Heating Effects in Overmoded Corrugated Waveguide for ITER

Abstract

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°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 HE 11 mode to low-order LP 11 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 LP 11 even mode has higher loss than the LP 11 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 HE 11 content in the waveguide is 93 % upstream from the misalignment and 85 % downstream.

Design and Performance of Microwave Components for ECH and ECE Applications at General Atomics

“…Work is underway to model and analyse a scaled water-cooled corrugated waveguide design at 50 mm diameter. Generally the attenuation losses scale as the inverse of the radius cubed, so the expected absorption becomes roughly twice that measured previously [4]. However, this increase is not expected to impact the approach to active waveguide cooling.…”

“…Factory acceptance tests of these gyrotrons have demonstrated 1 MW for 300 s [2], and 0.9 MW for 500 s [3]. Under such conditions, scaled measurements from QST's gyrotron facility in Naka, Japan [4], suggest that over 512 W/m of heating will be dissipated for the HE11 mode supported in 6061-T6 aluminium alloy corrugated waveguide of 50 mm diameter at 170 GHz far from a miter bend. Within the approximately 1 m decay length for higher order modes at a miter bend, power absorption increases by 4 to 5 times.…”

FEM analyses of the ITER EC H&CD torus diamond window unit towards the prototyping activity

“…The window unit shall withstand the loading conditions specified in the ITER load specifications during normal and off-normal events. In general, the FEM analyses of the window, carried out in ANSYS Workbench, can be grouped in four categories with reference to the external loads, internal loads and the thermo-structural loading during beam transmission and accidental events like the hot spot case (excess heating on one side of the EW wall versus the other side, [4]). The design criteria defined in the applicable ASME III-NC code are applied to the metallic parts of the unit.…”

Diamond Window Technology for Electron Cyclotron Heating and Current Drive: State of the Art

“…Several FEM analyses were carried out to check the behaviour of the window against the main design drivers and also to optimize the design itself. The analyses may be basically categorized in analyses for the external loads (mainly the severe SL-2 seismic event occurring during the vacuum vessel baking), for the internal loads (mainly the pressure loads acting on the disk) and for the thermo-structural loading during the beam transmission, considering also the offnormal event of hot spot 8 . The hot spots consist in non-axially symmetric modes in the WGs that lead to an excess heating on one side of the WG wall versus the other side.…”