Ultra-short pulse laser acceleration of protons to 80 MeV from cryogenic hydrogen jets tailored to near-critical density

Laser-driven ion accelerators hold immense promise across various fields, offering advantages in size, cost, and beam parameters compared to conventional methods. Their applications span warm dense matter research, archaeological surveys, fusion energy research, time-resolved studies, and radiation oncology, benefiting from high dose rates. However, current laser accelerators haven't reached their full potential due to limitations in achieving simultaneously high energies and high radiation doses. A major bottleneck is the lack of suitable high-repetition-rate targets that allow for precise control over plasma conditions. This paper investigates using cryogenic hydrogen jets as a target to address these challenges. The use of cryogenic hydrogen jets offers a unique opportunity to control the density gradient and achieve optimal conditions for ion acceleration. By carefully controlling the density profile, different acceleration mechanisms can be explored and optimized, leading to more efficient and higher-energy proton beams. The ability to precisely control the target density profile is a key step towards unlocking the full potential of laser-driven ion acceleration and opening new avenues in various scientific fields and applications.

Several ion acceleration schemes have been predicted through particle-in-cell (PIC) simulations, but experimental realization has lagged behind, particularly in achieving high energies and doses simultaneously. Target-normal sheath acceleration (TNSA) is a well-studied mechanism, but others like hole-boring radiation pressure acceleration (HB-RPA), light-sail radiation pressure acceleration (LS-RPA), acceleration from targets undergoing relativistic transparency, synchronized acceleration at the relativistic critical density surface (RTF-RPA), collisionless shock acceleration (CSA), and magnetic vortex acceleration (MVA) hold potential for higher energies. These mechanisms often compete, and transitions between them can improve performance, as seen in transparency-enhanced hybrid TNSA-RPA achieving over 90 MeV. The challenge lies in controlling the target density profile to selectively access these acceleration mechanisms. Previous methods have used overdense foils, targets exploded by heater pulses, near-critical density targets using nanoscopic structuring, and low-density materials like solid hydrogen or underdense systems from gas jets. However, full control and accurate characterization have remained elusive due to target geometry and the need for ultrafast X-ray probing techniques.

This research utilized the Draco PW laser system, focusing 18 J, 30 fs pulses onto a 5 µm diameter cryogenic hydrogen jet, achieving intensities of 5.4–10²¹ W cm⁻². The target density, when fully ionized, reached 5.1–10²² cm⁻³ (30nc). A single plasma mirror enhanced the temporal contrast, ensuring an unperturbed target until two picoseconds before the main pulse. A low-intensity pre-pulse (variable delay) hydrodynamically pre-expanded the target, adjusting its core density. On-shot high-resolution shadowgraphy using a synchronized optical probe (515 nm) measured the target expansion at the time of the main pulse arrival. Two Thomson parabola spectrometers (TPS) characterized proton emission, and radiochromic films (RCF) measured proton beam profiles. Hydrodynamic simulations, using the 1D-FLASH code, modelled the pre-pulse-induced target expansion, connecting the simulated density profiles with measured shadow diameters via ray tracing (using ZEMAX). Three-dimensional PIC simulations (PIConGPU) modeled the laser-plasma interaction, using the derived density profiles and laser pulse parameters. The simulations identified the dominant acceleration mechanisms across the density scan.

The experiment demonstrated proton energies up to 80 MeV, a two-fold increase compared to the unexpanded jet. Optimal performance occurred at a shadow diameter of 11 µm. The study showed a correlation between target pre-expansion, increased target transparency (measured by transmitted light), a shift in proton emission direction from isotropic to more laser-forward, and increased proton energies. Hydrodynamic simulations showed that the measured shadow diameter matched simulations with an initial plasma temperature of 150 eV. PIC simulations confirmed the experimental trend, showing significantly higher maximum proton energies (exceeding 100 MeV in simulations) for intermediate expansion levels. The simulations accurately reproduced the transition from isotropic proton emission (TNSA-dominated regime) to laser-forward directed emission. The simulations revealed the transition through various acceleration regimes: Initially, TNSA dominated, followed by enhanced acceleration at the relativistic transparency front (RTF) and, in the underdense regime, magnetic vortex acceleration (MVA). The optimal regime, showing the 80 MeV protons, corresponded to the RTF mechanism where the laser reflects inside the target bulk at the moving relativistic critical density front. Simulations showed a higher sensitivity and instability in the near-critical regime, while the unexpanded and largely expanded cases were stable.

The findings demonstrate a significant advancement in laser-driven proton acceleration, achieving high energies (up to 80 MeV) with a high-repetition-rate, debris-free target. The controlled pre-expansion of the cryogenic hydrogen jet enabled the tuning of different acceleration regimes, resulting in a two-fold increase in proton energy compared to the unexpanded case. The good agreement between experimental results and the integrated simulation framework (hydrodynamic and 3D PIC simulations) confirmed the dominant role of the relativistic transparency front in the optimized near-critical regime. The remaining discrepancies between experiment and simulation are attributed to minor limitations in laser-target overlap and model assumptions regarding electron distribution. Future studies could further improve performance by optimizing target density profiles for even higher energies and exploring the use of wider sheet-like jets for improved laser-target overlap.

This study presents a significant advance in laser-driven ion acceleration, achieving 80 MeV proton beams using a high-repetition-rate cryogenic hydrogen jet target and petawatt-class laser pulses. The controlled pre-expansion of the target allowed for the exploration and optimization of various acceleration mechanisms. The results demonstrate that this combination offers a viable pathway towards developing high-repetition-rate, 100 MeV-class proton accelerators. Future research should focus on further optimizing the target density profiles and improving laser-target coupling for enhanced performance and a better understanding of the complex interplay between different acceleration mechanisms in the near-critical density regime.

The main limitation of the current study is the slight discrepancy between the experimentally observed maximum proton energy (80 MeV) and the higher energies predicted by simulations (over 100 MeV). This discrepancy can be primarily attributed to the imperfect overlap between the laser focus and the hydrogen jet target. Fluctuations in the laser-target alignment from shot to shot introduced some variability in the experimental results. Further, the hydrodynamic simulations assumed a thermal electron population, which may not precisely represent the actual electron distribution in the experiment. Finally, the energy resolution of the Thomson Parabola Spectrometer limited the precision of some energy measurements.