Evolution of the Electron Energy Distribution in Nearly Collisionless Plasma Under Pulsed Magnetic Field

Summary form only given. The interaction of fast-rising magnetic field with plasma is a fundamental topic in research of laboratory and space plasmas. In particular, a central problem is the magnetic energy dissipation in a nearly collisionless plasma. We describe recent laboratory measurements of the evolution of the free-electron energy distribution during the propagation of a pulsed magnetic field through such a plasma. Experiments are performed using a system of pulsed currents driven through a plasma bridge between two electrodes. Time-dependent, spatially -resolved measurements of spectral line intensities of various ions sensitive to various electron energies are utilized to establish the evolution of the electron energy distribution. Measurements resolved in 3D, which are made possible by injecting locally the desired ions into the plasma, clearly show a rapid electron heating that is correlated with the propagation of the magnetic field front through the plasma. We find that the initial thermal (or nearly thermal) electron population becomes non-Maxwellian, consisting of at least two components: a relatively cold component with a Maxwellian distribution and a second in the form of a hot quasi-beam. We discuss these observations in the context of a two-decade puzzle of the energy balance during rapid magnetic field penetration into nearly collisionless plasma that was first observed in experiments performed in devices used for plasma opening switches. These early results have invoked several theoretical efforts based on electron magnetohydrodynamics (EMHD) to explain the field penetration in the absence of significant collisions. This led to a debate regarding the EMHD predictions that the electrons should acquire the dissipated magnetic energy, while there was no clear signature for the existence of hot electrons in the experiments. Present results combined with previous measurements of the ion dynamics that showed that the ions also acquire a substantial part of the magnetic energy, bring us closer to a complete understanding of the energy balance.