MW gyrotron development for fusion plasma applications

High power gyrotron oscillators are mainly used as millimetre wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control and diagnostics of magnetically confined plasmas for energy generation by controlled thermonuclear fusion. The maximum pulse length of commercially available 1.0 MW gyrotrons, employing open-ended cylindrical resonators and chemical vapour deposition diamond output windows is 12 s at 140 GHz and 9 s at 170 GHz, with efficiencies slightly above 30%. The energy world record of 160 MJ at a power level around 0.9 MW is held by the European FZK-CRPP-CEA-TED collaboration. Total efficiencies of up to 50% have been obtained using a single-stage depressed collector (SDC). To achieve output powers of around 2 MW in continuous wave operation at the ITER reference frequency 170 GHz, it is necessary to use a coaxial cavity geometry. A maximum output power of 2.2 MW at 165 GHz was obtained at FZK with an efficiency of 28% (48% in SDC operation). The availability of gyrotrons with fast frequency tunability would permit the use of a simple, fixed, non-steerable mirror antenna for local current drive experiments on ITER. This work reports on the progress in gyrotron development and the status of advanced coaxial cavity gyrotrons and step-wise frequency tunable gyrotrons