Enhanced energy localization and heating in high contrast ultra-intense laser produced plasmas via novel conical micro-target design

We report new experiments showing enhanced laser-target coupling and energy localization using nano-fabricated micro-conical Cu targets performed at the 100 TW CPA laser at LULI. A comparison was made between 1ω (λ = 1.057 μm, I = 10¹⁹ W/cm²) and 2ω (λ = 0.53 μm, I = 4-8 × 10¹⁸ W/cm²) irradiation to determine the effect of ASE induced preformed plasma filling the cone, using as principal diagnostics 2D Cu K_α imaging (transverse and rear-side), and high-resolution conical crystal spectroscopy of the Cu K_α bands. The 2ω irradiation exhibits laser absorption up to 50 μm deeper into the cone tip (versus at 1 ω), with a commensurately smaller K_α emission zone. Spectroscopy indicates a higher average charge state for the Cu emission at 2ω, with some shots exhibiting up to at least O-like emission. We deduce that micro-cone targets have similar performance in terms of material heating as a 50 μm diameter reduced mass target, despite a 900-fold larger mass. The observed enhancement in energy localization and heating in the cone geometry is supported by 2D collisional PIC simulations which indicate the presence of self-generated resistive magnetic field structures (≥ 10 MG) which confine the energetic electrons to the tip region.