Excitation temperature measurements in glow and arc discharges in hydrogen

Abstract

Experiments on high-pressure glow discharges in hydrogen (Gambling & Edels 1956) have shown that by either increasing the current or gas pressure of the discharge, a sudden marked column constriction can be produced which results in the formation of an arc column. It was also found that this columnar glow to arc transition could be induced or reversed by control of the thermal conditions of the discharge, indicating that the transition depends upon the discharge temperature values. Relatively few temperature determinations have been made on hydrogen arc discharges, and no values are available for the glow discharge. We have measured therefore the relative intensities of the α, β and γ Balmer lines emitted by the discharge column section and derived the variation of the excitation temperatures T αβ and T βγ through the discharge transition region. The radiation measurements were made using a photomultiplier technique and yielded derived temperature values accurate to ± 3 % . Temperatures have been obtained for currents between 0.25 and 10 A and for pressures from 0.33 to 2 atm . In all cases, except at 2 atm, the current range included the transition condition. The results show that with current variation the excitation temperature curves always exhibit minima at the transition current, even though the latter is changed by the change in pressure conditions. Further, the values of the minima are constant at 3900 ± 100 °K for T αβ and 2800 ± 100 °K for T βγ . The results indicate a thermally dependent mechanism of arc column formation in hydrogen. The results also show that when the column is in the glow state T αβ > T βγ . We conclude that in the glow state the distribution of excited state concentrations is non-thermal, being mainly governed by electron collisions. Above the transition current value the results show that the excitation temperatures rapidly converge, so that for the arc a thermal distribution of concentrations exists in the quantum states n = 3, 4 and 5. However, from considerations of the derived values of electron concentration and electron drift velocity and from calculations of the excess of the electron temperature over the gas temperature, we conclude that despite the equality of T αβ and T βγ , true thermal equilibrium does not exist in the newly formed low- current arc column in hydrogen. The transition in the column from glow to arc is explained in terms of the change in effective thermal conductivity produced by the variations of dissociation with temperature.