Switching behaviour of prebreakdown field emission in vacuum

Prebreakdown field emission from extended copper and molybdenum cathodes is measured with time resolution down to 100 ns. The molybdenum cathodes could be heated up to 2500 K. At constant voltage, sudden transitions to higher emission levels were observed, having elementary time steps <100 ns. They are referred to as switching. Often a series of steps led to long switching times up to 80 ms. In these cases the emission increased over several orders of magnitude. Such switching acts could also be provoked by mechanical shocks without any breakdown. On a time scale of seconds, slow enhancement of the emission could be produced by deposition of metal vapour from neighbouring arc discharges or by heating the cathode to temperatures >1000 K. Both types of changes are reversible. The fast transitions cause a flip-flop behaviour of the current, while the slow changes disappear after a pause of about one hour. The effects are explained by migration of surface atoms under the influence of the electric field. Thus, switching does not necessarily mean the presence of nonmetallic emitters