EARLY PHENOMENA OF CAPILLARY DISCHARGES IN DIFFERENT AMBIENT PRESSURES

An 80 kJ electrothermal gun facility was assembled at Kumamoto Univers ity in order to investigate the dependence of capillary discharges on ambient pressure. The observation of capillary discharges using an ima ge converter camera shows that early phenomena of the discharges vary with ambient pressure. At a pressure of one atmosphere, discharges beg in to occur near the radial center of the capillary. On the other hand , at a lower pressure of about 1 Pa, discharges occur near the insulat or surface of the capillary. Plasma parameters, such as electron densi ty, electron temperature, and the species of the plasma and neutral ga s, are measured by spectroscopic techniques. The electron density at l ow pressure is more than 10(17) cm(-3) at a current of only about 1 kA because of severe wall ablation by the surface discharge. At high pre ssure, the electron density is quite low for electron currents less th an 2 kA. For currents greater than 2 kA, the electron density is over 10(18) cm(-3) at both pressures. The temperature estimated from a Bolt zmann plot is in the range of 0.6 eV to 0.7 eV at low pressure.