Abstract
A wiggler-focused, sheet beam free electron laser (FEL) amplifier utilizing a short-period wiggler magnet has been proposed as a millimeter-wave source for current profile modification and/or electron cyclotron resonance heating of tokamak plasmas. As proposed, such an amplifier would operate at a frequency of approximately 100–200 GHz with an output power of 1–10 MW CW. Electron beam energy would be in the range 500–1000 keV. To test important aspects of this concept, an initial sheet beam FEL amplifier experiment has been performed using a 1 mm×2 cm sheet beam produced by a pulse line accelerator with a pulse duration of 100 ns. The 500–570 keV, 4–18 A sheet beam is propagated through a 56 period uniform wiggler (λw=9.6 mm) with a peak wiggler amplitude of 2–5 kG. Linear amplification of a 5–10 W, 94 GHz signal injected in the TE01 rectangular mode is observed. All features of the amplified signal, including pulse shape and duration, are in accordance with the predictions of numerical simulation. Amplified signal gain has been measured as a function of injected beam energy, current, and wiggler field amplitude and is also in good agreement with simulation results. Continuation of this experiment will involve studying nonlinear amplifier operation and adding a section of tapered wiggler.
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