Recent development of the step-tunable gyrotron at FZK

Summary form only given. Multi-frequency step-tunable gyrotrons would be a great advantage for electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) in magnetically confined plasma devices. At the ASDEX Upgrade tokamak at IPP in Garching there is an interest in step-tunable gyrotrons operating at frequencies between 105 GHz and 143 GHz. A mode selection for nine modes in this frequency range was done in. Currently a step-tunable gyrotron with a frequency range from 105 GHz to 143 GHz with nine frequencies is developed at FZK. The current focus is on the quasi-optical mode converter, the Brewster-type output window and the fast-tuning super-conducting magnet. The quasi-optical mode converter of the 2nd generation is optimised for the set of nine modes using an optimised launcher, a quasi-elliptical mirror and a combination of two toroidal mirrors. A Gaussian mode content of more than 90% is achieved in simulations for all modes. This could be enhanced to 93% by the 3rd generation mode converter using adapted phase correcting mirrors based on a toroidal contour. To confirm these designs currently low power measurements on the quasi-optical mode converter are performed, which will be presented in this paper. A fast tunable super-conducting magnet with a tuning range of 4.1-5.7 T with a slope of 130 mT/0.5s corresponding to a frequency change of approx. 3 GHz is needed. A feasibility study was made and an order was placed at Cryomagnetics, Inc. The maximum field is specified to 7.2 T to allow an operation also at the ITER frequecy of 170 GHz. The delivery is scheduled within 2008.