Loss Estimate for ITER ECH Transmission Line Including Multimode Propagation

The ITER ECH Transmission Lines (TLs) are 63.5 mm diameter corrugated waveguides that will each carry 1 MW of power at 170 GHz with a transmission efficiency that should exceed 83%. The losses on the ITER TL have been calculated for four possible cases corresponding to having HE11 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 in detail using a numerical code and analytical approaches. Estimates of the calorimetric loss on the line show that the output power is reduced by about 5 ±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 HE11 mode is about 3% smaller than the fraction of input power in the HE11 mode. High output mode purity therefore can only be achieved with significantly higher input mode purity. Combining both Ohmic and mode conversion loss, for 1 MW of power generated by the gyrotron, the output power in the HE11 mode at the end of the ITER TL can be roughly estimated in theory as 920 kW times the fraction of input power in the HE11 mode.