Heating Effects in Overmoded Corrugated Waveguide for ITER

The latest testing of International Thermonuclear Experimental Reactor (ITER)-class electron cyclotron heating (ECH) transmission line components at the Japan Atomic Energy Agency (JAEA) has revealed regions of significant heating. Temperature measurements taken along sections of the waveguide wall during a 350 kW, 500-s gyrotron pulse are higher than previously recorded, up to nearly 70 A degrees C above room temperature in some cases. One difference between these results and previous measurements is that the heating occurs in areas far away from miter bends (> 1 m), further than higher-order-mode decay lengths. The estimated ohmic losses are large enough that they may be potentially damaging at ITER power levels. The explanation is traced back to misalignment of upstream components created by heating effects under high-power conditions. These misalignments cause mode conversions from the desired HE11 mode to low-order LP11 modes. As the combination of modes propagate forward in the transmission line, the field interactions lead to localized hot spots along the waveguide wall. The heating could have implications for the design of ECH components as well as the layout of transmission lines at ITER. Furthermore, the effects are discovered to be dependent on polarization since the LP11 even mode has higher loss than the LP11 odd mode. This discovery leads to a novel way of estimating mode purity in the waveguide. By applying the method to the current data set, the estimated HE11 content in the waveguide is 93 % upstream from the misalignment and 85 % downstream.