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Combination of the power of two (or more) gyrotrons is an attractive feature for large ECRH systems, e.g. for ITER. Fast switching of the power from continuously operating gyrotrons between two antennas synchronous to the rotation of the magnetic islands in tokamaks maximizes the efficiency for stabilization of neoclassical tearing modes. Both tasks can be fulfilled with a high-power four-port diplexer, utilizing small frequency differences or small frequency-shift keying (some tens of MHz, by modulation of a supply voltage) of the gyrotrons. In the paper, high-power diplexer designs for quasi-optical and corrugated waveguide transmission systems including numerical and experimental results are discussed. In more detail, the principle and the design of a quasi-optical resonator diplexer is presented. Low-power measurements at 140 GHz of switching contrast, mode purity and efficiency are shown to agree well with theory. Results from high-power experiments on fast switching and power combination of two gyrotrons using the ECRH system for the stellarator W7-X are presented. Near-term plans for high-power experiments at ASDEX Upgrade and FTU are described (Note that there are also plans for in-line ECE on ASDEX Upgrade). Finally, applications of diplexers for the ECRH system on ITER assuming conventional gyrotrons are discussed, and future possibilities offered by phase-controlled gyrotrons like multibeam combining and switching are highlighted.

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

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To combine powers from a set of gyrotrons and to switch the combined wave beam between a number of output channels, various kinds of multiplexers can be used Especially, narrow-bandfrequency diplexers in connection with small frequency-shift keying of gyrotrons can be used to switch the millimeter-wave power between two output channels. This technique can, for example, be used for fast beam steering for synchronous stabilization of rotating neoclassical tearing modes in tokamaks. Beam steering can be performed by a multistage 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. A resonant diplexer experiment is at present being prepared to be tested at high power in the electron cyclotron resonance heating (ECRH) system for Wendelstein 7X (W7-X). Requirements and techniques for frequency control of the gyrotrons are discussed, and the results of preliminary frequency modulation experiments are shown. Finally, future prospects for the application of diplexers in large ECRH systems are discussed.