systems due to the limited extinction ratio of polarization gating systems. By matching the round trip times of the oscillator and the subsequent regenerative amplifiers, leaking pulses are hidden below the temporal intensity pedestal of the main pulse. With this method, prepulses at the temporal position equal to the time difference of the round trip times of the cavities could be suppressed completely.
2010
Abstract: At the Institute of Optics and Quantum Electronics in Jena, Germany, the currently most powerful diode-pumped solid-state laser system with 25-TW peak power Polaris is in operation. In this paper we give an overview about the dispersion management of the chirped pulse amplification in order to minimize the pulse duration and thus to maximize the intensity available for experiments. A detailed description of the stretcher and compressor design with a novel alignment routine is given as well as measurements for the pulse duration and the temporal contrast. The far field measurement of the beam focussed by an off-axis parabola yields a nearly diffraction limited focal spot.
Abstract: We present a novel, cascaded acceleration scheme for the generation of spectrally controlled ion beams using a laser-based accelerator in a 'double-stage' setup. An MeV proton beam produced during a relativistic laser-plasma interaction on a thin foil target is spectrally shaped by a secondary laser-plasma interaction on a separate foil, reliably creating well-separated quasi-monoenergetic features in the energy spectrum. The observed modulations are fully explained by a one-dimensional (1D) model supported by numerical simulations. These findings demonstrate that laser acceleration can, in principle, be applied in an additive manner.
Abstract: We present experimental results from an all-optical diagnostic method to directly measure the evolution of the hot-electron distribution driving the acceleration of ions from thin foils using high-intensity lasers. Central parameters of laser ion acceleration such as the hot-electron density, the temperature distribution and the conversion efficiency from laser pulse energy into hot electrons become comprehensively accessible with this technique.
Abstract: Experimental measurements of magnetic fields generated in the cavity of a self-injecting laser-wakefield accelerator are presented. Faraday rotation is used to determine the existence of multimegagauss fields, constrained to a transverse dimension comparable to the plasma wavelength similar to lambda(p) and several lambda(p) longitudinally. The fields are generated rapidly and move with the driving laser. In our experiment, the appearance of the magnetic fields is correlated with the production of relativistic electrons, indicating that they are inherently tied to the growth and wave breaking of the nonlinear plasma wave. This evolution is confirmed by numerical simulations, showing that these measurements provide insight into the wakefield evolution with high spatial and temporal resolution.
Abstract: We report the first experimental observation of a long-wavelength hosing modulation of a high-intensity laser pulse. Side-view images of the scattered optical radiation at the fundamental wavelength of the laser reveal a transverse oscillation of the laser pulse during its propagation through underdense plasma. The wavelength of the oscillation lambda(hosing) depends on the background plasma density n(e) and scales as lambda(hosing) similar to n(e)(-3/2). Comparisons with an analytical model and two-dimensional particle-in-cell simulations reveal that this laser hosing can be induced by a spatiotemporal asymmetry of the intensity distribution in the laser focus which can be caused by a misalignment of the parabolic focusing mirror or of the diffraction gratings in the pulse compressor.
Abstract: The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of mu m-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given.
Abstract: We describe a method that can be used for the coherent addition of laser pulses. As different laser pulses are initially generated in a laser-pulse compressor equipped with a tiled grating, such a coherent addition is indispens able in order to maximize the intensity in the laser far field. We present measurements in this context where, up to now, an unavoidable difference in the grating constants between the phased gratings reduced the maximum achievable intensity. The method significantly facilitates the high-precision alignment of a tiled grating compressor and could also be used for a coherent addition of laser pulses.
Abstract: Using state-of-the-art high-power laser systems, we are able to routinely generate extreme energy densities and focused light intensities in a controlled laboratory environment. During the interaction of these laser pulses with matter a plasma is generated that can both sustain and support huge electric fields. These can be used as a novel type of accelerator structure for electrons and ions having properties favorable for a large number of future applications.
Abstract: In this paper, a new system for improvement of the pulse contrast in CPA laser systems by use of an ultrafast OPA is reviewed together with a scheme to create sub‐picosecond synchronized OPA pump pulses. The scheme is being implemented at the PHELIX facility at GSI‐Darmstadt, Germany.
Abstract: At the Institute of Optics and Quantum Electronics, University of Jena, a fully diode pumped ultrahigh peak power laser system—POLARIS—has been realized. Presently, this laser system reaches a peak power of some ten terawatt. The last amplifier, which will boost the output energy to the 100 J level, is nearly completed and will be soon commissioned. The applied technologies and the basic design are reviewed here.