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Publications by
Stephan Kuschel

All publications of HI Jena

2019

A. Blinne, S. Kuschel, S. Tietze, and M. Zepf
Efficient retrieval of phase information from real-valued electromagnetic field data
Journal of Computational Physics: X 1, 100019 (2019)

Abstract: While analytic calculations may give access to complex-valued electromagnetic field data which allow trivial access to envelope and phase information, the majority of numeric codes uses a real-valued representation. This typically increases the performance and reduces the memory footprint, albeit at a price: In the real-valued case it is much more difficult to extract envelope and phase information, even more so if counter propagating waves are spatially superposed. A novel method for the analysis of real-valued electromagnetic field data is presented in this paper. We show that, by combining the real-valued electric and magnetic field at a single point in time, we can directly reconstruct the full information of the electromagnetic fields in the form of complex-valued spectral coefficients (k→-space) at a low computational cost of only three Fourier transforms. The method allows for counter propagating plane waves to be accurately distinguished as well as their complex spectral coefficients, i.e. spectral amplitudes and spectral phase to be calculated. From these amplitudes, the complex-valued electromagnetic fields and also the complex-valued vector potential can be calculated from which information about spatiotemporal phase and amplitude is readily available. Additionally, the complex fields allow for efficient vacuum propagation allowing to calculate far field data or boundary input data from near field data. An implementation of the new method is available as part of PostPic1, a data analysis toolkit written in the Python programming language.

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.

K. T. Behm, J. M. Cole, A. S. Joglekar, E. Gerstmayr, J. C. Wood, C. D. Baird, T. G. Blackburn, M. Duff, C. Harvey, A. Ilderton, S. Kuschel, S. P. D. Mangles, M. Marklund, P. McKenna, C. D. Murphy, Z. Najmudin, K. Poder, C. P. Ridgers, G. Sarri, G. M. Samarin, D. Symes, J. Warwick, M. Zepf, K. Krushelnick, and A. G. R. Thomas
A spectrometer for ultrashort gamma-ray pulses with photon energies greater than 10 MeV
Review of Scientific Instruments 89, 113303 (2018)

Abstract: We present a design for a pixelated scintillator based gamma-ray spectrometer for non-linear inverse Compton scattering experiments. By colliding a laser wakefield accelerated electron beam with a tightly focused, intense laser pulse, gamma-ray photons up to 100 MeV energies and with few femtosecond duration may be produced. To measure the energy spectrum and angular distribution, a 33 × 47 array of cesium-iodide crystals was oriented such that the 47 crystal length axis was parallel to the gamma-ray beam and the 33 crystal length axis was oriented in the vertical direction. Using an iterative deconvolution method similar to the YOGI code, modeling of the scintillator response using GEANT4 and fitting to a quantum Monte Carlo calculated photon spectrum, we are able to extract the gamma ray spectra generated by the inverse Compton interaction.

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.

T. Kurz, J. Couperus, J. Krämer, H. Ding, S. Kuschel, A. Köhler, O. Zarini, D. Hollatz, D. Schinkel, R. D’Arcy, J.-P. Schwinkendorf, J. Osterhoff, A. Irman, U. Schramm, and S. Karsch
Calibration and cross-laboratory implementation of scintillating screens for electron bunch charge determination
Review of Scientific Instruments 89, 093303 (2018)

Abstract: We revise the calibration of scintillating screens commonly used to detect relativistic electron beams with low average current, e.g., from laser-plasma accelerators, based on new and expanded measurements that include higher charge density and different types of screens than previous work. Electron peak charge densities up to 10 nC/mm2 were provided by focused picosecond-long electron beams delivered by the Electron Linac for beams with high Brilliance and low Emittance (ELBE) at the Helmholtz-Zentrum Dresden-Rossendorf. At low charge densities, a linear scintillation response was found, followed by the onset of saturation in the range of nC/mm2. The absolute calibration factor (photons/sr/pC) in this linear regime was measured to be almost a factor of 2 lower than that reported by Buck et al. retrospectively implying a higher charge in the charge measurements performed with the former calibration. A good agreement was found with the results provided by Glinec et al.. Furthermore long-term irradiation tests with an integrated dose of approximately 50 nC/mm2 indicate a significant decrease of the scintillation efficiency over time. Finally, in order to enable the transfer of the absolute calibration between laboratories, a new constant reference light source has been developed.

W. Schumaker, T. Liang, R. Clarke, J. M. Cole, G. Grittani, S. Kuschel, S. P. D. Mangles, Z. Najmudin, K. Poder, G. Sarri, D. Symes, A. G. R. Thomas, M. Vargas, M. Zepf, and K. Krushelnick
Making pions with laser light
New Journal of Physics 20, 073008 (2018)

Abstract: The interaction of high intensity short pulse laser beams with plasmas can accelerate electrons to energies in excess of a GeV. These electron beams can subsequently be used to generate short-lived particles such as positrons, muons, and pions. In recent experiments, we have made the first measurements of pion production using 'all optical' methods. In particular, we have demonstrated that the interaction of bremsstrahlung generated by laser driven electron beams with aluminum atoms can produce the long lived isotope of magnesium (²⁷Mg) which is a signature for pion (π⁺) production and subsequent muon decay. Using a 300 TW laser pulse, we have measured the generation of 150 ± 50 pions per shot. We also show that the energetic electron beam is a source of an intense, highly directional neutron beam resulting from (γ, n) reactions which contributes to the ²⁷Mg measurement as background via the (n, p) process.

K. Poder, M. Tamburini, G. Sarri, A. D. Piazza, S. Kuschel, C. Baird, K. Behm, S. Bohlen, J. Cole, D. Corvan, M. Duff, E. Gerstmayr, C. Keitel, K. Krushelnick, S. Mangles, P. McKenna, C. Murphy, Z. Najmudin, C. Ridgers, G. Samarin, D. Symes, A. Thomas, J. Warwick, and M. Zepf
Experimental Signatures of the Quantum Nature of Radiation Reaction in the Field of an Ultraintense Laser
Physical Review X 8, 031004 (2018)

Abstract: The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensity is one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date, there is no unanimously accepted theoretical solution for ultrahigh intensities and little or no experimental data. The basic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electron itself—the so-called radiation reaction force. We report here on the experimental evidence of strong radiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons (maximum energy exceeding 2 GeV) through the field of an ultraintense laser (peak intensity of 4×10²⁰  W/cm²). In their own rest frame, the highest-energy electrons experience an electric field as high as one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kinetic energy during the propagation through the laser field. The experimental data show signatures of quantum effects in the electron dynamics in the external laser field, potentially showing departures from the constant cross field approximation.

G. A. Becker, S. Tietze, S. Keppler, J. Reislöhner, J. H. Bin, L. Bock, F.-E. Brack, J. Hein, M. Hellwing, P. Hilz, M. Hornung, A. Kessler, S. D. Kraft, S. Kuschel, H. Liebetrau, W. Ma, J. Polz, H.-P. Schlenvoigt, F. Schorcht, M. B. Schwab, A. Seidel, K. Zeil, U. Schramm, M. Zepf, J. Schreiber, S. Rykovanov, and M. C. Kaluza
Ring-like spatial distribution of laser accelerated protons in the ultra-high-contrast TNSA-regime
Plasma Physics and Controlled Fusion 60, 055010 (2018)

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.

A. Blinne, D. Schinkel, S. Kuschel, N. Elkina, S. G. Rykovanov, and M. Zepf
A systematic approach to numerical dispersion in Maxwell solvers
Computer Physics Communications 224, 273 (2018)

Abstract: The finite-difference time-domain (FDTD) method is a well established method for solving the time evolution of Maxwell’s equations. Unfortunately the scheme introduces numerical dispersion and therefore phase and group velocities which deviate from the correct values. The solution to Maxwell’s equations in more than one dimension results in non-physical predictions such as numerical dispersion or numerical Cherenkov radiation emitted by a relativistic electron beam propagating in vacuum. Improved solvers, which keep the staggered Yee-type grid for electric and magnetic fields, generally modify the spatial derivative operator in the Maxwell–Faraday equation by increasing the computational stencil. These modified solvers can be characterized by different sets of coefficients, leading to different dispersion properties. In this work we introduce a norm function to rewrite the choice of coefficients into a minimization problem. We solve this problem numerically and show that the minimization procedure leads to phase and group velocities that are considerably closer to c as compared to schemes with manually set coefficients available in the literature. Depending on a specific problem at hand (e.g. electron beam propagation in plasma, high-order harmonic generation from plasma surfaces, etc.), the norm function can be chosen accordingly, for example, to minimize the numerical dispersion in a certain given propagation direction. Particle-in-cell simulations of an electron beam propagating in vacuum using our solver are provided.

J. M. Cole, K. T. Behm, E. Gerstmayr, T. G. Blackburn, J. C. Wood, C. D. Baird, M. J. Duff, C. Harvey, A. Ilderton, A. S. Joglekar, K. Krushelnick, S. Kuschel, M. Marklund, P. McKenna, C. D. Murphy, K. Poder, C. P. Ridgers, G. M. Samarin, G. Sarri, D. R. Symes, A. G. R. Thomas, J. Warwick, M. Zepf, Z. Najmudin, and S. P. D. Mangles
Experimental Evidence of Radiation Reaction in the Collision of a High-Intensity Laser Pulse with a Laser-Wakefield Accelerated Electron Beam
Physical Review X 8, 011020 (2018)

Abstract: The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today’s lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (ϵ>500  MeV) with an intense laser pulse (a₀>10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (γ rays), consistent with a quantum description of radiation reaction. The generated γ rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy ϵ꜀ᵣᵢₜ>30  MeV.

S. Kuschel
Erzeugung dichter Elektronenpulse mit Laser-Plasma-Beschleunigern für QED-Experimente in hohen Feldern
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: Quantum electrodynamics (QED) is widely considered to be one of the most accurately tested theories. Nevertheless fundamental processes such as pair production from the vacuum or the motion of the electron in extreme fields have not been measured in the laboratory to date. Their measurement requires a high intensity laser together with a high intensity electron or γ-beam, which can be produced by a high density electron bunch.

A recent development within the last two decades are plasma based accelerators. The high fields that can be sustained by a plasma are used to deliver extremely short and dense electron bunches while shrinking size and costs of the device. Importantly, they are automatically co-located with and synchronized to a high intensity laser pulse, providing an ideal basis for investigating QED in high fields.The availability of generating dense electron bunches brings new QED experiments within reach. However, the quality and stability of laser wake field accelerated (LWFA) electron beams still has to be improved to make these experiments possible. Beyond the tests of QED, the stability and quality of the electron beam is also crucial for highly demanding applications such as LWFA-driven free-electron lasers.

The first part of this thesis is devoted to the LWFA process and its improvements with a particular emphasis on improving the stability of laser plasma accelerators. It is shown that the gas dynamics on a 10 μm scale plays an important role in LWFA, which has not been fully appreciated yet. Density modulations on a 10 μm scale were measured in a gas jet using a few-cycle probe pulse. It is shown that self-injection can be triggered by these modulations. Particle-in-cell (PIC) simulations and analytical modeling confirm the experimental results. A gas cell providing a homogeneous plasma density has been developed in order to reduce self-injection. Using this gas cell, it was possible to suppress self-injection. The experiments show that self-injection was suppressed in the gas cell. Using ionization injection and the gas cell, the beam shape as well as the pointing stability were strongly improved. This finding paves the way towards self-injection free acceleration in a plasma based accelerator. It also establishes a new requirement on the homogenouity of the plasma density – not only for LWFA, but also in a broader context, for example in particle driven plasma wake field acceleration (PWFA).

In the second part of this, the possibility of focusing the ultra-short electron bunch by passive plasma lensing is studied. LWFA-beams typically have a very small source size and a divergence of the order or a few mrad, resulting in a rapid drop in electron beam density during free-space propagation. Many of the envisioned experiments, however, require intense focused electron bunches. Therefore, the concept of passive plasma lensing has been applied to ultra-short LWFA-bunches for the first time. The passive plasma lens effect was demonstrated experimentally by using a second gas target with predefined density. PIC simulations and analytical modeling describe the measured effect. Notably, the observed focusing strength of the passive plasma lens is larger compared to a conventional magnetic quadrupole lens. The analytical model predicts that the focusing strength can be further enhanced by increasing the bunch charge.

2017

G. Sarri, J. Warwick, W. Schumaker, K. Poder, J. Cole, D. Doria, T. Dzelzainis, K. Krushelnick, S. Kuschel, S. P. D. Mangles, Z. Najmudin, L. Romagnani, G. M. Samarin, D. Symes, A. G. R. Thomas, M. Yeung, and M. Zepf
Spectral and spatial characterisation of laser-driven positron beams
Plasma Physics and Controlled Fusion 59, 014015 (2017)

Abstract: The generation of high-quality relativistic positron beams is a central area of research in experimental physics, due to their potential relevance in a wide range of scientific and engineering areas, ranging from fundamental science to practical applications. There is now growing interest in developing hybrid machines that will combine plasma-based acceleration techniques with more conventional radio-frequency accelerators, in order to minimise the size and cost of these machines. Here we report on recent experiments on laser-driven generation of high-quality positron beams using a relatively low energy and potentially table-top laser system. The results obtained indicate that current technology allows to create, in a compact setup, positron beams suitable for injection in radio-frequency accelerators.

2016

M. Yeung, S. Rykovanov, J. Bierbach, L. Li, E. Eckner, S. Kuschel, A. Woldegeorgis, C. Rödel, A. Sävert, G. G. Paulus, M. Coughlan, B. Dromey, and M. Zepf
Experimental observation of attosecond control over relativistic electron bunches with two-colour fields
Nature Photonics 32, 11 (2016)

Abstract: Energy coupling during relativistically intense laser–matter interactions is encoded in the attosecond motion of strongly driven electrons at the pre-formed plasma–vacuum boundary. Studying and controlling this motion can reveal details about the microscopic processes that govern a vast array of light–matter interaction phenomena, including those at the forefront of extreme laser–plasma science such as laser-driven ion acceleration, bright attosecond pulse generation and efficient energy coupling for the generation and study of warm dense matter. Here we experimentally demonstrate that by precisely adjusting the relative phase of an additional laser beam operating at the second harmonic of the driving laser it is possible to control the trajectories of relativistic electron bunches formed during the interaction with a solid target at the attosecond scale. We observe significant enhancements in the resulting high-harmonic yield, suggesting potential applications for sources of ultra-bright, extreme ultraviolet attosecond radiation to be used in atomic and molecular pump–probe experiments.

S. Kuschel, D. Hollatz, T. Heinemann, O. Karger, M. B. Schwab, D. Ullmann, A. Knetsch, A. Seidel, C. Rödel, M. Yeung, M. Leier, A. Blinne, H. Ding, T. Kurz, D. J. Corvan, A. Sävert, S. Karsch, M. C. Kaluza, B. Hidding, and M. Zepf
Demonstration of passive plasma lensing of a laser wakefield accelerated electron bunch
Physical Review Accelerators and Beams 19, 071301 (2016)

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.

B. Dromey, M. Coughlan, L. Senje, M. Taylor, S. Kuschel, B. Villagomez-Bernabe, R. Stefanuik, G. Nersisyan, L. Stella, J. Kohanoff, M. Borghesi, F. Currell, D. Riley, D. Jung, C.-G. Wahlström, C. L. S. Lewis, and M. Zepf
Picosecond metrology of laser-driven proton bursts
Nature Communications 7, 10642 (2016)

Abstract: Tracking primary radiation-induced processes in matter requires ultrafast sources and high precision timing. While compact laser-driven ion accelerators are seeding the development of novel high instantaneous flux applications, combining the ultrashort ion and laser pulse durations with their inherent synchronicity to trace the real-time evolution of initial damage events has yet to be realized. Here we report on the absolute measurement of proton bursts as short as 3.5±0.7 ps from laser solid target interactions for this purpose. Our results verify that laser-driven ion acceleration can deliver interaction times over a factor of hundred shorter than those of state-of-the-art accelerators optimized for high instantaneous flux. Furthermore, these observations draw ion interaction physics into the field of ultrafast science, opening the opportunity for quantitative comparison with both numerical modelling and the adjacent fields of ultrafast electron and photon interactions in matter.

2015

M. Yeung, J. Bierbach, E. Eckner, S. Rykovanov, S. Kuschel, A. Sävert, M. Förster, C. Rödel, G. Paulus, S. Cousens, M. Coughlan, B. Dromey, and M. Zepf
Noncollinear Polarization Gating of Attosecond Pulse Trains in the Relativistic Regime
Physical Review Letters 115, 193903 (2015)

Abstract: High order harmonics generated at relativistic intensities have long been recognized as a route to the most powerful extreme ultraviolet pulses. Reliably generating isolated attosecond pulses requires gating to only a single dominant optical cycle, but techniques developed for lower power lasers have not been readily transferable. We present a novel method to temporally gate attosecond pulse trains by combining noncollinear and polarization gating. This scheme uses a split beam configuration which allows pulse gating to be implemented at the high beam fluence typical of multi-TW to PW class laser systems. Scalings for the gate width demonstrate that isolated attosecond pulses are possible even for modest pulse durations achievable for existing and planned future ultrashort high-power laser systems. Experimental results demonstrating the spectral effects of temporal gating on harmonic spectra generated by a relativistic laser plasma interaction are shown.

J. Bierbach, M. Yeung, E. Eckner, C. Roedel, S. Kuschel, M. Zepf, and G. G. Paulus
Long-term operation of surface high-harmonic generation from relativistic oscillating mirrors using a spooling tape
Optics Express 23, 12321 (2015)

Abstract: Surface high-harmonic generation in the relativistic regime is demonstrated as a source of extreme ultra-violet (XUV) pulses with extended operation time. Relativistic high-harmonic generation is driven by a frequency-doubled high-power Ti:Sapphire laser focused to a peak intensity of 3·10^19 W/cm2 onto spooling tapes. We demonstrate continuous operation over up to one hour runtime at a repetition rate of 1 Hz. Harmonic spectra ranging from 20 eV to 70 eV (62 nm to 18 nm) were consecutively recorded by an XUV spectrometer. An average XUV pulse energy in the µJ range is measured. With the presented setup, relativistic surface high-harmonic generation becomes a powerful source of coherent XUV pulses that might enable applications in, e.g. attosecond laser physics and the seeding of free-electron lasers, when the laser issues causing 80-% pulse energy fluctuations are overcome.

T. Hahn, J. Bierbach, C. Rödel, D. Hemmers, M. Yeung, B. Dromey, S. Fuchs, A. Galestian, S. Kuschel, M. Zepf, G. Paulus, and G. Pretzler
Broadband XUV polarimetry of high harmonics from plasma surfaces using multiple Fresnel reflections
Applied Physics B 118, 241 (2015)

Abstract: High-harmonic generation (HHG) by nonlinear interaction of intense laser pulses with gases or plasma surfaces is the most prominent way of creating highly coherent extreme ultraviolet (EUV/XUV) pulses. In the last years, several scientific applications have been found which require the measurement of the polarization of the harmonic radiation. We present a broadband XUV polarimeter based on multiple Fresnel reflections providing an extinction rate of 5–25 for 17–45 nm which is particularly suited for surface harmonics. The device has first been tested at a gas harmonic source providing linearly polarized XUV radiation. In a further experiment using HHG from plasma surfaces, the XUV polarimeter allowed a polarization measurement of high harmonic radiation from plasma surfaces for the first time which reveals a linear polarization state as predicted for our generation parameters. The generation and control of intense polarized XUV pulses - together with the availability of broadband polarizers in the XUV - open the way for a series of new experiments. For instance, dichroism in the XUV, elliptically polarized harmonics from aligned molecules, or the selection rules of relativistic surface harmonics can be studied with the broadband XUV polarimeter.

B. Aurand, S. Kuschel, C. Rödel, O. Jäckel, J. Polz, B. Elkin, H. Zhao, A. Karmakar, P. Gibbon, M. Kaluza, and T. Kühl
Reduction of X-ray generation in high-intensity laser ion acceleration
Applied Physics B 118, 247 (2015)

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.

2014

L. Senje, M. Yeung, B. Aurand, S. Kuschel, C. Rödel, F. Wagner, K. Li, B. Dromey, V. Bagnoud, P. Neumayer, M. Roth, C.-G. Wahlström, M. Zepf, T. Kuehl, and D. Jung
Diagnostics for studies of novel laser ion acceleration mechanisms
Review of Scientific Instruments 85, 113302 (2014)

Abstract: Diagnostic for investigating and distinguishing different laser ion acceleration mechanisms has been developed and successfully tested. An ion separation wide angle spectrometer can simultaneously investigate three important aspects of the laser plasma interaction: (1) acquire angularly resolved energy spectra for two ion species, (2) obtain ion energy spectra for multiple species, separated according to their charge to mass ratio, along selected axes, and (3) collect laser radiation reflected from and transmitted through the target and propagating in the same direction as the ion beam. Thus, the presented diagnostic constitutes a highly adaptable tool for accurately studying novel acceleration mechanisms in terms of their angular energy distribution, conversion efficiency, and plasma density evolution.

B. Aurand, S. Kuschel, O. Jäckel, C. Rödel, H. Zhao, S. Herzer, A. Paz, J. Bierbach, J. Polz, B. Elkin, A. Karmakar, P. Gibbon, M. C. Kaluza, and T. Kühl
Enhanced radiation pressure-assisted acceleration by temporally tuned counter-propagating pulses
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 740, 83 (2014)

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.

B. Aurand, B. Elkin, L.-O. Heim, B. Lommel, B. Kindler, M. Tomut, C. Rödel, S. Kuschel, O. Jäckel, and T. Kühl
Ultra-thin polymer foils for laser-ion acceleration
Journal of Radioanalytical and Nuclear Chemistry 299, 965 (2014)

Abstract: We report on the development of new materials for laser-ion acceleration applicable for the advanced mechanism of radiation-pressure-acceleration. These targets are ultra-thin with thicknesses of just a few nm. For several years, diamond-like carbon foils in this thickness range can be produced. An alternative material containing more than one ion-species has the potential to further improve the acceleration mechanism. The fabrication and characterization of self-supporting polymer-based targets will be described in this paper. Furthermore, we show the significant influence on a radiation-pressure induced acceleration process by experimental data.

2013

B. Aurand, B. Elkin, L.-O. Heim, B. Lommel, B. Kindler, M. Tomut, C. Rödel, S. Kuschel, O. Jäckel, J. Barz, and T. Kühl
Preparation and characterization of nanometer-thin freestanding polymer foils for laser-ion acceleration
Journal of Polymer Science Part B: Polymer Physics 51, 1355 (2013)

Abstract: We report on the production and characterization of polymer-based ultra-thin (sub 10 nm) foils suited for experiments on laser-ion acceleration in the regime of radiation pressure acceleration. Beside the remarkable mechanical stability compared with commonly used diamond-like-carbon foils, a very homogeneous layer thickness and a small surface roughness have been achieved. We describe the technical issues of the production process as well as detailed studies of the mechanical stability and surface roughness tests. The capability of producing uniform targets of large area is essential for advanced laser-ion acceleration projects which are dealing with high repetition rate and extended measurement series, but might also be useful for other applications which require ultra-thin and freestanding substrates of high quality.

B. Aurand, S. Kuschel, O. Jäckel, C. Rödel, H. Y. Zhao, S. Herzer, A. E. Paz, J. Bierbach, J. Polz, B. Elkin, G. G. Paulus, A. Karmakar, P. Gibbon, T. Kühl, and M. C. Kaluza
Radiation pressure-assisted acceleration of ions using multi-component foils in high-intensity laser-matter interactions
New Journal of Physics 15, 033031 (2013)

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.

S. Fuchs, C. Rödel, M. Krebs, S. Hädrich, J. Bierbach, A. E. Paz, S. Kuschel, M. Wünsche, V. Hilbert, U. Zastrau, E. Förster, J. Limpert, and G. G. Paulus
Sensitivity calibration of an imaging extreme ultraviolet spectrometer-detector system for determining the efficiency of broadband extreme ultraviolet sources
Review of Scientific Instruments 84, 023101 (2013)

Abstract: We report on the absolute sensitivity calibration of an extreme ultraviolet (XUV) spectrometer system that is frequently employed to study emission from short-pulse laser experiments. The XUV spectrometer, consisting of a toroidal mirror and a transmission grating, was characterized at a synchrotron source in respect of the ratio of the detected to the incident photon flux at photon energies ranging from 15.5 eV to 99 eV. The absolute calibration allows the determination of the XUV photon number emitted by laser-based XUV sources, e.g., high-harmonic generation from plasma surfaces or in gaseous media. We have demonstrated high-harmonic generation in gases and plasma surfaces providing 2.3 μW and μJ per harmonic using the respective generation mechanisms.

2012

A. Paz, S. Kuschel, C. Rödel, M. Schnell, O. Jäckel, M. C. Kaluza, and G. G. Paulus
Thomson backscattering from laser-generated, relativistically moving high-density electron layers
New Journal of Physics 14, 093018 (2012)

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.

B. B. Pollock, J. Meinecke, S. Kuschel, J. S. Ross, J. L. Shaw, C. Stoafer, L. Divol, G. R. Tynan, and S. H. Glenzer
Simultaneous imaging electron- and ion-feature Thomson scattering measurements of radiatively heated Xe
Review of Scientific Instruments 83, 10E348 (2012)

Abstract: Uniform density and temperature Xe plasmas have been produced over > 4 mm scale-lengths using x-rays generated in a cylindrical Pb cavity. The cavity is 750 μm in depth and diameter, and is heated by a 300 J, 2 ns square, 1054 nm laser pulse focused to a spot size of 200 μm at the cavity entrance. The plasma is characterized by simultaneous imaging Thomson scattering measurements from both the electron and ion scattering features. The electron feature measurement determines the spatial electron density and temperature profile, and using these parameters as constraints in the ion feature analysis allows an accurate determination of the charge state of the Xe ions. The Thomson scattering probe beam is 40 J, 200 ps, and 527 nm, and is focused to a 100 μm spot size at the entrance of the Pb cavity. Each system has a spatial resolution of 25 μm, a temporal resolution of 200 ps (as determined by the probe duration), and a spectral resolution of 2 nm for the electron feature system and 0.025 nm for the ion feature system. The experiment is performed in a Xe filled target chamber at a neutral pressure of 3–10 Torr, and the x-rays produced in the Pb ionize and heat the Xe to a charge state of 20 ± 4 at up to 200 eV electron temperatures.

J. Bierbach, C. Rödel, M. Yeung, B. Dromey, T. Hahn, A. Pour, S. Fuchs, A. E. Paz, S. Herzer, S. Kuschel, O. Jäckel, M. C. Kaluza, G. Pretzler, M. Zepf, and G. G. Paulus
Generation of 10 µW relativistic surface high-harmonic radiation at a repetition rate of 10 Hz
New Journal of Physics 14, 065005 (2012)

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.

B. Aurand, C. Rödel, H. Zhao, S. Kuschel, M. Wünsche, O. Jäckel, M. Heyer, F. Wunderlich, M. C. Kaluza, G. G. Paulus, and T. Kühl
Note: A large aperture four-mirror reflective wave-plate for high-intensity short-pulse laser experiments
Review of Scientific Instruments 83, 036104 (2012)

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.

2011

B. Aurand, S. Kuschel, C. Rödel, M. Heyer, F. Wunderlich, O. Jäckel, M. C. Kaluza, G. G. Paulus, and T. Kühl
Creating circularly polarized light with a phase-shifting mirror
Optics Express 19, 17151 (2011)

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.