Plasma scattering measurement by using a submillimeter wave gyrotron as a radiation source

An FU II gyrotron was applied as a submillimeter wave radiation source for plasma scattering measurements of the compact helical system (CHS) in the National Institute for Fusion Science (NIFS) in Japan. The stabilization of the gyrotron output and the high quality probe beam are required to improve the performance of the measurements. The outputs (cw operation, f=301 GHz, P∼20 W) of the FU IV gyrotron are stabilized (ΔP/P∼1%, Δf∼10 kHz) by decreasing the fluctuations of the potentials (ΔV_k∼0.6 V, ΔV_a∼0.2 V), by introducing smoothing circuits consisting of a resistor, an induction coil and a capacitor. Further stabilization of output frequency (Δf<1 kHz) is attained by applying a phase lock loop. A high purity mode operation is a prerequisite for effective conversion of the output radiation into the probe beam for scattering measurements. We have constructed a new gyrotron, namely, the FU VA gyrotron, which has a carefully designed resonant cavity. The measurements of the radiation patterns have confirmed high-purity mode operation.