A fast switch, combiner and narrow-band filter for high-power millimetre wave beams

Abstract

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.

High-power microwave diplexers for advanced ECRH systems

“…Well-known designs compatible with highpower millimetre waves are two-beam interferometer (e.g. the Mach-Zehnder type [2,6]), multibeam interferometers (Fabry-Perot type, or ring resonator [1,3]) and combinations thereof. As high-power compatible hybrids, low-loss dielectric beam splitters [2,6], metallic phase gratings (magic Y) [4,10], and square corrugated waveguides [5,11] have been identified up to now.…”

“…By choosing the appropriate L, the sinusoidal transmission characteristics can be easily matched to the application envisaged. The resonant diplexer in Fig 1b is a 4-mirror ring resonator [2,3,9] consisting of two focussing mirrors and two plane phase gratings for coupling of the incident beams to the resonator via the -1 st diffraction order. Additional mirrors provide matching to the input and output beam or waveguide.…”

“…Special features are the relatively narrow resonant transmission channels separated by broad non-resonant bands. Transmission loss of a few percent [3] affects mainly the resonant channel due to absorption of the mirrors in the resonator. The two-loop resonant diplexer (Fig.…”

“…Within the experimental possibilities of these mock-ups, a good agreement with theory is obtained for all set ups. Details can be found in the respective references [2][3][4][5][6][7][8][9][10][11][12]. The quasi-optical resonant diplexer was as well exposed to high-power tests, which were performed at the ECRH system for the stellarator W7-X [13].…”

“…Technology for sources, transmission and launchers is highly developed. Further improvements can be obtained by introduction of high-power diplexers [1,2,3,4,5,6] which might be used for several applications, leading to an increase of flexibility, performance or to a simplification of the system. Combining of powers from two (or more) conventional gyrotrons would essentially reduce the number of transmission lines and launchers for ECRH systems of large fusion devices, e.g.…”