Investigation of the effect of external magnetic perturbations upon divertor heat load

The heat load on a divertor target plate is spread over a wider region as the magnitude of the radial thermal conductivity in the edge plasma is increased. This observation gives rise to speculation that open-quotes ergodizationclose quotes of the edge plasma, through deliberate external magnetic field perturbations, could be employed to reduce the peak heat load on the divertor target plate in TPX or ITER. The authors have developed computer programs to model the effect of non-axisymmetric magnetic perturbations upon divertor heat load, and have explored what kind of externally applied perturbations are the most effective for heat load reduction without destroying core plasma confinement. The work relies on a grid-free random-walk method to solve a highly anisotropic model heat transport equation in a realistic perturbed tokamak magnetic field. The walkers are launched in the core plasma and make small steps across the magnetic field and long steps along the field lines; the latter steps are evaluated by accurate field line integration. The distribution of points where the walkers cross the boundary is interpreted as the heat load on the boundary. This is a model equation, because the authors do not consider the dependence of parallel conductivity on temperature, nor do they solve for a consistent density profile. Only the (constant) ratio of parallel to perpendicular diffusivity is important. They find that a carefully optimized set of coils located about 0.1 m outside the separatrix in TPX (or about 0.3 m outside the separatrix in ITER) can be used to spread the heat load over about 0.03 m in TPX (0.1 m in ITER) perpendicular to flux surfaces at the divertor plate, even at a very low level of anomalous cross-field heat transport. However, a similar beneficial effect is impossible to obtain if, for engineering reasons, the perturbing coils must be positioned significantly further away from the main plasma