Modelling of Cusp Gun for a Gyro-TWA

A gyro-TWA with a helically corrugated waveguide requires an axis encircling electron beam to operate. In the past this has been achieved using a `Pierce-like´ gun, which emitted a pencil beam that had to be `kicked´ with an asymmetric magnetic field. This `kicker´ coil requires cooling and represents an additional complication to the device. In order to remove the need for a `kicker´ a cusp gun has been investigated. A cusp gun produces an axis-encircling beam by passing an annular beam through a magnetic cusp causing the beam to rotate azimuthally. The magnetic cusp is produced using two solenoids, a cavity coil that produces the main field and a smaller solenoid behind the cathode surface, which produces a reverse field