Characteristics of the Glow to Arc Transition in Mercury Vapor

Abstract

The probability of the transition from glow discharge to arc discharge at graphite electrodes has been investigated under carefully controlled cathode conditions. The transition probability was found to increase in an approximately linear fashion with an increase in cathode current density and glow voltage. The curve of transition probability as a function of cathode temperature was found to have a distinct maximum at a temperature of about 400° or 450°C. The probability of the transition occurring at temperatures 50° lower or higher was not more than about one-fourth the maximum. It was also found that the frequency with which the glow to arc transitions occurred depended greatly on the previous history of the discharge and the amount of high voltage positive ion bombardment experienced by the cathode under test. A possible explanation of these results has been based on the hypothesis that it is never possible to de-gas completely the cathode electrode even after prolonged operation in a high voltage discharge. It has been shown that the nonuniform emission of this gas from the cathode in the form of bursts from minute patches can lead to a greatly increased probability for transition from glow to arc.

The initiation of arcs by ion bombardment

“…At the high power levels and long pulse length used in these experiments the electrode surfaces probably reached much higher temperatures than those attained by other workers (Maxfield andFredenhall 1938, Holliday andIsaacs 1966). This view is consistent with two other observations : incandescent spots seen on graphite and tungsten when bombarded at a voltage level where few arcs occurred and a marked tendency for arcs on fully conditioned specimens to occur at the end of the rf pulses.…”

“…Most present explanations of backfire do this by postulating abnormalities at localized regions of the electrode surface. These abnormalities may be mercury droplets which readily vaporize (Neretina 1962) ; semi-conducting or insulating inclusions (Griffiths 1961) and films (Haworth 1950) which release gas after heating or electrical breakdown; loose particles which may be accelerated by the electric field and vaporize on impact with an electrode (Cranberg 1952) ; surface contamination (Maskrey and Dugdale 1966) ; occluded gas (Maxfield and Fredenhall 1938) ; emitting whiskers which become unstable and vaporize (Brodie 1964) ; or from our own experiments, asperities or particles which are in poor thermal contact with the electrode. These become heated and outgas or vaporize.…”

“…The initiation of arcs in the presence of ion bombardment has been studied by other workers in connection with gas-filled valves and discharge vessels for thermonuclear research. Maxfield and Fredenhall (1938) examined the arcing behaviour of a graphite electrode in a mercury discharge at ion current densities in the region of 10 A m-2. For thermonuclear work, Craston et al (1958), Maskrey and Dugdale (1966) and Holliday and Isaacs (1966) worked with various forms of hydrogen discharge at current densities up to 200 kA m-2.…”

Аналіз Основних Нестабільностей Тліючого Розряду Середніх Тисків В Умовах Обробки Матеріалів

“…ТЕХНІЧНІ НАУКИ ТА ТЕХНОЛОГІЇ № 2 (12), 2018 TECHNICAL SCIENCES AND TECHNOLOGIES 105 Механізм утворення локалізованих просторових зарядів пояснюється в роботі [10] таким чином: будь-яка зміна величини γ (де γ -другий коефіцієнт Таунсенда, що характеризує кількість електронів, що падають на один іон, що підійшов до катода) зумовлює в різних точках на поверхні катода більший або менший струм. У місцях забруднень, окалини, окисної плівки робота виходу електронів менше, тому кожен іон вибиває електронів більше, ніж у середньому по катоду.…”

“…У роботі [12] підтверджується, що величина порогового струму, що приводить до утворення дугового розряду, залежить від чистоти металу й наявності на ньому плівок. Особливо істотним цей механізм переходу може виявитися для металів, які важко піддаються очищенню.…”