The short theoretical introduction

ARC DISCHARGE

The arc discharge is a self-sustaining one, and cathode drop of its potential has relatively low value of ionization potential or atom excitation, i.e. 10 eV. This is the main difference between arc discharge and glowing, which has the cathode drop of about hundred volts. The low value of cathode drop is the result of other mechanisms of cathode emission. These mechanisms are able to provide the higher values of electron current from cathode, close to the total current of a discharge. It means that there is no necessity to increase the electron current, which is the function of a great cathode drop in a glowing discharge. Cathodes of arcs release electrons as a result of thermo electronic, auto electronic and thermo autoelectronic emissions.

For arc discharges big currents are typical ( ), which considerably exceed the currents in glowing discharges ( - ). They are big in comparison with the current density of a discharge at a cathode. In one type of arc forms they are about , in others For the comparison let us define that at a high pressure of 1 atm for glowing discharge the normal density at a cooper cathode in the air is about 155 , which corresponds to the lowest value of a an arc range. Voltages of arc burning are low. In short arcs they do not exceed 20-30 V, in some other forms they are equal to some volts. Voltage-currents characteristics are mostly declining, but not in all the cases.

Cathodes of arcs obtain a lot of energy from current and have a high temperature. They are destroy with the erosion and evaporate. If the spectrum of emission of near cathode region of glowing discharge is concur with the environmental gas spectrum, it means that spectrum of arcs consists the spectrum of lines of vapors of metal electrodes. Vacuum arcs are burning in vapors of metals. Concerning the plasma of positive pole – the region between near electrode layers – there are both equilibrium and non-equilibrium arcs. It depends on the gas pressure. It can be pointed out that the equilibrium in plasma of a direct current are typical only for an arc, and the non-equilibrium are typical for both glowing and arc discharge, when the last takes place at a low pressure.

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Date: 2015-12-24; view: 822