Operation Of A 140 GHz Backward-wave Gyrotron Oscillator

Future FELs for electron cyclotron heating experiments require drivers that are continuously variable in frequency and can be tuned quickly compared to the tokamak pulse length. The backward-wave gyrotron oscillator is a voltage tunable fast-wave device that satisfies these requirements. Recent experimental studies1 of the backward-wave gyrotron oscillator have demonstrated a voltage tuning band width of 3% centered at 28 GHz in the TE
₀₁
rectangular waveguide mode with an annular beam. We have designed and constructed a 140 GHz voltage tunable backward-wave gyrotron oscillator at MIT. Novel features of this design include a 80 kV, 4 A Pierce-wiggler electron source and an overmoded (TE
_{12) cylindrical interaction region. Results from preliminary operation2 have demonstrated backward-wave oscillation from 133-143 GHz. However, these oscillations occur at discrete frequencies and do not display the desired continuous voltage tuning, possibly due to mode competition in the interaction region. A second phase of experiments will soon begin that will use a redesigned interaction region and beam tunnel, and a wide-band motheye window.}