Current characteristics of a high voltage 60 Hz crossed-field discharge

This paper deals with the continuation of research on vacuum pumping with 60 Hz ac voltages and permanent magnets arranged to form a crossed-field configuration. It is well known that dc voltages in a crossed-field discharge and in the presence of a titanium cathode will cause a decrease in residual gas pressure. In our previous research it was demonstrated that this result could also be achieved with 60 Hz ac voltages. Before applying this knowledge to commercial vacuum pumps it is important to gain an understanding of the underlying physical mechanisms. Of particular interest are the initiation and maintenance of the discharge under ac conditions. Results reported in this paper deal with a radial magnetic field in the butt-gap between parallel disk electrodes. In this particular crossed-field arrangement, it was found that the electron space charge is unstable, but ion production is more or less continuous with both dc and ac excitation. The pumping yield per ion was found to be low, but could probably be increased by replacing the glass tube with a metallic vessel so that fewer of the sputtered titanium atoms combine in situ with adsorbed gas molecules on the glass wall.