Design and Simulation of a Cusp Gun for use in Gyro-amplifiers

Gyro-amplifiers require relativistic electron beams with low velocity spread and with a high fraction of the electron energy associated with the cyclotron motion. For harmonic operation and mode control an axis-encircling beam is desirable. The passage of an electron beam through a non-adiabatic magnetic field reversal (cusp) converts part of the electron beam's axial velocity into axis-encircling transverse velocity. This magnetic field is produced using two solenoids wound in opposite sense. The main cavity coil produces the field required for interaction whilst a smaller secondary coil, which is positioned just behind the cathode surface, produces a reverse field. A cusp-based electron beam forming system, yielding a 10 MW, 150 kV, 70 A axis-encircling beam for use in a gyro-TWA will be presented. The latest results from numerical simulations and experiments are presented and compared.