Arc Phenomena at Low Pressure Atmosphere (Report 1)

Characteristics of Carbon Arc at Low Pressure Hydrogen Atmosphere

Abstract

Systematic research has been made on arc phenomena at low pressure atmosphere to clarify the electrode mechanism. Distinguished features of low pressure arc were found in cathode and anode phenomena, which appear eminently in the arc characteristics and appearance. For example, pure graphite cathode, which used to behave as a typical thermionic cathode (hot cathode), exhibits so called cold cathode under the condition of low pressure and small current. This cold cathode is characterized by random movement of a constricted cathode spot and strong vapour jet from the spot area.
This paper discusses the result of the systematic analysis on arc characteristics and their relation to transition of the cathode mode. The experiment was done under the condition of 10-150 Torr of pure Hydrogen atmosphere and 10-120 A of arc current. The electrode material was pure graphite rods (99.99%) of 6 mmφ.
Conclusions obtained are as follows:
(1) Transition of the cathode mode from the hot cathode to the cold cathode is mainly governed by arc current I, ambient gas pressure p and ionization p4tential V_i of the gas in the cathode fall region. Arc voltage suddenly falls by a few volts when the hot cathode changes to the cold one, though the average electric field of arc column E_p increases abruptly because of the shrinkage of column. This fact suggests that, in the cold cathode mode, Carbon rather than Hydrogen is ionized in the cathode fall region, and consequently the cathode drop V_σ reduces.
(2) Sum of the anode and cathode drops, (V_A+V_C), increases gradually in the range of the hot cathode with the decrease of ambient gas pressure and arc current. This value of (V_A+V_C) suddenly falls by 3-4 volts under the critical condition of ambient gas pressure and arc current when the cathode mode changes to the cold cathode. This abrupt change of (V_A+V_C) is due to the decrease of the cathode drop V_C.
(3) In the range of the cold cathode, (V_A +V_C) decreases with the reduction of ambient gas pressure, while (V_A+V_C) remains almost constant against arc current.
(4) Variation of the cathode drop V_C in both cathode. modes is explained qualitatively by Slepian's ionization mechanism in the cathode fall region.