Heat flux mitigation by impurity seeding in high-field tokamaks

The ability for tokamaks to exhaust power in the boundary via impurity radiation is explored using empirical scalings and a simple 0D exhaust model, focusing on the scaling with toroidal field and major radius. By combining a scaling for the heat flux width and the L-H threshold power, the parallel heat flux in the SOL is shown to scale strongly with magnetic field, q(parallel to) similar to B-T(2.52) while having little to no scaling with machine size, q(parallel to) similar to R-0.16. Despite the increased heat flux at high field, it is shown that target temperatures relevant to detachment can be reached with finite main- ion dilution for a variety of impurity seeding gases, although non- equilibrium ionization balance is required in most cases. The necessary impurity fractions are estimated to scale like f(Z) similar to (BTR1.33)-R-0.88,a result that is facilitated by an increase in upstream temperature at high q(parallel to) relative to peaks in the impurity cooling-curves. This scaling indicates that for optimizing reactors, minimizing device size while maximizing toroidal field, an approach shown to be consistent with energy confinement scaling, will also maximize the feasibility of reaching detachment at the lowest dilution. Despite this, analysis suggests an increase in the impurity fractions relative to existing devices will be required to exhaust power in a reactor-scale tokamak, with validation of impurity radiation physics required before both simple and detailed models can make reliable predictions of absolute f(z).