Investigation of the power balance in alternative divertor configurations in spherical tokamaks

In tokamak reactors of the future, the heat fluxes in the divertor regions will be higher than current materials are capable of withstanding. Instead of merely scaling up existing tokamak technology to reactor-relevant scales, a new method for reducing heat fluxes is needed. This will allow plasmas of sufficient temperatures for net energy gain in the reactor, while still giving acceptable component lifetimes in the divertor.

Changing divertor magnetic configuration has the potential to lower heat fluxes for equivalent core plasmas. The super-X concept shows promise in simulations, but can only be tested experimentally in a tokamak with a full divertor baffle. The first of these, MAST Upgrade, has just completed its first experimental campaign. Early results are presented herein, taking advantage of methods developed during other analysis.

A method for power balance analysis was developed utilising archive data from MAST. Two effects not accounted for previously were considered; tile shadowing and neutral beam power absorption. The method gives 85% power accounting for shots with 0 or 1 neutral beams, and 73% for 2 beams, with this discrepancy thought to result from fishbone instabilities.

The effect of tile plasma exposure on infrared camera measurements was determined with experiments on Magnum-PSI. The infrared reported a temperature decrease of 70C over three hours, despite neither the heat flux nor the true tile temperature decreasing, caused by uneven tile erosion of up to 100 μm. This will not occur in MAST Upgrade, but is an interesting result nonetheless.

Power balance on MAST Upgrade, utilising the method developed for MAST, was assessed. The effectiveness of a super-X with full a baffle was measured experimentally for the first time. The radiation in the divertor during a super-X was 3.1 times core radiation, compared to 1.1 with a conventional divertor, with a corresponding reduction in divertor heat flux.