Modification of divertor heat and particle flux profiles with applied 3D fields in NSTX H-mode plasmas

Abstract

Externally imposed non-axisymmetric magnetic perturbations are observed to alter divertor heat and particle flux profiles in the National Spherical Torus Experiment (NSTX). The divertor profiles are found to have a modest level of multiple local peaks, characteristic of strike point splitting or the ‘magnetic lobe’ structure, even before the application of the 3D fields in some (but not all) NSTX discharges. This is thought to be due to the intrinsic error fields. The applied 3D fields augmented the intrinsic strike point splitting, making the amplitude of local peaks and valleys larger in the divertor profile and striations at the divertor surface brighter. The measured heat flux profile shows that the radial location and spacing of the striations are qualitatively consistent with a vacuum field tracing calculation. 3D field application did not change the peak divertor heat and particle fluxes at the toroidal location of measurement. Spatial characteristics of the observed patterns are also reported in the paper.