FADIS a Fast Switch and Combiner for High-power Millimetre Wave Beams

Neoclassical tearing modes (NTMs) in tokamaks can be stabilized by electron cyclotron current drive (ECCD) at the corresponding resonant flux surface. Best efficiency is obtained if ECCD is applied in the O-point of the island, which requires modulation of the launched EC power synchronously with the rotating islands. An alternative to modulation, which makes full use of the installed gyrotron power could be synchronous switching of the millimetre waves from a continuously operating source: A fast directional switch (FADIS) toggles the beam between two launchers in different poloidal or toroidal planes, which are 180° apart from each other with respect to the phase of the NTM. Generally, the device can be used to share the installed EC power between different types of launchers or different applications (e.g. in ITER, midplane / upper launcher), whichever is given priority during a plasma discharge. The switching is performed electronically without moving parts by a small frequency-shift keying of the gyrotron (some tens of MHz), and a narrow-band diplexer, which directs an input beam to one of the two output channels. The device can be operated as a beam combiner also, which offers attractive transmission perspectives in multi-megawatt ECRH systems. The principle and the design of a four-port quasi-optical resonator diplexer is presented. Low-power measurements of switching contrast, mode purity and efficiency are shown and are compared with theory. First results from high-power tests using the ECRH system for W7-X are presented. Requirements and techniques for frequency control of the gyrotrons are discussed, and the results of preliminary frequency modulation experiments of two different types of gyrotrons are shown. Finally, the integration of this type of diplexer into corrugated waveguide transmission lines, as well as alternative waveguide diplexer concepts are discussed. This work is carried out in the frame of the virtual institute "Advanced ECRH for ITER" (collaboration between IPP Garching and Greifswald, FZK Karlsruhe, IHE Karlsruhe, IPF Stuttgart, IAP Nizhny Novgorod, and IFP Milano), which is supported by the Helmholtz-Gemeinschaft deutscher Forschungszentren