Beam combination and routing at high power with a ring-type waveguide millimeter-wave resonator

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 lambda, 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. Infact, 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 lambda, or varying the source frequency by a fraction Delta nu/nu << 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.