Dynamics of Laser-Plasma Expansion Across Strong Magnetic Field

Abstract

Summary form only given. Explosive plasma expansion in an external magnetic field occurs in a variety of space and astrophysical events that include supernova explosions in the interstellar magnetic field and the interaction of solar coronal mass ejections with planetary magnetospheres. We investigated in the laboratory the interaction of plasma winds with magnetic obstacles for various degrees of ion magnetization. In these experiments, an azimuthal magnetic field with flux density up to 30 T was generated by high current discharges of the pulsed-povver generator Zebra (0.6 MA maximum current with 200 ns rise time) in a reusable short-circuit load. The plasma wind was created by ablation of a solid CH2 target with the pulsed laser Tomcat (1 mum wavelength). Laser pulses 6 ns long with energy up to 5 J and irradiance up to 1014 W/cm2 were used to investigate the large Larmor radius regime. The regime in which ions are magnetized was studied with short laser pulses (4 J in 5 ps, with irradiance up to 1016 W/cm2). In both experiments the plasma and magnetic field were characterized with multi-frame laser shadow and schlieren imaging and magnetic probes, respectively. Without the magnetic field, the laser diagnostics show a plasma plume expanding quasi-spherically. When a magnetic field is present, it decelerates the plasma flow and a steep density gradient forms at the interaction front. The boundary forms in regions where the density would be too low to observe with laser diagnostics without the magnetic field. When observing the plasma evolution over times much longer than the inverse ion gyrofrequency, the density gradient decreases at the plasma front in the direction normal to the target and the plasma flows beyond the initial boundary. In the transverse direction, the boundary becomes unstable. Most likely, this is a result of the electrons being trapped in the magnetic field along the interface. As a result they decelerate the ions producing a region of velocity shear across the interface. This configuration is favorable for the onset of instabilities. Three-dimensional ideal magnetohydrodynamic (MHD) modeling describes well the formation of a diamagnetic cavity, the dynamics of the plasma plume during the interaction with the magnetic field, and the onset of instabilities at the plasma-field interface. Two dimensional particle-in-cell modeling describes well details of the plasma-field interaction at the boundary and the plasma penetration across the field beyond the limit predicted by MHD.