Comparison of ITER Single-Null and Double-Null Operation

The nominal International Thermonuclear Experimental Reactor (ITER) configuration is a double-null (DN) divertor, which requires precise plasma vertical position control. Vertical displacements of only about 1 cm (out of a plasma height of 4.7 m) are estimated to destroy the up/down symmetric distribution of power flow to the divertor plates. As an alternate configuration to avoid this difficulty, we look at the single-null (SN) option, where all the charged power flow is deposited on the lower divertor plate. The primary consideration in this study is that of technology phase performance (maximum neutron wall load) for the ITER divertor heat load and plasma constraints. With regard to the divertor heat loads, the SN case has the advantages of (a) longer scrape-off field line connection lengths and (b) more vertical space, which allows a greater spreading of the heat load on the divertor plates. These advantages offset the SN case disadvantage of having fewer divertor plates, and therefore the potential for higher heat fluxes for a given core plasma condition. The attainable wall loads for the SN and DN divertors are found to be similar for steady-state and hybrid operation scenarios.