Studies of near-critical density laser plasma interactions for ion acceleration

• Main Author: Ettlinger, Oliver
• Other Authors: Najmudin, Zulfikar ; Dangor, Bucker
• Published: Imperial College London 2017
• Subjects: 530
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.739658

This thesis presents experimental research, complemented by numerical particle-in-cell simulations, studying the interaction of a high power CO2 laser with near-critical density plasmas. The experiments all occurred around relativistic intensities, a0 ≃ 1, where radiation pressure effects are important. Experiments with a high intensity, 3.5 ps beam with peak intensities IL > 1016 Wcm−2, focussed on to a shaped, over-critical density hydrogen gas target were studied. The accelerated proton beams showed spectral peaking, indicative of radiation pressure or collisionless shock driven acceleration. Higher than previously observed proton energies for this laser system were observed, with peak energies > 1.8 MeV, and energy spreads as low as ∼ 5%. The peak proton energy showed good agreement with the predicted energy scaling for hole-boring RPA, with Ep ∝ IL/ni. Experiments were also conducted at lower intensities, with a 5 ps beam of peak intensity IL∼ 1015 Wcm−2 again focussed on to a shaped hydrogen gas target. Here, the unique laser and target conditions lead to a plasma grating structure being formed in the density ramp preceding the critical surface, from which radiation pressure driven acceleration could occur. The limited mass of these grating structures, along with the suppressed background density, results in enhanced acceleration when compared to that at the unmodified critical surface. Experimentally, a dependence on the peak proton energy compared with the scale length of the plasma preceding the critical surface was observed, attributed to an optimal density profile for the grating formation. Finally, using the same experimental conditions, an alternative method for producing thin gas targets was explored, through two colliding blast waves. Proton acceleration was studied for relative levels of separation between the shock fronts, with the optimal case being at the point of collision. Numerical simulations suggest that acceleration was again enhanced by the creation of grating structures in the sharpened density profile.