An electron cyclotron resonance heating (ECRH) system capable of delivering 2.4 MW cw has been designed to be built at Commissariat à l'Energie Atomique, Cadarache, for the Tore Supra (TS) experiment, to provide plasma heating and current drive by electron cyclotron resonance interaction.
The planned system was composed of a generator using six gyrotrons 500 kW for 5 s or 400 kW cw working at 118 GHz. Six transmission lines made of corrugated waveguide, 63.5-mm diameter, carry the HE11 mode to one antenna composed of six fixed mirrors and three independently movable mirrors for the adjustment of the injection angles of the rf beams.
The antenna was built and installed in TS, and all transmission line components ordered and installed between the gyrotron locations and the antenna. In the same way, the required six oil tanks, the six cryomagnets, and the six modulating anode devices were designed and manufactured.
In parallel, after demonstration in the factory of proper operation of the prototype gyrotron, the manufacture of a first so-called series gyrotron was made. But this gyrotron experienced hard limitations (overheating inducing prohibited outgassing, parasitic oscillations) during the long-pulse tests in Cadarache, and the achieved performance was 300 kW for 110 s. A new study was then carried out in collaboration with Thales Electron Devices, the EURATOM-CEA Association, and the EURATOM-Confédération Suisse Association to understand and overcome the limitations, which led to the construction of a new modified gyrotron.
During the tests in factory of this new gyrotron, the output beam showed two peaks, a pattern never predicted by simulations. The gyrotron was nevertheless transferred to Cadarache for long-pulse testing, but an arc on the windows definitely stopped the tests.
To understand the cause of the observed two peaks, various low-level tests were then performed on a model of the mode converter with different shapes for the launcher, but without real improvement. Besides measurements, the use of a new software, Surf3D, based on integral equations and providing a complete three-dimensional modeling, showed that the problem mainly comes from the third mirror, whose curvature is too high and consequently not well taken into account by the calculation.
These technological problems have seriously delayed the development of the gyrotrons; as a consequence, only two tubes (intermediate developments) are presently available on TS to inject 700 kW in 5-s pulses.
In spite of this relatively low power, the localized absorption property of electron cyclotron waves has been used on TS in a wide variety of experiments, such as stabilization and control of the sawtooth period, perturbative transport studies by ECRH modulations, and ECRH-assisted plasma start-up.