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Electron cyclotron resonance heating (ECRH) is the main heating system for W7-X. A 10-MW ECRH plant with continuous wave (cw) capability is under construction to support the W7-X operation, which aims at demonstrating the steady-state capability of stellarators at reactor-relevant plasma parameters. The ECRH system consists often radio-frequency (rf) modules with 1 MW power each at 140 GHz. The rf beams of the individual gyrotrons are transmitted in common to the W7-X torus via open multibeam mirror lines. The losses of individual components of the transmission system were measured with both low- and high-power methods. Integrated full-power, cw measurements of the long-distance transmission losses are reported and compared to theoretical design estimates.

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-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. 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.

This paper reports the results of the high-power test of a remote steering launcher mock-up at 140 GHz, which were performed at the ECRH installation for the future stellarator W7-X at IPP Greifswald. The mock-up test system consists of a 6.62 m long corrugated square waveguide with a steerable optic at the entrance and various diagnostics at the exit of the waveguide. A straight and a dog-leg version of the launcher were investigated. The high-power tests of the straight setup have been performed with powers up to P0 = 700 kW (typically 500 kW) and pulse lengths of up to 10 seconds. For both polarizations (parallel and perpendicular to the steering plane), no arcing was observed in spite of the fact, that the experiments were performed under ambient atmospheric conditions. After the integration of 2 mitre bends in the setup, arcing limited the usable parameter range. The ohmic loss PΩ of the waveguide was measured via the temperature increase of the waveguide wall, and was used to calibrate the calculated angular dependence of the total ohmic losses of the waveguide. Short-pulse radiation pattern measurements with thermographic recording show high beam quality and confirm the steering range of −12° < < 12°.

The 10 MW, 140 GHz, CW ECRH‐plant for W7‐X is in an advanced state of commissioning and the installation was used to investigate advanced applications for extended heating‐ and current drive scenarios. The operation of the TED gyrotrons was recently extended to a 2nd frequency of 103.6 GHz at reduced output power and first results are presented. An improved collector sweep system for the W7‐X gyrotrons with enhanced power capability and smooth power distribution was developed, results are reported.

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The virtual German-Italian-Russian Helmholtz- Institute "Advanced ECRH for ITER" runs a program FADIS aimed to develop a system for combining outputs from a set of gyrotrons and toggling the combined beam with kHz-frequency between two launcher for dedicated electron cyclotron heating and current drive in fusion experiments. A diplexer prototype fed by a 140 GHz gyrotron with ~0.5- MW, ~ 1-s pulses operated in good agreement with theory.

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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Experiments on the combination of the high-power wave beams from two gyrotrons and fast switching of the combined beam between two transmission channels are described. The measurements were performed using a high-power resonator diplexer in the optical transmission line of the electron cyclotron heating system for W7-X. The principle and the engineering design of the prototype four-port quasi-optical diplexer is presented. The wave beams from two gyrotrons with output powers of 370 and 560 kW, respectively, have been combined for pulse lengths up to 10 s, limited only by the uncooled mirrors used in the diplexer. By modulating the gyrotron frequency using a fast high-voltage body modulator, controlled toggling of the combined power between the two outputs of the diplexer was demonstrated with switching frequencies of up to 20 kHz. The experiments are compared to theory, showing good agreement when the limited stability of the free-running gyrotrons is taken into account.

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.