Arc Phenomena in Low Pressure Atmospheres (Report 3)

Cathode and Andoc Modes in Low Pressure Arc with Nonconsumable Electrodes

Abstract

In the previous works, the authors pointed out that the change of cathode mode of a graphite electrode could be explained rationally by considering the energy balance and ionization process in the cathode fall region. There are still left, however, some difficulties, in a carbon arc, to clarify the cathode mechanism, since there is evaporation of carbon out of electrodes and, consequently, the composition of a plasma at each condition is not consistent. Therefore, systematic researches on low pressure arcs were made with nonconsumable electrodes, such as a thoriated tungsten cathode.and a water-cooled copper anode, to eliminate the influence of vapours from electrodes. Experimental conditions were: the current range of 20-100 A, and the pressure of 20-100 Torr in a pure hydrogen atmosphere.
A tungsten cathode did not show the cold cathode mode which was recognized with a graphite cathode, but it took the modes of the plasma cathode and the hot one under this experimental conditions. In an arc with a tungsten cathode, however, the plasma cathode mode clearly appeared in wide ranges of arc currents and pressures, and the mode transition was more exactly observed than in a carbon arc.
The anode mode, which was not described in the previous papers, was systematically studied. Under the conditions of low pressure and small current, a glow-like discharge with light violet colour emerged on a water cooled copper anode, and this anode glow gave eminent effects on electrical and thermal characteristics of an arc. Thus, two kinds of anode modes, i.e. the normal and the glow modes, were recognized in low pressure arcs with a copper anode.
The anode drop V_A in the range of normal anode mode gradually increased, particularly it showed remarkable increase when the current or the pressure came near to the critical conditions of the mode transition. When the glow anode mode emerged, anode energy V_A^*, as well as arc voltage V_A, increased abruptly by 10 volts or so, and this change in V_A^* and V_A was due to the increase of the anode fall V_A.
Governing factors of the anode modes and their correlations were clarified. They are (a) arc current I, (b) ambient gas pressure p, (3) kinds of atmosphere and (d) anode material. These facts were very similar to the governing factors of the cathode modes and the correlations among factors.