ELECTRON-BEAM SIMULATIONS OF A MULTISTAGE DEPRESSED COLLECTOR, INCLUDING SECONDARY AND SCATTERED ELECTRONS

The Fusion FEM is the prototype of a high power, rapid-tunable mm-wave source, operating in the range 130-260 GHz (Urbanus, Nucl. Instr. and Meth. A 375 (1996) 401) [1]. The device is driven by a 2 MeV, 12 A DC electron beam. A decelerator and depressed collector will be mounted behind the mm-wave cavity, for energy recovery of the unspent electron beam. This way the device will have a high system efficiency of over 50%. The unspent beam has a high-energy spread due to the FEM interact ion. We report on the design of the depressed collector and on the bea m transport system from the decelerator into the collector. The curren t streaming back has to be below 0.1%, Secondary electrons with low en ergy have little influence. Much attention had to be paid to prevent s cattered electrons with high energies from streaming back. Two design solutions will be presented, one completely cylindrical symmetric and one with an a-symmetric bending scheme. (C) 1998 Elsevier Science B.V. All rights reserved.