Accessing high normalized current in an ultra-low-aspect-ratio torus

The attainment of high normalized current, IN, in the P EGASUS Toroidal Experiment device is studied. This work assesses the plasma current-carrying capabilities of an ultra-low-aspect-ratio-tokamak, or ULART. This is directly related to minimizing the confining and stabilizing fields of this magnetic confinement system for fusion energy applications. The particular issues addressed in this work are: (1) numerical studies which show access to high IN is possible; (2) mitigation of tearing modes through manipulation of the plasma current profile; and (3) access to high IN through the manipulation of the plasma current profile and/or operation at very low toroidal field.; This work presents the first demonstration of high IN in a large-scale, ohmically-driven ULART. Values of IN = 8-14 are attained through three specific means. The first utilizes a plasma preionization source to allow plasma breakdown at very low toroidal field. This method, coupled with plasma shaping, gives the ability to access high IN before the discharge is limited by low-q resistive MHD activity. The second method uses DC helicity injection as a means to drive strong edge plasma current, which gives hollow plasma current profiles and reversed magnetic shear q-profiles. The reversed shear q-profile is shown to transiently suppress resistive MHD activity, which otherwise restricts access to high IN. The last method uses vacuum toroidal field ramps with ramprates ∼50 MA/sec. This method again drives edge plasma current to provide modest plasma current manipulation and suppression of resistive MHD activity.; Earlier studies indicated a non-disruptive limit of IN ∼6. The present work confirms this was not an intrinsic physics limit of a ULART, but was instead due to limitations in the experimental capabilities of the facility. It has also confirmed the hypothesis that plasma current profile manipulation and plasma shaping are strong influences on the plasma stability and, hence, access to high IN.