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

摘要

Large stable values of normalized current I_N 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 I_N > 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 (I_p/I_(tf)) was found to be limited roughly to unity by the onset of large-scale low-order tearing modes during the current ramp, which limited I_N to roughly 6 MA/ m-T. Three techniques have now been developed that allow access to higher values of I_N, 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 I_(tf). 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 he-licity sources to form ST plasmas non-inductively. Using these techniques, values of I_p/I_(tf) > 2 have been obtained.