First lasing of the Dutch fusion-FEM in the long-pulse configuration

The Dutch Fusion-FEM is the prototype of a high-power, long-pulse, rapid-tunable free-electron maser. The target is to generate mm-wave power in a frequency range of 130–260 GHz, e.g. for tokamak heating and diagnostics experiments in fusion devices. For these applications a high system efficiency is needed. The electron beam is first DC-accelerated to the FEL interaction region. The unused electron beam energy is recovered by a DC-decelerator and a three-stage depressed collector. In short-pulse regime, without energy recovery system, 730 kW, 200 GHz of net output power was generated. Single-frequency operation and tunability have been demonstrated. In the present set-up, with the energy recovery system being operational, initial experiments showed a net output power of 110 kW on average and 140 kW peak power at a pulse length of 40 μs. During the full-pulse length, a stable-frequency operation around 170 GHz has been observed.