Energetic ion acceleration from relativistically transparent prepulse-expanded films driven by ultra-intense femtosecond laser pulses

State-of-the-art high-power lasers are reaching ever-higher focal intensities, enabling discoveries in high-field science. One key application is the generation of high energy particles from irradiation of solid foils. We have recently investigated laser driven ion acceleration using two different femtosecond-class laser systems operating with ultra-high focal intensities exceeding 10²¹ W/cm²:  J-KAREN-P, at KPSI, and DRACO PW at HZDR.  Using advanced pulse characterisation techniques, we tuned both lasers to generate similar intensity and laser contrast profiles, enabling proof-of-principle repeatability experiments.

We irradiated sub-micron thickness formvar foils to explore ion acceleration in the relativistic transparency regime driven on femtosecond timescales.  Despite a modest laser energy (~10 J) and no contrast-enhancing plasma mirrors we have generated high energy protons (>50 MeV) and carbon ions (>30 MeV/nucleon) at an optimum thickness of ~250 nm.  Transmitted light and electron diagnostics show that this optimum thickness is related to the first onset of relativistic transparency.  

Hydrodynamic and 3D particle-in-cell modelling reveals that the laser prepulse plays an integral part in pre-expanding the targets.  Acceleration is optimised when the prepulse driven expansion primes the target density to be matched to the relativistic critical density threshold.  The laser ponderomotively blows out electrons from the transparent target, causing a strong transient space charge in the densest region. Ions accelerated from this region are post-accelerated in large-scale sheath fields.  The quantitative replication of the results on both laser facilities demonstrates the robustness of the mechanism.  These results pave the way for the establishment of repetitive laser driven ion sources using current femtosecond-class high power lasers, providing high energy and high peak current beams ideal for applications in radiobiology and materials science.