High-power pulsed gyrotron for 300 GHz-band collective Thomson scattering diagnostics in the Large Helical Device

A high-power pulse gyrotron was developed to generate a probe wave for 300 GHz-band collective Thomson scattering (CTS) diagnostics in the Large Helical Device. In this frequency range, avoiding mode competition is critical to realizing high-power and stable oscillation with a narrow frequency bandwidth. A moderately over-moded cavity was investigated to ensure sufficient isolation of a desired mode from neighbouring modes, and to achieve high power output simultaneously. A cavity with the TE14,2 operation mode, a triode electron gun with an intense laminar electron beam, and an internal mode convertor were designed to construct a prototype tube. It was experimentally observed that oscillation of the TE14,2 mode was strong enough for mode competition, and provided high power with sufficient stability. The oscillation characteristics associated with the electron beam properties were compared with the numerical characteristics to find an optimum operating condition. As a result, single-mode operation with maximum output power of 246kW was demonstrated at 294 GHz with 65 kV/14A electron beam, yielding efficiency of similar to 27%. The radiation pattern was confirmed to be highly Gaussian. The duration of the 130kW pulse, which is presently limited by the power supply, was extended up to 30 mu s. The experimental results validate our design concept and indicate the potential for realizing a gyrotron with higher power and longer pulse toward practical use in 300 GHz CTS diagnostics.