Ionization and current growth in an E B

The discharge buildup in a long Homopolar-type rotating device with mirror-shaped magnetic field was investigated by time-resolved measurements of the current and potential distributions, in combination with image- converter camera pictures. The ratio of cyclotron and collision frequency for electrons was much larger than unity, and the filling gases were H 2 , N 2 , He, Ne and Ar at 10 -2 torr. An exponential increase of the current was observed between 10 -1 and 10 2 A, within a wide range of E/B. The rate of current growth in H 2 could be compared with known values of the electron impact ionization rate in crossed fields. For the other gases it is shown that under certain experimental conditions ionization by positive ions can be important.