Electrical conductance time constants for freely decaying arcs

Electrical conductance time constants for the early stages of free decay after current modulation have been calculated from experimental measurements on a 5 mm diameter cascade arc at atmospheric pressure. The time constants were found by measuring the electric field response of the asymptotic portion of the arc column immediately after a sudden step decrease of arc current. The electric field strength was monitored by means of the copper cooling disks of the cascade, whose probe characteristics were studied thoroughly. The initial high current was supplied by a capacitor discharge circuit which was inductively compensated to produce a square wave pulse of ～ 2 msec duration. Time constants for initial decay were measured in both argon and nitrogen for initial currents ranging from 100 to 400 amperes. The initial free decay time constants of nitrogen were found to increase weakly from approximately 25 to 35 usec over the initial current range considered. The time constants of argon decreased from approximately 100 to 60 Μsec over the same initial current range.