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Abstract.A fast directional switch (FADIS) is described, which allows controlled switching of high-power microwaves between two outputs. A possible application could be synchronous stabilization of neoclassical tearing modes (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). 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. The device can be operated as a beam combiner also, which offers attractive transmission perspectives in multi-megawatt ECRH systems. In addition, these diplexers are useful for plasma diagnostic systems employing high-power sources due to their filter characteristics. The principle and the design of a four-port quasi-optical resonator diplexer is presented. Lowpower measurements of switching contrast, mode purity and efficiency show good agreement with theory. Preliminary frequency modulation characteristics of gyrotrons are shown, and first results from high-power switching experiments using the ECRH system for W7-X are presented.

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Narrow-band frequency diplexers in connection with small frequency-shift keying of gyrotrons can be used to switch the millimeter wave power between two output channels or to combine the power from different sources. This technique can e.g. be used for fast beam steering for synchronous stabilization of rotating neoclassical tearing modes (NTMs) in tokamaks. Beam steering can be performed by a multi-stage multiplexer, provided that phase-controlled sources are available. In the paper, various concepts for fast directional switches as well as their integration into transmission lines are discussed. Calculations and low-power measurements of prototypes are presented. Requirements and techniques for frequency control of the gyrotrons are discussed, and the results of preliminary frequency modulation experiments are shown. Future prospects for the application of diplexers in large ECRH systems are discussed. A resonant diplexer experiment in the beam duct of the high-power ECRH system for W7-X is presently being prepared.