Proton acceleration from ultrahigh-intensity short-pulse laser-matter interactions with Cu micro-cone targets at an intrinsic ~10-8 contrast

In this paper, we report on experiments performed on the 200 TW Trident laser (80 J, 600 fs, ~7 µm spot size, S-polarization and ~1.5x10²⁰ W/cm²) at an intrinsic (to the system’s regenerative amplifier) ASE contrast of 10-8, using various geometries of conical Cu targets, as well as Cu flat foils for comparison. The work presented in this paper follows on some earlier work on proton acceleration on Trident (20 J, ~14 µm spot size, P-polarization, ~1019 W/cm² and also at the intrinsic contrast of 10-8), using Flat Top Cone (FTC) targets [1] and which demonstrated an enhancement in both proton energies and conversion efficiencies. In the current experiment, an electron spectrometer was added, which shows linear correlation of electron temperatures and proton energies, as well as a Cu Kα imaging diagnostic, to determine the characteristics of laser absorption in the FTC, which demonstrates that the laser is absorbed in the preplasma filling the cone, preventing the previously observed enhancement in proton energies.