Improved magnetron injection guns for high power gyro devices

Summary form only given. A continuing problem for high power gyrotrons is nonuniform current emission from the electron gun. This problem was identified at the University of Maryland several years ago during operation of a gyroklystron operating at 600 kV. More recently, measurements of an electron gun at the Plasma Fusion Center at NUT indicated that the current was varying by a factor of 6 azimuthally around the electron beam. Communications and Power Industries, Inc. recently tested a 140 GHz gyrotron where the power deposited in the collector varied azimuthally by a factor of 10. Not only do these asymmetries create problems in the collector, but the operating efficiency is degraded and, in extreme cases, excitation of undesired modes results in unacceptable tube performance. Nonuniform emission can be caused primarily by two factors. Gyrotron guns operate temperature limited, so variations in temperature can result in significant differences in the emitted current. Temperature variations were identified in the Maryland device mentioned above. In other cases, bell jar measurements indicated very small variations in temperature, yet large differences in current were measured during operation. If the temperature of the cathode is essentially uniform, then variations in cathode work function are probably causing the emission nonuniformity.