Progress on Gyrotrons for ITER and Future Thermonuclear Fusion Reactors

Abstract

The prototype of the Japan 170-GHz ITER gyrotron holds the energy world record of 2.88 GJ (0.8 MW, 3600 s and 1 MW, 800 s) and the efficiency record of 57%, whereas the Russian 170-GHz ITER prototype tube achieved 0.8 MW with a pulse duration of 800 s at 55% efficiency and 1 MW at 280 s and 53%. The record parameters of the European megawatt-class 140-GHz gyrotron for the stellarator Wendelstein W7-X are as follows: 0.92-MW output power at 1800-s pulse duration, nearly 45% efficiency, and 97.5% Gaussian mode purity. These gyrotrons employ a cylindrical cavity, a quasi-optical output coupler, a synthetic diamond window, and a single-stage depressed collector (SDC) for energy recovery. In order to reduce the costs of the ITER 24-MW 170-GHz ECH&CD system, 2-MW millimeter-wave power per gyrotron tube is desirable. Cylindrical gyrotron cavities are not suitable for the 2-MW power regime because of high ohmic wall losses and mode competition problems. However, in coaxial cavities, the existence of the longitudinally corrugated inner conductor reduces the problem of mode competition, thus allowing one to use even higher order modes with lower ohmic attenuation than in cylindrical cavities. Synthetic diamond windows with a transmission capability of 2-MW CW are feasible. A 2-MW CW 170-GHz coaxial-cavity gyrotron for ECH&CD in ITER is under development in cooperation with European research institutions (EGYC, collaboration among the CRPP, Switzerland, the KIT, Germany, the HELLAS, Greece, the CNR, Italy, and the ENEA, Italy). At the Karlsruhe Institute of Technology (KIT), the short-pulse (1-ms) preprototype tube delivered 2.2 MW at 30% efficiency (without SDC) with 96% Gaussian output mode purity. Design studies for a 4-MW 170-GHz coaxial-cavity gyrotron with two synthetic diamond output windows and two 2-MW millimeter-wave output beams for future fusion reactors are currently being performed at KIT. The availability of sources with fast frequency tunability would permit the use of simple fixed nonsteerable mirror antennas for local current drive experiments and plasma stabilization. IAP Nizhny Novgorod develops in collaboration with IPP Garching and KIT an industrial multifrequency 1-MW gyrotron with approximately 50% efficiency (SDC). A four-frequency tube (105, 117, 127, and 140 GHz) delivered 0.8 MW at 105 GHz and 0.95 MW at 140 GHz in 10-s pulses. After the installation of a broadband diamond window, this gyrotron will be operated also at the two intermediate frequencies.