Accessing novel ion acceleration mechanisms, such as Radiation Pressure Acceleration (RPA), is a promising route to generate high energy beams of both light and heavy ions [1]. In particular, the Light Sail (LS) regime predicts high efficiency, mono-energetic beams and can be accessed with currently available high power laser facilities with the use of ultra-thin foils and circular polarisation [2-4]. In recent experiments at the GEMINI laser facility (RAL, UK), target bulk (carbon) ions were favourably accelerated in the LS-RPA regime up to 33MeV/nucleon at an optimal carbon foil thickness of 15nm, whereas protons only reached energies of 18 MeV. This result, which differs from what is typically observed in laser-solid interactions, where protons are always accelerated more efficiently than heavier ions, is interpreted with the support of multi-dimensional Particle in Cell (PIC) simulations. While the 40fs pulse was temporally cleaned by a double plasma mirror arrangement to increase the laser contrast to 10-14 at the ns timescale, it is shown that the limited preceding laser fluence incident on the target on the ps scale causes target expansion, with protons, being lighter, escaping from the interaction region. This leaves a pre-dominantly carbon plasma which, for circular polarization, is accelerated by RPA, with proton energies determined instead by plasma expansion and sheath effects. It is shown through simulations that controlling the laser temporal profile and plasma mirror activation opens up a promising route for controlling which ion species is preferentially accelerated in the RPA regime. This has particular importance as <1PW systems are coming online currently where these accelerations will begin to inherently dominate, and the preceding laser intensity will need to be suitably controlled.
The spatio-temporal and polarisation properties of intense light is important in wide-ranging topics at the forefront of intense light-matter interactions, including laser-driven particle acceleration. In the context of experiments to optimize transparency-enhanced ion acceleration in expanding ultrathin foils, we investigate the polarisation and temporal properties of intense light measured at the rear of the target. An effective change in the angle of linear polarisation of the light results from a superposition of coherent radiation, generated by a directly accelerated bipolar electron distribution, and the light transmitted due to the onset of relativistic self-induced transparency. Simulations show that the generated light has a high-order transverse electromagnetic mode structure in both the first and second laser harmonics that can evolve on intra-pulse time-scales. The mode structure and polarisation state vary with the interaction parameters, opening up the possibility of developing this approach to achieve dynamic control of structured light fields at ultrahigh intensities [1].
We also report on frequency-resolved optical gating measurements of the light which demonstrate a novel and simple approach to diagnose the time during the interaction at which the foil becomes transparent to the laser light. This is a key parameter for optimising ion acceleration in expanding ultrathin foils. Coherent transition radiation produced at the foil rear interferes with laser light transmitted through the foil producing spectral fringes. The fringe spacing enables the relative timing of the onset of transmission with respect to the transition radiation generation to be determined. This self-referencing approach to spectral interferometry provides a route to optically controlling and optimising ion acceleration from ultrathin foils undergoing transparency [2].
[1] M.J. Duff et al., Scientific Reports 10, 105 (2020)
[2] S.D.R. Williamson et al., Phys. Rev. Applied 14, 034018 (2020)
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