Accelerating Ions with High-Repetition-Rate Lasers on optically shaped gas jets

Near-critical density (NCD) target profiles are considered a promising alternative to conventional, over-dense foil targets for high-repetition-rate, debris-free laser-driven ion acceleration. State-of-the-art simulations predict ion acceleration to energies of up to hundreds of MeV in the NCD regime. However, this regime remains experimentally challenging for intense fs laser systems due to the precise control required over the target peak density and density gradients, with only scarce experimental demonstrations reported to date.

Here, we present a novel method of optically shaping under-dense gaseous profiles into NCD profiles as high-repetition-rate targets for laser-driven ion acceleration. Experimental validation of the proof-of-principle of our previous simulations1 is demonstrated. Ion acceleration experiments are conducted using the 45 TW, fs laser system Zeus, hosted at the Institute of Plasma Physics and Lasers (IPPL) of the Hellenic Mediterranean University (HMU). Experimental results on the shaping of the target along with measurements of multi-MeV ion energy spectra and high-dose electron emission are presented.

The optical shaping scheme employs dual, intersecting laser-generated, counterpropagating blast waves that compress the gaseous medium, upon their shock front’s collision, forming steep density gradient slabs of a few microns scale length. The compression of the target is maintained over several ns, mitigating laser synchronization issues.

3D HydroDynamic (HD) simulations are performed to optimize the target density profile optical shaping, along with exploiting the effects of the inherent Amplified Spontaneous Emission (ASE) of the accelerating fs laser pulse. Matching of simulation to experimental results is achieved using an ‘in-house’ developed synthetic optical probing algorithm that performs ray-tracing on the 3D HD simulation results.

The super-intense laser-target interaction is investigated using 3D Particle-In-Cell simulations. The simulations reveal the generation of multi kTesla, azimuthal magnetic fields, strongly suggesting that Magnetic Vortex Acceleration is the dominant underlying acceleration mechanism.

Acknowledgments

This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion) and the Hellenic National Program of Controlled Thermonuclear Fusion. Views and opinions expressed are however those of the author only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them. We acknowledge the support with computational time granted by the Greek Research & Technology Network (GRNET) in the National HPC facility ARIS-under project ID pr016025-LaMPIOS IIΙ.

References

1.       Tazes, I., et. all, Sci Rep 14, 4945 (2024).