Abstract
In many natural phenomena in space (cosmic-rays, fast winds), non-thermal ion populations are produced, with wave-particle interactions in self-induced electromagnetic turbulence being suspected to be mediators. However, the processes by which the electromagnetic energy is bestowed upon the particles is debated, and in some cases requires field compression. Here we show that laboratory experiments using high-power lasers and external strong magnetic field can be used to infer magnetic field compression in the interpenetration of two collisionless, high-velocity (0.01–0.1c) quasi-neutral plasma flows. This is evidenced through observed plasma stagnation at the flows collision point, which Particle-in-Cell (PIC) simulations suggest to be the signature of magnetic field compression into a thin layer, followed by its dislocation into magnetic vortices. Acceleration of protons from the plasma collision is observed as well. As a possible scenario, with 1D and 2D PIC simulations we consider a compression of the vortices against dense plasma remnants.
Highlights
An interesting new scenario for the source of the ion acceleration resulting from the collision observed in the experiment is supported by simulations, and results from the evolution of the magnetic vortices, formed during the breakup of the compressed magnetic barrier at the center of the colliding plasmas, and which interact with the remaining solid-density targets at the system edge. Colliding with these high density remnants, the magnetic vortices could be compressed due to the impossibility for the magnetic field to fully penetrate into the dense and cold regions; only a fraction of the magnetic field penetrates, corresponding to the pressure equilibrium between the cold plasma region and the incoming TNSA plasma flow toward it, in between which the magnetic field is compressed. We tested this in separate 1D PIC simulations, looking at the evolution of one part of the system comprising the magnetic field of the vortex, the TNSA plasma flowing toward it and one of the solid-density targets
We observed that the electrons within the TNSA plasma continuously flowing through the dissolved magnetic barrier can be trapped and accumulated in the compressed magnetic field
An electrostatic potential builds up, leading to an ambipolar electrostatic field which in turn can accelerate the cold ions present in the density gradients of the high density remnants. This dynamics takes place very rapidly: simulating only 1 ps duration after the edge of the TNSA flow arrives at the original position of the opposite target, we observe that the acceleration process is already saturated
Summary
These plasmas are produced following the irradiation of the external surfaces of two opposing Al solid targets using two coincident, short-pulse (0.65 ps), high-intensity (7 × 1019 W cm−2) lasers of the Titan laser at the Jupiter Laser Facility (Lawrence Livermore National Laboratory, USA), aligned in time to within the laser pulse duration, a
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