Electron energy distribution across a magnetic field front propagating through plasma

Summary form only given. Understanding the interaction of fast-rising magnetic field with plasma is of fundamental importance for research of laboratory and space plasmas. A topic of particular interest is the problem of the magnetic energy dissipated by the various particles. Here we describe recent laboratory measurements of the evolution of the electron energy distribution that allow also determining the portion of the magnetic field energy dissipated by the free electrons. The experiments are performed using a system of pulsed currents driven through a plasma bridge between two electrodes (similar to a configuration known as plasma opening switch). The currents generate magnetic fields of up to 10 kG with a rise time of ~300 ns. Observations are based on time-dependent, spatially-resolved spectroscopic techniques. Previous studies of the magnetic field evolution and ion dynamics in this system showed that the two basic, competing processes of magnetic field penetration and plasma pushing can occur simultaneously. The present work focuses on the electron energy evolution. Temperature-sensitive spectral line ratios clearly show rapid electron heating that is correlated with the propagation of the magnetic field front through the plasma. Analysis of spectral line intensities emitted from various ions that are injected locally into the plasma and correspond to different energy ranges are used to establish the free electron energy distribution. The results indicate that the initial thermal (or nearly thermal) electron population becomes non-Maxwellian, with at least two components of relatively cold and hot populations. Present measurements further indicate that the mean energy acquired by the free electrons corresponds only to ~50% of the magnetic field energy they are expected to dissipate, based on energy balance calculations