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Publications by
Dr. Alexander Sävert

All publications of HI Jena

2022

L. Doyle, P. Khademi, P. Hilz, A. Sävert, G. Schäfer, J. Schreiber, and M. Zepf
Experimental estimates of the photon background in a potential light-by-light scattering study
New Journal of Physics 24, 025003 (2022)

Abstract: High power short pulse lasers provide a promising route to study the strong field effects of the quantum vacuum, for example by direct photon-photon scattering in the all-optical regime. Theoretical predictions based on realistic laser parameters achievable today or in the near future predict scattering of a few photons with colliding Petawatt laser pulses, requiring single photon sensitive detection schemes and very good spatio-temporal filtering and background suppression. In this article, we present experimental investigations of this photon background by employing only a single high power laser pulse tightly focused in residual gas of a vacuum chamber. The focal region was imaged onto a single-photon sensitive, time gated camera. As no detectable quantum vacuum signature was expected in our case, the setup allowed for characterization and first mitigation of background contributions. For the setup employed, scattering off surfaces of imperfect optics dominated below residual gas pressures of 1 x 10(-4) mbar. Extrapolation of the findings to intensities relevant for photon-photon scattering studies is discussed.

2021

H. Abramowicz, U. Acosta, M. Altarelli, R. Aßmann, Z. Bai, T. Behnke, Y. Benhammou, T. Blackburn, S. Boogert, O. Borysov, M. Borysova, R. Brinkmann, M. Bruschi, F. Burkart, K. Büßer, N. Cavanagh, O. Davidi, W. Decking, U. Dosselli, N. Elkina, A. Fedotov, M. Firlej, T. Fiutowski, K. Fleck, M. Gostkin, C. Grojean, J. Hallford, H. Harsh, A. Hartin, B. Heinemann, T. Heinzl, L. Helary, M. Hoffmann, S. Huang, X. Huang, M. Idzik, A. Ilderton, R. Jacobs, B. Kämpfer, B. King, H. Lahno, A. Levanon, A. Levy, I. Levy, J. List, W. Lohmann, T. Ma, A. J. Macleod, V. Malka, F. Meloni, A. Mironov, M. Morandin, J. Moron, E. Negodin, G. Perez, I. Pomerantz, R. Pöschl, R. Prasad, F. Quéré, A. Ringwald, C. Rödel, S. Rykovanov, F. Salgado, A. Santra, G. Sarri, A. Sävert, A. Sbrizzi, S. Schmitt, U. Schramm, S. Schuwalow, D. Seipt, L. Shaimerdenova, M. Shchedrolosiev, M. Skakunov, Y. Soreq, M. Streeter, K. Swientek, N. Hod, S. Tang, T. Teter, D. Thoden, A. I. Titov, O. Tolbanov, G. Torgrimsson, A. Tyazhev, M. Wing, M. Zanetti, A. Zarubin, K. Zeil, M. Zepf, and A. Zhemchukov
Conceptual design report for the LUXE experiment
European Physical Journal Special Topics 230, 2445 (2021)
No abstract availableLinkBibTeX

2020

M. B. Schwab, E. Siminos, T. Heinemann, D. Ullmann, F. Karbstein, S. Kuschel, A. Sävert, M. Yeung, D. Hollatz, A. Seidel, J. Cole, S. P. D. Mangles, B. Hidding, M. Zepf, S. Skupin, and M. C. Kaluza
Visualization of relativistic laser pulses in underdense plasma
Physical Review Accelerators and Beams 23, 032801 (2020)

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

G. Becker, M. Schwab, R. Lötzsch, S. Tietze, D. Klöpfel, M. Rehwald, H.-P. Schlenvoigt, A. Sävert, U. Schramm, M. Zepf, and M. Kaluza
Characterization of laser-driven proton acceleration from water microdroplets
Scientific Reports 9, 17169 (2019)

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.

2018

A. Bernhard, V. A. Rodríguez, S. Kuschel, M. Leier, P. Peiffer, A. Sävert, M. Schwab, W. Werner, C. Widmann, A. Will, A.-S. Müller, and M. Kaluza
Progress on experiments towards LWFA-driven transverse gradient undulator-based FELs
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 909, 391 (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.

S. Kuschel, M. B. Schwab, M. Yeung, D. Hollatz, A. Seidel, W. Ziegler, A. Sävert, M. C. Kaluza, and M. Zepf
Controlling the Self-Injection Threshold in Laser Wakefield Accelerators
Physical Review Letters 121, 154801 (2018)

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.

O. N. Rosmej, Z. Samsonova, S. Höfer, D. Kartashov, C. Arda, D. Khaghani, A. Schoenlein, S. Zähter, A. Hoffmann, R. Loetzsch, A. Saevert, I. Uschmann, M. E. Povarnitsyn, N. E. Andreev, L. P. Pugachev, M. C. Kaluza, and C. Spielmann
Generation of keV hot near-solid density plasma states at high contrast laser-matter interaction
Physics of Plasmas 25, 083103 (2018)

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.

A. K. Arunachalam, M. B. Schwab, A. Sävert, and M. C. Kaluza
Observation of non-symmetric side-scattering during high-intensity laser-plasma interactions
New Journal of Physics 20, 033027 (2018)

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.

2017

D. Adolph, M. Möller, J. Bierbach, M. Schwab, A. Sävert, M. Yeung, A. M. Sayler, M. Zepf, M. C. Kaluza, and G. G. Paulus
Real-time, single-shot, carrier-envelope-phase measurement of a multi-terawatt laser
Applied Physics Letters 110, 081105 (2017)

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.

M. Kasim, L. Ceurvorst, N. Ratan, J. Sadler, N. Chen, A. Sävert, R. Trines, R. Bingham, P. Burrows, M. Kaluza, and P. Norreys
Quantitative shadowgraphy and proton radiography for large intensity modulations
Physical Review E 95, 023306 (2017)

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.

2016

S. Keppler, A. Sävert, J. Körner, M. Hornung, H. Liebetrau, J. Hein, and M. Kaluza
The generation of amplified spontaneous emission in high-power CPA laser systems
Laser & Photonics Reviews 10, 264 (2016)

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.

A. Sävert
Few-cycle microscopy of a laser wakefield accelerator
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)

Abstract: This thesis describes the development and first application of a novel diagnostic for a laser driven wakefield accelerator. It is termed Few‐Cycle Microscopy (FCM) and consists of a high-resolution imaging system and probe pulses with a duration of a few optical cycles synchronized to a high intensity laser pulse. Using FCM has opened a pristine view into the laser‐plasma interaction and has allowed to record high‐resolution images of the plasma wave in real time. Important stages during the wave’s evolution such as its formation, its breaking and finally the acceleration of electrons in the associated wake fields were observed in the experiment as well as in simulations, allowing for the first time a quantitative comparison between analytical and numerical models and experimental results. Using this diagnostic, the expansion of the wave’s first period, the so‐called ‘bubble’, was identified to be crucial for the injection of electrons into the wave. Furthermore, the shadowgrams taken with FCM in combination with interferograms and backscatter spectra have revealed a new acceleration regime when using hydrogen as the target gas. It was found that in this scheme electron pulses are generated with a higher charge, lower divergence and better pointing stability than with helium gas. The underlying pre‐heating process could be attributed to stimulated Raman scattering, which has been thought up till now to be negligible for short (t < 30 fs) laser pulses. However, as it is shown in this thesis, the interplay of the temporal intensity contrast of the laser pulse 1 ps before the peak of the pulse together with a sufficiently high plasma electron density can provide suitable conditions for this instability to grow, resulting in improved electron pulse parameters.

2014

M. Hornung, H. Liebetrau, A. Seidel, S. Keppler, A. Kessler, J. Körner, M. Hellwing, F. Schorcht, D. Klöpfel, A. K. Arunachalam, G. A. Becker, A. Sävert, J. Polz, J. Hein, and M. C. Kaluza
The all-diode-pumped laser system POLARIS – an experimentalist’s tool generating ultra-high contrast pulses with high energy
High Power Laser Science and Engineering 2, e20 (2014)

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.

S. Keppler, M. Hornung, R. Bödefeld, A. Sävert, H. Liebetrau, J. Hein, and M. C. Kaluza
Full characterization of the amplified spontaneous emission from a diode-pumped high-power laser system
Optics Express 22, 11228 (2014)

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.

A. Kessler, M. Hornung, S. Keppler, F. Schorcht, M. Hellwing, H. Liebetrau, J. Körner, A. Sävert, M. Siebold, M. Schnepp, J. Hein, and M. C. Kaluza
16.6 J chirped femtosecond laser pulses from a diode-pumped Yb:CaF2 amplifier
Optics Letters 39, 1333 (2014)

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

S. Keppler, C. Wandt, M. Hornung, R. Bödefeld, A. Kessler, A. Sävert, M. Hellwing, F. Schorcht, J. Hein, and M. C. Kaluza
Multipass amplifiers of POLARIS
Proceedings of SPIE 8780, 87800I (2013)

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.

M. Hornung, S. Keppler, R. Bödefeld, A. Kessler, H. Liebetrau, J. Körner, M. Hellwing, F. Schorcht, O. Jäckel, A. Sävert, J. Polz, A. K. Arunachalam, J. Hein, and M. C. Kaluza
High-intensity, high-contrast laser pulses generated from the fully diode-pumped Yb:glass laser system POLARIS
Optics Letters 38, 718 (2013)

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.

2011

S. Keppler, R. Bödefeld, M. Hornung, A. Sävert, J. Hein, and M. C. Kaluza
Prepulse suppression in a multi-10-TW diode-pumped Yb:glass laser
Applied Physics B 104, 11 (2011)

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.