Shocks and instabilities in collisionless electron-ion plasma
||Shocks and instabilities in collisionless electron-ion plasma|
||NAISS Medium Compute|
||Mark Eric Dieckmann <Mark.E.Dieckmann@liu.se>|
||2023-10-01 – 2024-10-01|
||10303 10305 |
Charged particles in collisionless plasma interact via electromagnetic fields, which are induced by the collective motion of plasma particles. Since binary collisions between particles like their deflection by random angles rarely occur, the plasma does not reach a thermal equilibrium on the time scales of interest. Yet, we observe many structures in collisionless plasma that resemble hydrodynamic ones, where particle collisions enforce thermal equilibrium at all times.
We address with particle-in-cell simulations shocks in collisionless plasma that propagate across the magnetized plasma. We found previously that if we perturb these shocks by a local variation of the plasma density ahead of them, they start oscillating even after they enter into a uniform plasma. We could determine the frequency of these oscillations, but we could not identify their exact cause based on one simulation. We will perform further simulations, where we vary the relative strength of the upstream magnetic field and observe how it affects the shock oscillations. This will reveal if the oscillations are caused by magnetic tension or by properties of the lower-hybrid waves that mediate the shock. We will also study how different shapes of the plasma density perturbation affect the shock oscillations. Perturbations of the plasma upstream of the shock exist in laser-generated plasma. Examples are the channel the laser drives in the ionized residual gas while it ablates the target or solid obstacles like grids. Another example is plasma structures like waves in the Solar wind that flow towards the Earth's bow shock. We will also continue our investigation of plasma instabilities downstream of shocks, which are driven by the ablation of a solid target by an ultra-intense laser pulse. One such instability develops in the thermally anisotropic electron distribution in the density ramp of the laser-generated blast shell. We found that a background magnetic field can turn the Weibel instability into a Whistler wave instability. It yields propagating waves that can escape from the density ramp and enter the plasma upstream of the laser-generated blast shell where their amplitude is large enough to be detectable in experimental settings. We shall explore with 1- and 2-dimensional particle-in-cell simulations experimentally relevant plasma configurations and determine those that lead to observable Whistler wave activity. Finally, we will examine aspects of Raman scattering; a process where an ultra-intense laser beam is converted into an electron density wave and an electromagnetic wave that propagates in the opposite direction of the laser beam. This instability reduces the efficiency, with which laser energy can be deposited into a plasma, posing a major obstacle to inertial confinement fusion. By comparing simulations of Raman scattering in a particle-in-cell simulation, which produces statistical fluctuations of the particle's charge density that resemble thermal plasma noise, and our noise-free Vlasov code, we will obtain insight into how this instability is affected by thermal fluctuations.