Attainment of High Normalized Current by Current Profile Manipulation in the Pegasus Toroidal Experiment

Large stable values of normalized current IN are achievable in spherical tori due to the large gradient of the toroidal field across the plasma. This allows access to high Troyon beta limits. Values of IN > 12 MA/m-T are expected to be stable to ideal MHD modes in the ultra-low-A Pegasus Toroidal Experiment. In previous experiments, the toroidal field utilization (Ip/Itf) was found to be limited roughly to unity by the onset of large-scale low-order tearing modes during the current ramp, which limited IN to roughly 6 MA/m-T. Three techniques have now been developed that allow access to higher values of IN, primarily through modification of the current density profile. The first of these techniques employs a fast rampdown of the low-inductance toroidal field coil to rapidly decrease Itf. The other techniques employ gas-fueled washer stack guns as plasma sources. The second technique uses these guns to provide preionization at low toroidal field, while the third relies on the guns as helicity sources to form ST plasmas non-inductively. Using these techniques, values of Ip/Itf > 2 have been obtained.