Progress in gyrotron development

Abstract

Gyrotrons are microwave oscillators based on the Electron Cyclotron Maser (ECM) instability. The free energy is the rotational energy of weakly relativistic electrons in a longitudinal magnetic cavity field. In contrast, to klystrons the interaction circuit is a high-order-mode cavity allowing higher power at higher frequencies. At present gyrotrons are mainly used as high power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control and diagnostics of magnetically confined plasmas for generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 1.0 MW gyrotrons employing open-ended cylindrical resonators and synthetic diamond output windows is 5 s at 110 GHz, 12 s at 140 GHz and 9 s at 170 GHz, with efficiencies slightly above 30%. The energy world record of 160 MJ (0.89 MW at 180 s pulse length and 140 GHz) at power levels higher than 0.8 MW has been achieved by the European FZK-CRPP-CEA-TED collaboration. Total efficiencies around 50% have been obtained using a single-stage depressed collector (SDC). To reduce the costs of the ECRH system on ITER and to make its poloidal launcher for neoclassical-tearing-mode stabilization more compact, a further increase of the output power per gyrotron is desirable. To achieve output powers in excess of around 2 MW in continuous wave (CW) 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 (1 ms pulse length) was obtained at FZK with an efficiency of 28%. At the nominal output power of 1.5 MW the efficiency increases from 30 to 48% in operation with a SDC. The availability of sources with fast frequency tunability would permit the use of a simple fixed, non-steerable mirror antenna for local current drive experiments. This work reports on the progress in gyrotron development and the status of advanced coaxial cavity gyrotrons and step-wise frequency tunable gyrotrons.