A new modulational instability in a crossed-field gap

Summary form only given. Cycloidal electron flows in a gap with a crossed electric and magnetic field are found to be violently unstable when a small AC voltage is imposed across the gap. This instability is electrostatic, one dimensional. It occurs over a wide band of frequencies . Its excitation is insensitive to the precise values of the electron current density, and may occur even if the AC voltage V/sub 1/ is less than one per cent of the DC voltage V/sub 0/. This instability is quite violent. In the low frequency regime, breakdown of the flow may occur in less than one rf cycle of the AC voltage. The physical origin of this instability is the formation of a virtual cathode right in front of the cathode. These unexpected results were obtained from the electrostatic, one dimensional code PDP1. An analytic theory is developed which accurately predicts the onset of the instability, especially in the low frequency limit.