Directional couplers-polarimeters for high-power corrugated waveguide transmission lines

Compact directional couplers-polarimeters were developed for the circular corrugated waveguide transmission lines of the joint IFP-ENEA Electron Cyclotron Resonance Heating experiment at 140 GHz, 2 MW on the Frascati Tokamak Upgrade in Frascati. A linear array of cutoff holes was drilled in the mirrors of the quasi-optical miter bends. The radiated pattern preserves the symmetry and polarization of the waveguide mode in the plane of the array. The direction of propagation is preserved too. Two pairs of standard gain horns with detectors are placed in the plane of the array along the propagation axes of incident and reflected radiation to detect both linear components. The whole assembly is enclosed in a shielded anechoic box of suitable geometry. be exerted in preventing radio frequency (rf) arcing. Moreover, performing the measurement as close as possible to the tokamak is highly desirable, but this involves strong electromagnetic interference and sometimes a nonnegligible neutron load. Directional couplers based on traditional design have been available for smooth-walled, moderately oversized waveguides for many years. More recently, components based on the k-spectrometer(1) have been proposed. We adopted the approach originally proposed in Refs. 2, 3, and 4, and built a suitably modified coupling mirror to be used on the quasi-optical miter bends of the transmission line. A similar concept is used in Refs. 5 and 6, where the emphasis is on the polarimetric measurements though, and the mirror is deposited on a diamond substrate acting also as vacuum barrier, since the waveguide is evacuated.