Collisionless electrostatic shock formation and ion acceleration in intense laser interactions with near critical density plasmas

TL;DR

This study investigates how laser-driven collisionless electrostatic shocks form and accelerate ions in near critical density plasmas, revealing enhanced shock speeds, unique ion reflection phenomena, and the importance of electric field oscillations.

Contribution

It demonstrates the effects of near critical density on shock formation and ion acceleration, highlighting differences from overdense targets and uncovering anomalous ion reflection mechanisms.

Findings

Shock speed and ion energies are greatly enhanced in near critical density plasmas.

Shock formation requires longer response times compared to overdense targets.

Ion reflection can occur even with smaller electrostatic potential jumps due to electric field oscillations.

Abstract

Laser-driven collisonless electrostatic shock formation and the subsequent ion acceleration have been studied in near critical density plasmas. Particle-in-cell simulations show that both the speed of laser-driven collisionless electrostatic shock and the energies of shock-accelerated ions can be greatly enhanced due to fast laser propagation in near critical density plasmas. However, a response time longer than tens of laser wave cycles is required before the shock formation in a near critical density plasma, in contrast to the quick shock formation in a highly overdense target. More important, we find that some ions can be reflected by the collisionless shock even if the electrostatic potential jump across the shock is smaller than the ion kinetic energy in the shock frame, which seems against the conventional ion-reflection condition. These anomalous ion reflections are attributed to the strongly time-oscillating electric field accompanying laser-driven collisionless shock in a near critical density plasma.