Techniques for the measurement of disruption halo currents in the National Spherical Torus Experiment

This paper describes techniques for measuring halo currents, and their associated toroidal peaking, in the National Spherical Torus Experiments [M. Ono et al., Nucl. Fusion 40, 557 (2000)]. The measurements are based on three techniques: (1) measurement of the toroidal field created by the poloidal halo current, either with segmented Rogowski coils or discrete toroidal field sensors, (2) the direct measurement of halo currents into specially instrument tiles, and (3) small Rogowski coils placed on the mechanical supports of in-vessel components. For the segmented Rogowski coils and discrete toroidal field detectors, it is shown that the toroidal peaking factor inferred from the data is significantly less than the peaking factor of the underlying halo current distribution, and a simple model is developed to relate the two. For the array of discrete toroidal field detectors and small Rogowski sensors, the compensation steps that are used to isolate the halo current signal are described. The electrical and mechanical design of compact under-tile resistive shunts and mini-Rogowski coils is described. Example data from the various systems are shown.