Experimental Investigation of a cusp Gun gyro-TWA with Helical Interaction Region

Abstract

Summary form only given. The latest results of experimental investigations of an X-band gyro-TWA using a cusp gun and a helically corrugated interaction region are presented. Results will be presented from two experimental investigations, one using a 150 kV, 35 A beam from a cold field emission cusp gun and one with a 150 kV, 70 A beam from a thermionic cusp gun. Designs for a gridded thermionic cusp gun currently under construction will also be presented. Numerical investigations of the gyro-TWA have been completed and used to predict the saturated and small signal gain of several Gyro-TWA devices, these predictions are compared with the experimental results obtained. Of the many types of microwave tube developed to operate using the CRM instability, the gyro-TWA has generated particular interest due to its potential to efficiently generate high power, broadband microwave radiation. 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. In the past, axis- encircling beams have been achieved using a "Pierce-like" gun emitting a pencil beam that was then "kicked" with an asymmetric magnetic field. The kicker magnet is the limiting factor for duty cycle and increases in pulse length. A cusp gun has been investigated as a possible alternative which does not require a kicker. An axis-encircling beam is formed by passing an annular electron beam through a non-adiabatic magnetic field reversal (cusp), converting part of the electron beam's axial velocity into axis-encircling transverse velocity. The complex nature and sensitivity of cusp gun design to magnetic field profile creates the need for accurate simulation. To this end, several computer codes were used and their results compared. Excellent agreement was seen, both between the codes themselves and with experiment. The small signal gain of the gyro-TWA was calculated using the linearised averaged equations. The saturated gain of the amplifier was investigated using the FDTD PiC code MAGIC. The results from the two methods were compared, both with each other and with experimental results.