Laser-Plasma Accelerator Workshop 2019

Multi-Species dynamics in the radiation pressure acceleration of ions from ultra-thin foils

Nicht eingeplant

20m

MedILS

Poster Contribution Laser-driven ion acceleration

Sprecher

Aodhan McIlvenny (Queen's University Belfast)

Beschreibung

As the community prepares for the next generation of laser facilities coming online in the near future, attention will shift towards advanced mechanisms such as the radiation pressure acceleration (RPA) which has been predicted to be the dominant ion acceleration mechanism at intensities >10${}^{22}$W/cm2 [1]. Recent studies have shown that current facilities can also enter this regime by irradiating ultra-thin foils with circularly polarised (CP) pulses at intensities [2], I ~6x10${}^{20}$W/cm2 on target with the use of double plasma mirrors for contrast enhancement [2]. The use of CP light helps to reduce electron heating thus mitigating relativistic transparency and allowing the target to remain opaque and efficiently accelerated by RPA in the Light Sail mode.
The work presented here will focus on a recent campaign on the GEMINI laser system at the Rutherford Appleton laboratory which has advanced the results reported in [2], by improving the efficiency of the bulk species (Carbon) acceleration and demonstrating the existence of an optimal thickness for Light Sail acceleration. Additionally, the data highlight the importance of multispecies dynamics during the acceleration with clear evidence for a different acceleration mechanism for Carbon ions and protons ions.
Ultra-thin (2-100nm) amorphous carbon foils were irradiated at normal incidence with an f/2 parabola by a high contrast 40fs laser pulse with ~6J on target, producing an intensity of ~5x10${}^{20}$W/cm2. The data shows a clear difference between the effects of linearly polarized and CP light on the ion energies with CP generating significantly higher carbon energies for thinner targets, with an optimum thickness of 15nm.
For this type of target, experimental data shows the acceleration of C6+ up to 33MeV/n (400MeV) while the corresponding proton energies are less than 18MeV. 2D PIC simulations (carried out with the EPOCH code) suggest that this may be associated to a non-negligible laser pedestal on the sub 6ps timescale (within the reflection window of the plasma mirror). Protons, with the higher q/m ratio, will expand much faster than C6+ beyond the short Rayleigh range associated with the f/2 parabola before the peak of the pulse arrives. The remaining plasma will remain an over-dense, sub-wavelength scale, carbon-electron plasma that can still be efficiently accelerated by RPA.

[1] A. Macchi, M. Borghesi, and M. Passoni, “Ion acceleration by superintense laser-plasma interaction,” Rev. Mod. Phys., vol. 85, no. 2, pp. 751–793, 2013.
[2] C. Scullion et al., “Polarization Dependence of Bulk Ion Acceleration from Ultrathin Foils Irradiated by High-Intensity Ultrashort Laser Pulses,” Phys. Rev. Lett., vol. 119, no. 5, pp. 1–6, 2017.