Study of some phenomena of arc discharges

The first part of this thesis presents a new approach to the understanding of the collision processes which occur in the vicinity of the cathode of the "cold cathode" arc of which Hg and Cu are taken as examples. The temperature at the cathode spots of such arcs is too low for electrons to be emitted thermionically. None of the theories hitherto suggested, such as the field emission and thermal ionisation theories, can account satisfactorily for the well-established features of the arc: the cathode fall in potential which is often lower than the ionisation potential of the cathode vapour; the concentration of current at the cathode into a small spot where the current density is very large; the short re-ignition tins; the force on the cathode; the evaporation of the cathode substance; the light emitted from the spot; the temperature of the cathode under the spot and the heat losses from the arc, and also the transition to the arc from the glow discharge. Because of the abundance of experimental information which exists, particularly about the mercury arc, this study is essentially a theoretical one. The starting point in the study is to consider the density distribution in the space above the cathode of the metal vapour originating from the spot. Whereas it was previously thought that atoms evaporating from the spot would cross the evacuated vessel without collisions before condensing on the walls, it is suggested here for the first time that the evaporation is controlled by back-scattering of atoms and hence is a diffusion problem. In setting up the equation of diffusion for the steady state, allowance is made for the variation of the diffusion coefficient with the density of the vapour. The solution shows that a large density, corresponding perhaps to 10 atmospheres or more can exist close to the evaporating surface although the density elsewhere in the vessel is very low. Both the magnitude of the force on the cathode and of the net evaporation from the spot are consistent with this picture. The excitation and ionisation processes in the cathode region are consequently regarded as taking place In a dense vapour. On account of the low cathode fall, collisions between electrons and neutral atoms can only lead to excitation: ionisation can therefore occur only by step-wise processes. Electrons from the cathode gain sufficient energy in the cathode fall to excite neutral atoms within a few mean free paths. Elastic losses are shown to be negligible. A large density of excited atoms is produced close to the cathode and these diffuse back to the cathode surface. Since the transit times of the excited atoms are shorter than their free life, they reach the cathode without losing their energy by radiation and there release electrons. This leads to a self-sustaining arc discharge. From a simultaneous consideration of energy balance and number balance, the yield for the emission of electrons by excited atoms is es- timated. It compares favourably with existing experimental and theoretical evidence. Since the cathode fall of a mercury arc is larger than the resonance potential, electrons which have lost energy in an exciting collision can gain further energy in the remainder of the cathode fall. It is shown that because of strong interaction between the electrons an energy distribution is established, so that each electron from the cathode produces, on the average, more than one excited atom in the vapour. Since the density of excited atoms is high, collisions between them are shown to produce electrons and molecular positive ions. The reverse process, that is recombination of ions and electrons to form ex- cited atoms approximately balances the former process. A small difference is sufficient to account for the current of positive ions to the cathode. This current is associated with a positive space charge which predominates close to the cathode, and the ensuing electric field causes the cathode fall in potential. From the known values of current density and cathode fall and the calculated density distribution of neutral vapour it has been possible to derive how the concentration of electrons, positive ions and excited atoms, and also the axial electric field vary as a function of the distance from the cathode. The transition of the glow discharge into the arc is discussed. It is shown that the final size of the cathode spot can be found from sta- bility considerations associated with the flow of excited atoms to the cathode. The short reignition time is explained in terms of the rapid neutralisation of the positive space charge near the cathode and the subsequent inability of the space-charge region to form when the external potential is re-applied. The light from the cathode region is attributed to the radiation from excited atoms which have diffused up into regions of lower vapour density. The observed spectrum of the spot is re-interpreted on the basis of the new picture. The above treatment can be applied to arcs other than the mercury arc with certain minor modifications. The copper arc is taken as an example. The second part of the thesis presents a new theory of the motion of a cathode spot in a transverse magnetic field. It is known that under certain conditions the spot will move in a direction contrary to Ampere's rule. This reverse motion usually occurs when the ambient gas or vapour pressure is low. If the pressure is increased, the direction of notion changes to the correct, or Ampere direction. The pressure at which the change of direction occurs is the greater the stronger the magnetic field and the lower the current. The effect is found only in cold-cathode arcs. From these observations it is proposed that the apparent violation of Anpere's rule is due to a local reversal of the magnetic field in the cathode region. The magnetic field acting at the cathode spot it thought to consist of two components. The first is the externally applied field, and the second is an opposing field due to the current loop in the deflected positive column. It is shown that the magnitude of the latter may exceed the applied field and thus the spot will be driven in the reverse direction. The behaviour of the arc at low pressure and with argon admitted is explained qualitatively. It is shown here that the reverse driving effect is not confined to low-pressure arcs, but occurs in open air between closely spaced copper electrodes. The separation of the electrodes is shown to be an important parameter of the effect, which supports the view that it is due to a distortion of the arc column rather than to some process occurring in the cathode fall region.