OPTICAL INVESTIGATION OF ARC-SPOT FORMATION ON COLD CATHODES IN NOBLE-GAS ATMOSPHERE

An are ignited by a high-voltage breakdown in pure argon at atmospheri c pressure is blown magnetically against a third so-called commutation electrode (CE) positioned perpendicularly to the are axis. Cathodic a re spot formation is initiated on this electrode by application of a p otential difference between the bulk plasma and the commutation electr ode. The first stage of this current transfer is investigated electric ally and optically. Comparing high-speed photographs with the starting phase of the commutation current, it turns out that an abrupt current increase is correlated to a plasma ball in front of the cathode, and a precurrent (less than 1 A) is correlated to a luminous layer on the cathode surface and the subsequent formation of a bright channel betwe en the cathode surface and the bulk plasma.