Effect of the drift gap between the undulator sections on the operation ofthe Fusion-FEM

The 'Fusion-FEM' is a free electron MASER based on an electrostatic acceler ator. An electron beam of 12 A, 1.35-2 MeV is injected into a step-tapered undulator to generate 1 MW of radiation in the range 130-250 GHz. The undul ator is built from two sections with different field strength (step-taperin g), separated by a field-free gap. Tapering enhances the efficiency at high output power. Proof of principle has been demonstrated in the short-pulse regime, without a beam- and energy recovery system. The latter is now being installed and will allow long-pulse operation. The experiments have shown that high output power can be generated and that the FEM has the tendency t o lock onto a single longitudinal mode. The influence of the length of the gap between the undulator sections has been investigated and is the subject of this paper. The experimental results are reasonably well explained by s imulations. (C) 2000 Elsevier Science B.V. All rights reserved.