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Beam Combination and Routing at High Power with a Ring-Type Waveguide Millimeter-Wave Resonator

A. Bruschi, S. Cirant, A. Moro, A. Simonetto

Fusion Science and Technology / Volume 53 / Number 1 / January 2008 / Pages 97-103

Technical Paper / Special Issue on Electron Cyclotron Wave Physics, Technology, and Applications - Part 2

A hybrid quasi-optical waveguide resonating device providing millimeter-wave beam switching and combination at high power is described in this paper. It can be realized, starting from the beam-splitting properties of the rectangular corrugated waveguide with aperture much greater than the wavelength , by arranging the waveguides in a resonating ring configuration. This kind of waveguide, cut at an appropriate length, has been proposed for the remote steering (RS) system of the ITER upper electron cyclotron resonance heating (ECRH) launcher, because of its imaging properties. In fact, beam steering can be performed far from the plasma edge since an input beam is transformed into an output beam with the same angle with the waveguide axis as the input one. Multiple imaging properties, derived by the fractional Talbot effect, are applied at waveguide sections cut at fractional lengths and lead to 3-dB beam-splitting properties for a length equivalent to half the length of an RS waveguide. Ring-type resonant devices with two outputs are obtained by setting two or more waveguides in properly arranged loops. The power distribution in the two output channels available can be controlled either mechanically, moving the mirrors used to couple the different sections by fractions of the wavelength , or varying the source frequency by a fraction / << 1. The exploitation of a second input port allows beams of different gyrotrons with nearly the same frequency to be coupled to the same transmission line. This relatively compact device can be evaluated for application into the ITER ECRH transmission line, with advantages on beam routing control.

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