Loss Estimate for ITER ECH Transmission Line

The ITER ECH Transmission Lines (TL) are 63.5 mm diameter corrugated waveguides that will each carry 1 MW of power at 170 GHz. The current specifications require that the transmission efficiency of the HE$_{11}$ mode from the gyrotron to the plasma should exceed 83{\%}. The transmission losses on the ITER TL have been calculated for four possible cases corresponding to having HE$_{11}$ mode purity at the input of the TL of 100{\%}, 97{\%}, 90{\%} and 80{\%}. The losses due to coupling, Ohmic and mode conversion loss are evaluated. Estimates of the calorimetric loss on the line show that the output power is reduced by about 5 $\pm $1 {\%} due to Ohmic loss in each of the four cases. Estimates of the mode conversion loss show that the fraction of output power in the HE$_{11}$ mode is about 3{\%} smaller than the fraction of input power in the HE$_{11}$ mode. High output mode purity therefore can only be achieved with significantly higher input mode purity. We also present a new conservation theorem relating the field distribution offset and propagation angle due to any two linearly polarized LP$_{mn}$ modes ~propagating in the corrugated waveguide.