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.
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