A moving belt divertor concept with applications to ITER

For near-term fusion devices such as ITER (the International Thermonuclear Experimental Reactor), the divertor system represents one of the most challenging problems in the design of the entire machine. During normal operation the divertor surface must remove the plasma transport power (-100 MW) conducted to it along the magnetic field lines. Also, the divertor surface acts to neutralize the impinging plasma ions for subsequent removal by the vacuum pumping system. Plasma contamination due to sputtering of divertor surface atoms by incident plasma ions must be limited. For ITER, peak heat fluxes to the divertor arc predicted to be 15-30 MW/m2, with accompanying particle fluxes of 4.0x1023 m"2s'1. The amount of uncertainty inherent in any prediction of the energy and particle fluxes to the divertor necessitates the design of as robust a divertor as possible. The current ITER reference divertor design illustrates the difficulty of designing a feasible divertor. The initial phase of ITER, the Physics Phase, calls for a carbon-carbon (C-O composite armor tile brazed to a molybdenum- or copper-alloy heat sink, which is water-cooled at 60°C and 3.5 MPa. For this design, separatrix sweeping is required if the peak plasma heat flux is higher than 15 MW/m2, to maintain adequate burnout margins and armor thickness. Separatrix sweeping involves oscillating the currents in the magnetic field coils which produce the diverted magnetic configuration, thus causing an oscillation of the strike point location on the divertor surface. If redeposition is assumed to reduce the steady-state erosion of the armor, then disruption erosion limits the lifetime of this design to 20-50 disruptions, necessitating frequent replacement of the divertor modules, a time- consuming procedure. Other critical design issues include the neutron-induced thermal conductivity degradation in carbon, the retention of tritium and carbon codeposited on the divertor surface, and the risk of severe damage by runaway electrons. Particularly crucial issues are the fatigue life and reliability of the thousands of brazes joining the C-C armor and the heat sink material, because the loss of a carbon tile will lead to the exposure of the metallic heat sink to the plasma. The performance of the overall tokamak is therefore strongly linked to the performance of the divertor system and its ability to effectively remove a large amount of heat while receiving an enormous particle flux.