High Power Discharges in an Intense Longitudinal Magnetic Field

Abstract

The properties of high power pulsed discharges in hydrogen have been studied. The time for the discharge to reach steady state conditions was investigated as a function of its parameters. Without a magnetic field the time was of the order of 100 μsec. At high pressures, longitudinal magnetic fields up to 23 000 oersteds caused little change in the time to reach equilibrium, but at low pressures (e.g. 0.46 mm Hg) the time was considerably increased and values up to 2.5 msec were recorded. Observations of the arc diameter showed that a strong uniform magnetic field produced a constriction, unless the natural arc diameter was much greater than the tube diameter. Non-uniform magnetic fields, on the other hand, caused the discharge to become diffuse where the field was diverging. The electrode falls and positive column gradient of the steady state discharges were measured. Without a magnetic field the electrode falls were 58 v, and the positive column gradient was 60p0 34i-0 5 where p is the pressure in mm Hg and i the current in amperes. This expression is in reasonable agreement with the theoretical value. The electron densities were found to lie in the range 1013 to 1015 electrons per cm3. These densities correspond to ionization of from 0.5% to 12% of the gas atoms. The addition of a magnetic field considerably increased the electrode falls and values as high as 400 v were recorded. At low pressures a magnetic field, as predicted by theory, reduced the positive column gradient. At high pressures, on the other hand, the gradient was increased. For example, with a 100 A arc at 10 mm pressure the gradient was increased from 13 v per cm to 25 v per cm by an intense magnetic field. Again this change is consistent with theory. This change in gradient was accompanied by a 13oo increase in the discharge temperature.