Optimization of wall perturbations in rippled wall mode converters for overmoded circular waveguide

Considerable improvement (higher efficiency, shorter length) of periodic-perturbation mode converters for overmoded circular high-power waveguides was accomplished by changing the perturbation period of the wall slightly from the beat wavelength between the input and the desired output mode, combined with superimposing additional, phase-matched, small perturbations, whose periods correspond to the beat wavelengths between the input and output mode and their related, strongest coupled unwanted modes, respectively. Numerical calculations and measurements on optimization of wall perturbations in TEOn-to-TEO,n-1 mode converters and TEO1-to-TE11 transducers for the 1 MW-ECRH system (five 200 kW/70 GHz-transmission lines with CW capability) at a stellerator are reported. For TEO1-to-TE11 mode conversion in transducers with constant diameter and periodically perturbed curvature (in one plane) multimode calculations show that the efficiency = 86.2% of a simple 8-period 70 GHz transducer can be raised to 97.4%.