Dynamic behavior of high-current arcs in a capillary

The optical and electrical characteristics of high-current discharge in a capillary in helium were studied in the range of initial pressure from 10 to 760 Torr. The discharge current increased to a peak of 8 kA in 1.5 μsec after breakdown. The duration of the current was about 15 μsec. Two different types of peak were observed during the temporal changes of arc voltage. No such peaks of arc voltage have ever been observed in free-burning arcs. To study the cause of these voltage peaks, temporal changes in the emission spectra, electron density, and temperature of the arc were measured and analyzed. One type of voltage peak appears at high initial pressures above 100 Torr and is related to the kink instability which occurs at very high pressures in the capillary. The pressure of the arc is estimated to be 90 atm when the initial pressure is 1 atm. The other type of voltage peak observed at low initial pressures below 100 Torr is attributed to wall evaporation which causes a reduction in the electron temperature in the arc. It has also been proven that local thermodynamic equilibrium (LTE) between the temperatures of heavy particles and electrons and also between the particle densities of singly and doubly ionized helium are not always established in arcs confined in capillaries.