Electron beam simulations of a multi-stage 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 interaction. We report on the design of the depressed collector and on the beam transport system from the decelerator into the collector. The current streaming back has to be below 0.1%, Secondary electrons with low energy have little influence. Much attention had to be paid to prevent scattered 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.