Electron Temperature and Arc Diameter in a Sand-Filled HBC Fuse

The electron temperature as a function of time in a model high-breaking-capacity fuse has been determined from measurement of the relative intensity of the Si II spectral lines at 505, 597, and 636 nm. The fuses used in this paper consisted of a 0.55-mm-diameter Ag wire fusible element surrounded by silica (SiO₂) sand. The spectra were resolved with a grating spectrometer and recorded by a gated image intensifier coupled to a linear photodiode array for prospective currents of 1.25- and 4.5-kA amplitudes and for arc lengths of 112 and 240 mm. The electron temperatures varied during the life of the arc from no significant change (for 1.25-kA peak prospective current through the long fuse) to ~50% decrease (for 4.5-kA peak prospective current through the short fuse). Average temperatures, excluding data points at the early and late times during the arc discharge when the most extreme temperatures were measured, were as follows: 1.8 × 10⁴ K and 1.1 × 10⁴ K at 1.25- and 4.5-kA peak prospective currents, respectively, for the 112-mm fuse and 1.4 × 10⁴ K and 1.5 × 10⁴ K at 1.25- and 4.5-kA peak prospective currents, respectively, for the 240-mm fuse. The individual data points, however, exhibited a wide scatter about the line of best fit. Detailed analysis of the data indicates that it is essential to include the intensity of all of the doublets previously listed when measuring the plasma electron temperature and that self-absorption at 636 nm is, at least for the present experiment, not a source of error. The measured electron temperatures were used to calculate the Spitzer conductivity of the plasma which, together with the measured electrical characteristics of the arc, enabled the variation of the diameter of the arc over time to be estimated.