The on axis coupled structure type RF gun

The fundamental design of this newly developed RF gun with a thermionic cat hode is the pi/2 mode standing wave structure. It has two accelerating cell s and a coupling cell located on the beam axis, a so-called on axis coupled structure (OCS). This structure offers a stable operation for high beam cu rrent, owing to high group velocity and wide bandwidth. It is important to reduce damage onto the cathode caused by back bombardment, especially for l ong macropulse operation, such as in an FEL injector. Back bombardment, as well as output beam profile was simulated by using the electromagnetic fiel d analytical codes 'EMSYS'(2D) and 'MAFIA'(3D), The cavity shape was optimi zed to reduce back bombardment power without sacrificing beam emittance. (C ) 1999 Elsevier Science B.V. All rights reserved.