Fast-frequency step-tunable high-power gyrotron with hybrid magnet system

Fast frequency-tunable gyrotrons are of interest for controlling instabilities in magnetically confined plasmas in large fusion reactors like e.g. ITER. The confining magnetic field depends on the radial position in tokamaks and stellarators. Hence, the electron cyclotron resonance interaction between the RF wave and the plasma electrons occurs only in a small plasma layer where the resonance condition is approximately fulfilled. Other plasma flux surfaces can be reached by a change of the gyrotron frequency or by mechanically steerable mirrors in the plasma vessel making use of the Doppler shift. Since it is difficult to find a material for such mirrors and since a reduction of the number of movable parts inside the plasma vessel is desired, it is obvious that the first solution is more advantageous. A 1 MW frequency-step-tunable gyrotron that operates between 114 and 166 GHz is under development at FZK. In a first step the frequency change has been performed by a slow variation of the magnetic field B by modification of the current in the superconducting (SC) coils. To prevent quenching of the SC solenoids, /spl part/B//spl part/t is limited to a maximum of /spl sim/0.1 Tesla/minute which results in a frequency change of /spl sim/42 MHz/s. To detect and suppress plasma instabilities in the ITER tokamak, the required frequency variation has been estimated to be of the order of seconds for several GHz. To fulfil these demands a hybrid magnet system including two normal conducting (NC) solenoids was build at FZK.