Thermal Analysis to Calculate the Vessel Temperature and Stress in Alcator C-Mod due to the Divertor Upgrade

Abstract

Alcator C-Mod is planning an upgrade to its outer divertor. The upgrade is intended to correct the existing outer divertor alignment with the plasma, and to operate at elevated temperatures. Higher temperature operation will allow study of edge physics behavior at reactor relevant temperatures. The outer divertor and tiles will be capable of operating at 600 °C. Longer pulse length, together with the plasma and RF heat of 9 MW, and the inclusion of heater elements within the outer divertor produces radiative energy which makes the sustained operation much more difficult than before. An ANSYS model was built for the global thermal analysis of C-Mod. It models the radiative surfaces inside the vessel and between the components, and also includes plasma energy deposition. Different geometries have been simulated and compared. Results show that steady state operation with the divertor at 600 °C is possible with no damage to major vessel internal components. The differential temperature between inner divertor structure, or “girdle” and inner vessel wall is ~70 °C. This differential temperature is limited by the capacity of the studs that hold the inner divertor backing plates to the vessel wall. At a 70 °C temperature differential the stress on the studs is within allowable limits. The thermal model was then used for a stress pass to quantify vessel shell stresses where thermal gradients are significant.