Early phenomena of capillary discharges in different ambient pressures

An 80 kJ electrothermal gun facility was assembled at Kumamoto University in order to investigate the dependence of capillary discharges on ambient pressure. The observation of capillary discharges using an image converter camera shows that early phenomena of the discharges vary with ambient pressure. At a pressure of one atmosphere, discharges begin 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 insulator surface of the capillary. Plasma parameters, such as electron density, electron temperature, and the species of the plasma and neutral gas, are measured by spectroscopic techniques. The electron density at low pressure is more than 10¹⁷ cm^-3 at a current of only about 1 kA because of severe wall ablation by the surface discharge. At high pressure, the electron density is quite low for electron currents less than 2 kA. For currents greater than 2 kA, the electron density is over 10¹⁸ cm^-3 at both pressures. The temperature estimated from a Boltzmann plot is in the range of 0.6 eV to 0.7 eV at low pressure