Enrichment of impurities seeded for exhaust control in a spherical tokamak power plant geometry
A strategy to prevent concentrated deposition of the exhaust power in a fusion device is to seed a localised ‘divertor’ region with impurity gas, which is chosen to radiate predominantly at the cooler electron temperatures associated with the plasma edge. If this impurity travels upstream to the main confined plasma, it can significantly impact fusion performance, by altering the upstream density through ionization, diluting the fuel and affecting the power balance through radiation losses. The aim is therefore to keep the impurity localized to the divertor, through a combination of magnetic geometry and seeding or fuelling locations. Measures of the success are given by the impurity compression and enrichment, quantifying the ratio of the amount of impurity found upstream to that in the divertor. Through SOLPS-ITER simulations, we have investigated the compression and enrichment of seeded argon in power-plant class connected double null diverted spherical tokamak geometry, with a well-baffled, extended outer divertor leg and a short, weakly baffled inner divertor leg. We find the argon remains well localized as the outer leg detaches, and the compression and enrichment in the inner leg can be improved by seeding there directly. Neon, by comparison, is found to be a much less suitable impurity for use at this scale.