Luke Ceurvorst

Compression of X-ray free electron laser pulses to attosecond duration

State of the art X-ray Free Electron Laser facilities currently provide the brightest X-ray pulses available, typically with mJ energy and several hundred femtosecond duration. Here we present one- and two-dimensional Particle-in-Cell simulations, utilising the process of stimulated Raman amplification, showing that these pulses are compressed to a temporally coherent, sub-femtosecond pulse at 8% efficiency. Pulses of this type may pave the way for routine time resolution of electrons in nm size potentials. Furthermore, evidence is presented that significant Landau damping and wave-breaking may be beneficial in distorting the rear of the interaction and further reducing the final pulse duration.

Quantitative shadowgraphy and proton radiography for large intensity modulations

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.

Optimization of plasma amplifiers

Plasma amplifiers offer a route to side-step limitations on chirped pulse amplification and generate laser pulses at the power frontier. They compress long pulses by transferring energy to a shorter pulse via the Raman or Brillouin instabilities. We present an extensive kinetic numerical study of the three-dimensional parameter space for the Raman case. Further particle-in-cell simulations find the optimal seed pulse parameters for experimentally relevant constraints. The high-efficiency self-similar behavior is observed only for seeds shorter than the linear Raman growth time. A test case similar to an upcoming experiment at the Laboratory for Laser Energetics is found to maintain good transverse coherence and high-energy efficiency. Effective compression of a 10kJ, nanosecond-long driver pulse is also demonstrated in a 15-cm-long amplifier.

Dense plasma heating by crossing relativistic electron beams

Here we investigate, using relativistic fluid theory and Vlasov-Maxwell simulations, the local heating of a dense plasma by two crossing electron beams. Heating occurs as an instability of the electron beams drives Langmuir waves, which couple nonlinearly into damped ion-acoustic waves. Simulations show a factor 2.8 increase in electron kinetic energy with a coupling efficiency of 18%. Our results support applications to the production of warm dense matter and as a driver for inertial fusion plasmas.

Machine learning applied to proton radiography of high-energy-density plasmas

Proton radiography is a technique extensively used to resolve magnetic field structures in high-energy-density plasmas, revealing a whole variety of interesting phenomena such as magnetic reconnection and collisionless shocks found in astrophysical systems. Existing methods of analyzing proton radiographs give mostly qualitative results or specific quantitative parameters, such as magnetic field strength, and recent work showed that the line-integrated transverse magnetic field can be reconstructed in specific regimes where many simplifying assumptions were needed. Using artificial neural networks, we demonstrate for the first time 3D reconstruction of magnetic fields in the nonlinear regime, an improvement over existing methods, which reconstruct only in 2D and in the linear regime. A proof of concept is presented here, with mean reconstruction errors of less than 5% even after introducing noise. We demonstrate that over the long term, this approach is more computationally efficient compared to other techniques. We also highlight the need for proton tomography because (i) certain field structures cannot be reconstructed from a single radiograph and (ii) errors can be further reduced when reconstruction is performed on radiographs generated by proton beams fired in different directions.

Attosecond-scale absorption at extreme intensities

A novel non-ponderomotive absorption mechanism, originally presented by Baeva et al. [Phys. Plasmas 18, 056702 (2011)] in one dimension, is extended into higher dimensions for the first time. This absorption mechanism, the Zero Vector Potential (ZVP), is expected to dominate the interactions of ultra-intense laser pulses with critically over-dense plasmas such as those that are expected with the Extreme Light Infrastructure laser systems. It is shown that the mathematical form of the ZVP mechanism and its key scaling relations found by Baeva et al. in 1D are identically reproduced in higher dimensions. The two dimensional particle-in-cell simulations are then used to validate both the qualitative and quantitative predictions of the theory.

Effect of preplasma on double pulse irradiation of targets for proton acceleration

Summary form only given. We report on the experimental and simulated characterization of proton acceleration from double-pulse irradiation of um-scale foil targets with varying preplasma conditions. Temporally separated pulses of less than a picosecond in duration have been shown to increase the conversion efficiency of laser energy to MeV protons1. The experiment utilized two 700 fs, 1054 nm pulses, separated by 1 to 5 ps; total beam energy was 100 J, with 5-20% of the total energy contained within the first pulse. In contrast to the ultraclean beams used in previous experiments1, prepulse energies on the order of 10 mJ were present. The resulting significant preplasma appears to have a moderating effect on the double pulse enhancement. Proton beam measurements were made with radiochromic film stacks and magnetic spectrometers.LSP 2D PIC simulations2 have been performed to better understand the double pulse enhancement mechanism, as well as the role of preplasma in modifying this effect. Simulation results will be shown for various target conditions, and compared to experimental data.

Characterization of x-ray lens for use in probing high energy density states of matter

By using polycapillary lenses to focus laser-produced x-ray sources to high intensities, an improvement in signal-to-noise ratio can be achieved. Here the He-α line emission produced by driving a titanium backlighter target is focused by a polycapillary lens and the output characterized. The x-ray spot is measured to have a peak intensity of 4.5 × 107 photons, with a total photon count of 8.8 × 108 in 0.13 ± 0.01 mm2. This setup is equivalent to placing the backlighter target 3 mm from the sample with a 600 μm diameter pinhole. The polycapillary lens enables the placement of the backlighter target at a much larger distance from the sample to be studied and therefore has the ability to greatly improve the signal-to-noise ratio on detectors. We demonstrate this with two simple diffraction experiments using pyrolytic graphite and polycrystalline aluminium.