SPECTROSCOPIC INVESTIGATION OF FAST (ns) MAGNETIC FIELD PENETRATION IN A PLASMA

The time dependent magnetic eld spatial distribution in a coaxial positive-polarity Plasma Opening Switch, carrying a current '135 kA during '100 ns, was investigated by two methods. In the rst, ionic line emission was observed simultaneously for two polarizations to yield the Doppler and Zeeman contributions to the line prooles. In the second method, the axial velocity distribution of ions was determined, giving the magnetic eld through the ion equation of motion. This method requires knowledge of the electron density, here obtained from the observed particle ionization times. To this end, a lower bound for the electron kinetic energy was determined using various line intensities and time-dependent collisional-radiative calculations. An important necessity for POS studies is the locality of all measurements in r; z, and. This was achieved by using laser-evaporation to seed the plasma non-perturbingly with the species desired for the various measurements. The Zeeman splitting and the ion motion showed magnetic eld penetration through the 3.5-cm-long plasma at a velocity '10 8 cm/s. The current density was found to be relatively high at the load-side edge of the switch plasma. It is suggested that this may cause plasma acceleration into the vacuum section towards the load, which is supported by charge-collector measurements. The fast magnetic eld penetration agrees with estimates based on the Hall-eld mechanism. I. INTRODUCTION Investigating the behaviour of plasmas carrying high short-duration currents is of major importance for improving the understanding of fundamental plasma physics phenomena , such as magnetic eld penetration into plasmas and plasma ow under magnetic and electric elds. Such studies are also essential for improving and scaling up of many pulsed-power systems. In previous studies, information on the evolution of the magnetic eld in Plasma Opening Switches (POS) was obtained by the use of magnetic loops inserted in the plasma 1. Recently, the time-dependent line-integrated plasma electron density, was studied in a microsecond POS using laser interferometry 2. More experimental investigations are highly required for studying the plasma behaviour in Plasma Opening Switches and for examining the various underlying theories 3?7. The question of the magnetic eld penetration into short-duration plasmas is of major importance for many pulsed-power systems. In particular in POS, the magnetic eld aaects both the plasma properties in the conduction phase and the switch opening. Field penetration due to diiusion appears unlikely since the classical diiusion time is >500 times longer than the pulse duration (100 ns) …