Development of terahertz gyrotrons and their application to CTS on LHD

Development of a high-power, sub terahertz pulse gyrotron has started in FIR FU for application to CTS on LHD. A new electron gun was designed to produce a laminar electron beam with a good quality. Second harmonic TE6,5 and TE8,5 modes were selected as oscillation modes well isolated from other competing modes. Single mode oscillation has been confirmed for both TE6,5 and TE8,5 modes. The maximum power is larger than 50 kW for TE6,5 mode (0.349 THz) and about 40 kW for TE8,5 mode (0.390 THz). These powers are new records of second harmonic gyrotron in this frequency range. Feasibility study of CTS with a 0.4 THz pulse gyrotron from a high density plasma in LHD was carried out. The CTS condition is satisfied for a wide operation regime and scattering angles large enough for good spatial resolution. Ray tracing calculation shows that the launched scattered beams are propagated almost straight. CTS spectra calculated with a newly developed code indicates that a large signal to noise ratio can be obtained against ECE for use of a 100 kW gyrotron.