R Holmes

Cathode spot motion in high current vacuum arcs on copper electrodes

The motion of vacuum arc cathode spots under the influence of the self-generated magnetic field has been investigated for copper electrodes at currents up to 6.9 kA. The spots were photographed with a high-speed framing camera. The magnetic flux density was measured. Surface contamination caused large changes in spot velocity. For velocities up to 15 ms-1 and corresponding flux densities up to 2*10-2T the velocity is proportional to the flux density. Above 20 ms-1 and 4*10-2T the velocity increases relatively slowly with increasing flux density. Velocities of more than 30 ms-1 were not observed.

Cathode spot division in vacuum arcs with solid metal cathodes

Studies of cathode spot division have been made for vacuum arcs with solid metal cathodes of zinc, lead, copper, aluminium and bismuth for discharge currents in the range 5-150 A. The distribution function of the number of spots is Gaussian for fixed experimental conditions. Above a certain current i0 the mean number of spots increases linearly with current except for bismuth. A model for spot splitting is proposed which is based on the energy balance at the cathode surface and temperature-field emission for electrons.

Cathode spot structure and dynamics in low-current vacuum arcs

A study has been made of cathode spot structure and dynamics in copper vacuum arcs for currents between 25 and 100 A using a high-magnification optical technique. The cathode spot consists of one or more electron emission zones or fragments with areas fluctuating between 10?10 m2 and 5?10?8 m2. The spot generally exists in one of two types. In the second type the average current density is about 5?1010 A m?2 and in the first type it is about (0?5-1)?1010 A m?2. The cathode spot undergoes continuous rebuilding in the form of fluctuations in the number and size of the fragments such that transitions in spot type, from type 1 to type 2 and vice versa, occur on average every 100 ?s. A possible explanation of the observations in terms of a cathode spot model based on temperature-field electron emission is discussed.

Arcing voltage of the metal vapour vacuum arc

Arcing voltage characteristics of metal vapour arcs in vacuum for seven metals (copper, aluminium, tin, magnesium, zinc, cadmium and bismuth) of high purity and low gas content have been investigated. These metals cover a fairly wide range of thermophysical parameters. A triggered vacuum switch with circular butt contacts of 75 mm diameter and 15 mm separation was used and the discharge current was varied up to about 10 kA. The low-current arcing voltage and average current per cathode spot are reported for these metals. The empirical relationships for arcing voltages are derived for the linear range of the voltage-current characteristics. Voltage-current characteristics in axial magnetic fields up to 0.2 T are presented and the observed similarities and differences in the behaviour of the metals investigated are explained. Finally, the axial magnetic field necessary for giving a minimum arcing voltage is discussed.

Cathode spot motion in high-current vacuum arcs under self-generated azimuthal and applied axial magnetic fields

The motion of vacuum arc cathode spots under the influence of self-generated azimuthal and externally applied axial magnetic fields has been investigated for several metals at currents up to about 10 kA. The spot motion was recorded by means of a high-speed framing camera, and the self-generated magnetic field was measured with an inductive magnetic probe. The axial magnetic field was applied by means of two coils round the vacuum chamber in an approximate Helmholtz arrangement. The data have been used to determine the relationships between the retrograde (anti-Amperian) spot velocity and the self and axial magnetic flux densities. At low self-generated fields, the velocity is directly proportional to the flux density but at high fields velocity saturation is observed in some cases. An axial magnetic field decreased the retrograde velocities on all metals investigated. The spontaneous spot formation observed with high rates of current rise is analysed; the velocity saturation phenomenon is explained semiquantitatively and the influence of the axial magnetic field on the cathode spot velocity discussed.

Measurement of the neutral vapour density decay following the extinction of a high-current vacuum arc between copper electrodes

The decay of neutral copper vapour density which follows the extinction of a high current (2-11 kA) vacuum arc between copper electrodes has been measured. The results show that during the period 1-8 ms from current zero the vapour density decays at least an order of magnitude slower than expected on the assumption that vapour atoms generated by the arc move freely to the electrode and vacuum chamber surfaces where they condense. The results can be explained, however, if it is assumed that the vapour is produced by evaporation from molten droplets which are emitted from the cathode spots.

Post-arc current mechanism in vacuum interrupters

The decay of plasma following the forced extinction (similar, equals 1 μs) of a 200 A DC vacuum arc with copper electrodes has been studied by an electrical probing technique. For times between 1 and 12 μs after current zero a constant current was injected into the decaying plasma and the time-dependent voltage across the resulting dynamic ion sheath at the negative electrode was measured. A model of the dynamic sheath is in good agreement with the measurements. The plasma density in the region of the negative electrode is about 4×1018 m−3 at 1 μs after current zero and falls to about 2×1017 m−3 after 12 μs.

The voltage, self-generated magnetic field and current distribution in a high-current vacuum arc

Measurements of the anode-cathode voltage of a high-current (>2 kA) vacuum arc are presented, attention being restricted to conditions which result in a smoothly varying voltage. It is found that under these conditions the voltage/current characteristics can be divided into three regions, namely linear, non-linear and irreproducible. The current distribution within the arc is derived from magnetic probe measurements, attention being further restricted to reproducible and axisymmetric conditions. Annular current distributions are found and a simplified theory of the current conduction process is based on an idealised annular current distribution. This theory leads to an expression for the arc voltage which is in reasonable agreement with the voltage measurements in the linear region.

Vacuum arc response to current transients

The transition of a vacuum arc from one steady state to another has been investigated by superposition of a current step. At low rates of change of current there is a decrease in the magnitude of the voltage oscillations and a slight increase in the arcing voltage in agreement with DC observations. At high rates of change of current the arc shows a transient response which includes an increase in the interelectrode voltage which persists for a few tenths of a microsecond. Possible explanations of this behaviour are suggested.

Temperature dependence of vibrational relaxation times in cyclopropane, ethylene and sulphur hexafluoride

Sound absorption and velocity measurements were made in cyclopropane, ethylene and sulphur hexafluoride at 400 and 700 kHz in the frequency/pressure range 0?2-30 MHz atm?1 and in the temperature range -70 to 150?C. Relaxation times for the transfer of vibrational energy were deduced and compared with previous measurements. These results are discussed in terms of the theories of molecular energy transfer. Evidence for double relaxation processes is found in the case of ethylene and sulphur hexafluoride, but the measurements in cyclopropane fit a single relaxation process within the estimated experimental error.