Abstract
A pair of collisionless shocks that propagate in the opposite directions are firstly observed in the interactions of laser-produced counter-streaming flows. The flows are generated by irradiating a pair of opposing copper foils with eight laser beams at the Shenguang-II (SG-II) laser facility. The experimental results indicate that the excited shocks are collisionless and electrostatic, in good agreement with the theoretical model of electrostatic shock. The particle-in-cell (PIC) simulations verify that a strong electrostatic field growing from the interaction region contributes to the shocks formation. The evolution is driven by the thermal pressure gradient between the upstream and the downstream. Theoretical analysis indicates that the strength of the shocks is enhanced with the decreasing density ratio during both flows interpenetration. The positive feedback can offset the shock decay process. This is probable the main reason why the electrostatic shocks can keep stable for a longer time in our experiment.
Highlights
Two shocks would form in the interaction region and oppositely propagate into the upstream region[17,18]
Our experiment was performed at the Shenguang II (SG-II) laser facility at the National Laboratory on High Power Lasers and Physics, which can deliver a total energy of 2.0 kJ in 1 ns at 3ω(351 nm)
The crimson area in the Abel inversion map stands for the plasma density higher than the critical density of the probe beam (~4 × 1021 cm−3), which corresponds to the no-fringe area in the raw image
Summary
Our experiment was performed at the Shenguang II (SG-II) laser facility at the National Laboratory on High Power Lasers and Physics, which can deliver a total energy of 2.0 kJ in 1 ns at 3ω(351 nm). Higher flow velocities (~100–1000 km/s) and lower flow densities (~1018–1019 cm−3) can lead to the formation of collisionless electrostatic shock[20], while the differences of the initial densities and temperatures between both flows can enhance the strength of generated shock (larger Mach number)[14]. The crimson area in the Abel inversion map stands for the plasma density higher than the critical density of the probe beam (~4 × 1021 cm−3), which corresponds to the no-fringe area in the raw image. Nab =αNcr is the ablation density, depending on the parameters of the incident laser, the Ncr = 8.9 × 1021 cm−3 is the critical density of the driven lasers, Cs is the sound velocity, ΔN is
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