Technique for relating internal plasma shape to plasma current profiles in noncircular tokamaks

A tangentially viewing soft x‐ray pinhole camera has been used to measure internal plasma shape on the PBX‐M tokamak. While several methods of deriving the poloidal emissivity profile have been investigated, the most robust consists of a forward modeling technique which permits the inference of the internal shape, axial safety factor, and the current profile. Using poloidal flux measurements to constrain the external shape, equilibria are calculated for different values of q(0). The mapping of the x‐ray emissivity to the internal poloidal flux value is obtained by Abel‐inverting the x‐ray image on the plasma midplane and assuming constant emissivity on a flux surface. Model camera images are then constructed and compared with the measured camera image. A least‐squares fit of these models to the measured image produces an inferred value of q(0). The effects of noise have been examined, and the resulting uncertainty in q(0) can range from 0.1 to 0.3, depending on details of the experimental conditions.