Recent development of high power gyrotrons and windows for EC wave applications Available to Purchase

Electron cyclotron heating is one of the major candidates for heating, current drive (170 GHz, 50 MW) and start-up (90–140 GHz, 3 MW) on ITER and will be the main start-up and heating method on W7-X (140 GHz, 10 MW). High unit power and high efficiency single-mode CW gyrotrons with conventional cylindrical (1 MW) and advanced coaxial (2 MW) cavities are worldwide under development. 140 GHz (118 GHz) conventional cavity gyrotrons with 0.55 MW (0.53 MW) output power at a pulse duration of 3s (5s) and 36% (32%) efficiency are commercially available. Advanced internal quasi-optical mode transducers generate linearly polarized output wave beams from high-order cavity modes with efficiencies of 90–95% and separate the mm-wave beam from the electron beam, thus allowing the use of large CW-relevant depressed collectors for energy recovery. Overall efficiencies around 50% have been already achieved at JAERI, FZK, and GYCOM employing single-stage depressed collectors (SDC). First successful experiments at FZK employing a broadband Brewster window gave 1 MW at around 45% efficiency (SDC) for all operating mode series in the frequency range from 114 to 166 GHz (frequency tuning in 3.7 GHz steps by variation of the magnetic field in the cavity). Gyrotrons with advanced coaxial cavities designed for operation in the modes $TE28,16$ and $TE31,17$ at 140 and 165 GHz, respectively, are under development and test at FZK and IAP Nizhny Novgorod. A maximum output power of 1.5 MW has been measured at 140 GHz with an efficiency of 33% (pulse duration 0.1 ms, IAP) and 1.2 MW with an efficiency of 26.7% at 165 GHz (pulse duration 0.5 ms, FZK). Cryogenically edge-cooled single-disk sapphire (⁠$T=77$ or 27 K) and silicon $(T=230 K)$ windows as well as CVD diamond windows with water edge-cooling are under investigation in order to solve the window problem.