Experimental imaging of separatrix splitting on DIII-D

Perturbations in the plasma boundary due to the application of non-axisymmetric n = 3 fields (δB/B ∼ 10−4−10−3) were measured in the X-point region in DIII-D by tangential imaging of extreme ultra-violet and soft x-ray emission. Vacuum-field calculations show that these perturbations take the form of lobes extending outwards from the unperturbed separatrix. These structures can be resolved in experiment by performing a differential measurement of the image for different toroidal phases of the applied perturbation. Forward modelling of the line-integrated image with a synthetic diagnostic is used to directly compare the measurement with vacuum-field calculations. Results indicate the existence of these lobes match well with the measured data. These lobes are observed to shift poloidally on the high-field side of the magnetic axis as the edge magnetic safety factor is varied, consistent with vacuum-field calculations. (letter)