Conceptual Design of a New Outer Divertor for C-Mod

Abstract

The next generation outer divertor target proposed for C-Mod is intended to operate with edge physics behavior that is ’Demo-like’, i.e., it will be capable of operating at a bulk tile and structure temperature of 600C. The proposed design exposes a vertical cylinder covered with tungsten lamellae tiles to the divertor heat flux. Heat load variation along the height of the cylindrical target has been specified and is being considered in the tile design. The design must allow for differential radial thermal expansion of the cylindrical structure. It is intended to be toroidally continuous with a high tolerance on axisymmetry to improve alignment with the plasma and limit interactions of disruption induced currents with the toroidal field. Inductively driven axisymmetric disruption currents are calculated using electromagnetic transient simulations previously employed for RF antennas and the cryopump. Disruption-induced halo currents are expected to flow though the structure, which have proved troublesome for the old outer divertor structure. The new toroidally continuous structure will be intrinsically strong with respect to axisymmetric mechanical loads, although the support hardware will also need to be robust to resist movement during non-axisymmetric halo loads. Halo current specifications for the outer divertor have been developed, and halo current paths that minimize loading are “forced” with appropriate use of insulation and grounding straps. Radiative energy transfer to other components in the vessel makes sustained operation of the outer divertor at elevated temperatures difficult.