Studying the interaction of ultrashort, intense laser pulses with solid targets

This thesis experimentally investigates laser-driven proton acceleration in the regime of target normal sheath acceleration (TNSA) using ultrashort (pulse duration τL = 30 fs), high power (∼100TW) laser pulses. The work focuses on how the temporal intensity profile of the ultrashort laser pulse influences the plasma formation during the laser-target interaction and the subsequent acceleration process. The corresponding experiments are performed at the Draco laser facility at the Helmholtz-Zentrum Dresden – Rossendorf. The main result of the thesis is the experimental observation of transverse spatial modulations in the laser-driven proton distribution. The onset of the modulations occurs above a target-dependent laser energy threshold and is found to correlate with parasitic laser emission preceding the ultrashort laser pulse. The analysis of the underlying plasma dynamics by using numerical simulations indicates that plasma instabilities lead to the filamentation of the laser-accelerated electron distribution. The resulting spatial pattern in the electron distribution is then transferred to the proton distribution during the acceleration process. The plasma instabilities, which the electron current is subjected to, are a surface-ripple-seeded Rayleigh-Taylor or a Weibel instability. Regarding their occurrence, both instabilities show a strong dependence on the initial plasma conditions at the target. This supports the experimentally observed connection between the temporal intensity profile of the laser pulse and the development of spatial modulations in the proton distribution. The study is considered the first observation of (regular) proton beam modulations for TNSA in the regime of ultrashort laser pulses and micrometer thick target foils. The experiments emphasize the requirement for TNSA laser power scaling studies under the consideration of realistic laser-plasma interaction conditions. In that way, the potential of the upcoming generation of Petawatt power lasers for laser-driven proton acceleration can be assessed and fully exploited. In the second part of the thesis, experimental pump-probe techniques are investigated. With an imaging method termed high depth-of-field time-resolved microscopy in a reflective probing setup, micrometer-size local features of the near-critical density plasma as well as the global topography of the plasma can be resolved. The spatio-temporal resolution of the target ionization and heating dynamics is achieved by probing the target reflectivity, whereas the angular distribution of the reflected probe beam carries signatures of the plasma expansion. The presented probing technique avails to correlate the temporal intensity profile of a laser pulse with the spatio-temporal plasma evolution triggered upon laser-target interaction

Studying the interaction of ultrashort, intense laser pulses with solid targets

This thesis experimentally investigates laser-driven proton acceleration in the regime of target normal sheath acceleration (TNSA) using ultrashort (pulse duration τL = 30 fs), high power (∼100TW) laser pulses. The work focuses on how the temporal intensity profile of the ultrashort laser pulse influences the plasma formation during the laser-target interaction and the subsequent acceleration process. The corresponding experiments are performed at the Draco laser facility at the Helmholtz-Zentrum Dresden – Rossendorf. The main result of the thesis is the experimental observation of transverse spatial modulations in the laser-driven proton distribution. The onset of the modulations occurs above a target-dependent laser energy threshold and is found to correlate with parasitic laser emission preceding the ultrashort laser pulse. The analysis of the underlying plasma dynamics by using numerical simulations indicates that plasma instabilities lead to the filamentation of the laser-accelerated electron distribution. The resulting spatial pattern in the electron distribution is then transferred to the proton distribution during the acceleration process. The plasma instabilities, which the electron current is subjected to, are a surface-ripple-seeded Rayleigh-Taylor or a Weibel instability. Regarding their occurrence, both instabilities show a strong dependence on the initial plasma conditions at the target. This supports the experimentally observed connection between the temporal intensity profile of the laser pulse and the development of spatial modulations in the proton distribution. The study is considered the first observation of (regular) proton beam modulations for TNSA in the regime of ultrashort laser pulses and micrometer thick target foils. The experiments emphasize the requirement for TNSA laser power scaling studies under the consideration of realistic laser-plasma interaction conditions. In that way, the potential of the upcoming generation of Petawatt power lasers for laser-driven proton acceleration can be assessed and fully exploited. In the second part of the thesis, experimental pump-probe techniques are investigated. With an imaging method termed high depth-of-field time-resolved microscopy in a reflective probing setup, micrometer-size local features of the near-critical density plasma as well as the global topography of the plasma can be resolved. The spatio-temporal resolution of the target ionization and heating dynamics is achieved by probing the target reflectivity, whereas the angular distribution of the reflected probe beam carries signatures of the plasma expansion. The presented probing technique avails to correlate the temporal intensity profile of a laser pulse with the spatio-temporal plasma evolution triggered upon laser-target interaction

Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets.

We report on recent experimental results deploying a continuous cryogenic hydrogen jet as a debris-free, renewable laser-driven source of pure proton beams generated at the 150 TW ultrashort pulse laser Draco. Efficient proton acceleration reaching cut-off energies of up to 20 MeV with particle numbers exceeding 109 particles per MeV per steradian is demonstrated, showing for the first time that the acceleration performance is comparable to solid foil targets with thicknesses in the micrometer range. Two different target geometries are presented and their proton beam deliverance characterized: cylindrical (∅ 5 μm) and planar (20 μm × 2 μm). In both cases typical Target Normal Sheath Acceleration emission patterns with exponential proton energy spectra are detected. Significantly higher proton numbers in laser-forward direction are observed when deploying the planar jet as compared to the cylindrical jet case. This is confirmed by two-dimensional Particle-in-Cell (2D3V PIC) simulations, which demonstrate that the planar jet proves favorable as its geometry leads to more optimized acceleration conditions

I-BEAT: New ultrasonic method for single bunch measurement of ion energy distribution

The shape of a wave carries all information about the spatial and temporal structure of its source, given that the medium and its properties are known. Most modern imaging methods seek to utilize this nature of waves originating from Huygens' principle. We discuss the retrieval of the complete kinetic energy distribution from the acoustic trace that is recorded when a short ion bunch deposits its energy in water. This novel method, which we refer to as Ion-Bunch Energy Acoustic Tracing (I-BEAT), is a generalization of the ionoacoustic approach. Featuring compactness, simple operation, indestructibility and high dynamic ranges in energy and intensity, I-BEAT is a promising approach to meet the needs of petawatt-class laser-based ion accelerators. With its capability of completely monitoring a single, focused proton bunch with prompt readout it, is expected to have particular impact for experiments and applications using ultrashort ion bunches in high flux regimes. We demonstrate its functionality using it with two laser-driven ion sources for quantitative determination of the kinetic energy distribution of single, focused proton bunches.Comment: Paper: 17 Pages, 3 figures Supplementary Material 16 pages, 7 figure

Spectral and spatial shaping of laser-driven proton beams using a pulsed high-field magnet beamline

Intense laser-driven proton pulses, inherently broadband and highly divergent, pose a challenge to established beamline concepts on the path to application-adapted irradiation field formation, particularly for 3D. Here we experimentally show the successful implementation of a highly efficient (50% transmission) and tuneable dual pulsed solenoid setup to generate a homogeneous (8.5% uniformity laterally and in depth) volumetric dose distribution (cylindrical volume of 5 mm diameter and depth) at a single pulse dose of 0.7 Gy via multi-energy slice selection from the broad input spectrum. The experiments have been conducted at the Petawatt beam of the Dresden Laser Acceleration Source Draco and were aided by a predictive simulation model verified by proton transport studies. With the characterised beamline we investigated manipulation and matching of lateral and depth dose profiles to various desired applications and targets. Using a specifically adapted dose profile, we successfully performed first proof-of-concept laser-driven proton irradiation studies of volumetric in-vivo normal tissue (zebrafish embryos) and in-vitro tumour tissue (SAS spheroids) samples.Comment: Submitted to Scientific Report