A Chopped Electron Beam Driver for H-Type Cavities

The acceleration of heavy ions with H-type structures is usually powered by tube driven rf transmitters. These are limited to about 1 MW in pulsed operation and at frequencies above 200 MHz. This paper reports about an rf amplifier development, which should allow to improve the rf power level as well as the reliability significantly. An electron beam is directly coupled with one girder of an H-type structure by a two gap-array. In contrast to the bunch formation in klystrons a chopper technique is applied to get reasonable mechanical dimensions for the amplifier at frequencies below 400 MHz. A high perveance beam from a pierce-type electron gun is chopped in two steps by passage along the magnetic field of a solenoid and superposition of a transverse rf electric field (two harmonics) and of a transverse electrostatic field, respectively. The deflected particles are dumped in a depressed collector while the penetrating electron bunch may be post-accelerated additionally before entering the H-type cavity. MAFIA-PIC-code simulations have demonstrated the efficiency of the suggested chopper array and have allowed to improve the design of this key element.