Ion acceleration by compact laser-plasma sources promises a variety of applications, but reaching the required beam quality parameters demands a high level of understanding and control over the laser-plasma interaction process. Several advanced acceleration schemes, including the Relativistically Induced Transparency (RIT) regime, have been proposed and investigated in search of a stable acceleration regime for proton energies beyond 100 MeV.
Central component in the RIT scheme is the absorption of the electromagnetic laser field by the target and the generated plasma respectively. In joint studies, we experimentally explore these interactions at the onset of transparency at the two laser systems DRACO PW (HZDR) and J-KAREN (KPSI). With our transmission diagnostics, we characterize this onset with respect to the laser temporal profile in order to learn about the sensitivity of laser input parameters to increase the process’s robustness. Using ultra-short, high and low-contrast laser pulses on thin solid density foil targets, we observe high performance proton beams in an expanded foil case. The results of spectral, spatial, and energy analysis of the effects on the transmission and its correlation with the acceleration performance indicate changes in the plasma interaction itself.
The ultrahigh laser intensities enabled by high power lasers facilitate the generation of high energy ions using accelerating gradients many million times that of conventional accelerators. The maturation of these sources relies on breakthroughs in the generated beam parameters and improved reproducibility and repetition rate. We used two independent state-of-the-art femtosecond laser systems capable of repetitive operation to accelerate protons and carbons to high energies (>50 MeV and 30 MeV/nucleon respectively) in the relativistically induced transparency (RIT) regime. We demonstrate that acceleration is optimised for different laser prepulse levels by varying the initial target thickness, relaxing laser requirements for energetic ion generation. We elucidated the acceleration dynamics with cutting-edge 3D simulation, showing a) the role of the laser prepulse in pre-expanding the target, and b) radiation pressure assisted electron expulsion from the target during relativistically induced transparency, generating a strong space charge field which rapidly accelerates ions. Our demonstration of a robust acceleration mechanism that does not require complicated targetry nor a single-shot prepulse suppressing plasma mirror is an important step forward for developing high repetition rate applications of laser driven ion sources.
We report on the time-resolved observation of transient laser-induced breakdown (LIB) during the leading edge of high-intensity petawatt-class laser pulses with peak intensities up to 6x10^21 W/cm^2 in interaction with dielectric cryogenic hydrogen jet targets. The results show that LIB occurs much earlier than what is typically expected following the concept of barrier suppression ionization and that the laser pulse duration dependence of LIB and laser-induced damage threshold (LIDT) is very relevant to high-intensity laser-solid interactions. We demonstrate an effective approach to determine the onset of LIB, i.e. the starting point of target pre-expansion, by comparing a laser contrast measurement with a characterization study of the target specific LIB thresholds.
We report on the measurement of XUV harmonic spectra between 53nm and 17nm wavelength emitted from solid targets driven by a short pulse (30fs FWHM) PW laser with peak intensity up to 6 x 10^21 W/cm2. Experiments were carried with a variety of target materials (metal foils, plastic foils, glass substrates), thicknesses (tens of nm to micron range) and laser parameters. This allowed us to study the influence of these parameters on the harmonic emission and gain insight into the interaction of the pump laser with the target front surface, where most of the energy absorption takes place. We explore the correlation between the dynamics on this target region and the proton acceleration from the laser target interaction by complementing the XUV spectrum measurements with simultaneous proton spectra for the different aforementioned conditions.
In this contribution, we present the results of laser-target interaction studies with intensities ranging from the relativistic regime down to the intensities of dielectric breakdown of hydrogen. They were conducted using the cryogenic hydrogen jet platforms together with the high-resolution optical probing capabilities at the Draco laser facility at Helmholtz-Zentrum Dresden-Rossendorf and the HiBEF facility at European XFEL. Changing the laser parameters enables to utilize specific plasma processes for controlled plasma density tailoring. These results, together with technical advancements of the target, pave the way towards a stable platform for near-critical density targets that will enable stable, repetition-rated proton sources for a multitude of applications at superb energies.
Karl Zeil, Constantin Bernert, Florian-Emanuel Brack, Marco Garten, Lennart Gaus, Thomas Kluge, Stephan Kraft, Florian Kroll, Josefine Metzkes-Ng, Thomas Pueschel, Martin Rehwald, Hans-Peter Schlenvoigt, Ulrich Schramm
We report on experimental investigations of proton acceleration from laser-irradiated solid foils with the DRACOPW laser, where highest proton cut-off energies were achieved for temporal pulse parameters that varied significantlyfrom those of an ideally Fourier transform limited (FTL) pulse. Controlled spectral phase modulation of the driverlaser by means of an acousto-optic programmable dispersive filter enabled us to manipulate the temporal shape ofthe last picoseconds around the main pulse and to study the effect on proton acceleration from thin foil targets. Theresults show that short and asymmetric pulses generated by positive third order dispersion values are favourable forproton acceleration and can lead to maximum energies of 60 MeV at 18 J laser energy for thin plastic foils, effectivelydoubling the maximum energy compared to ideally compressed FTL pulses. The talk will further prove the robustnessand applicability of this enhancement effect for the use of different target materials and thicknesses as well as laserenergy and temporal intensity contrast settings. Assuming appropriate control over the spectral phase of the laser andcomparable temporal contrast conditions, we believe that the presented method can be universally applied to improveproton acceleration performance using any other laser system, particularly important when operating in the PW regime.
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