Mode conversion losses in ITER transmission lines

Mode conversion in miter bends and polarizers is the main contributor of loss in the transmission lines (TLs) for the ITER 170 GHz ECH system, which should transport one megawatt of power with the smallest possible loss. Previous loss estimates assumed that the power in the TL was carried by a pure HE11 mode; however, in practice, there is significant power in higher order modes (HOMs). It is shown that the mode conversion loss of the power in an HE11 mode at a miter bend is greatly altered by the presence of even a small proportion of HOMs in the TL, and is a strong function of both the magnitude of the HOM and its phase relative to that of the HE11 mode. The resulting total loss in the ITER transmission lines is expected to be very different from the loss previously predicted using single mode theory.