Development of multimegawatt gyrotrons for fusion plasma heating and current drive

High-frequency gyrotrons; with high output power are mainly used for microwave heating and current drive in plasmas for thermonuclear fusion. The development of high-power gyrotrons; in continuous wave (CW) operation has been in progress for several years in a joint collaboration between different European research centers and an industrial partner. The status of the development of the 140-GHz continuously operating gyrotrons with an output power of 1 MW for the stellarator Wendelstein 7-X will be described. An output power of 890 kW has been achieved with a pulse length of 3 min. limited by the available high-voltage power supply at an electron. beam current of 40 A. At a reduced beam current of 27 A, an output power of 540 kW was measured with a pulse length of 939 s. For the next fusion plasma device international thermonuclear experimental reactor, gyrotrons with a higher output power of about 2 MW are desirable. In short-pulse experiments, the feasibility of fabrication of coaxial cavity gyrotrons with an output power up to 2 MW, CW, has been demonstrated, and the information necessary for a technical design has been obtained. An output power of 2.2 MW has been reached in stable operation (without mode competition). At the nominal output power of 1.5 MW an efficiency of 48% could be obtained with single-stage depressed collector. The development of frequency tunable gyrotrons; operating in the range from 105 to 140 GHz; for stabilization of current driven plasma instabilities in fusion plasma devices (neoclassical tearing modes) is another task in the development of gyrotrons at the Forschungszentrum Karlsruhe.