A Study of HIT-SI Plasma Dynamics Using Surface Magnetic Field Measurements

Steady Inductive Helicity Injection (SIHI) is a method to both form and sustain magnetically confined plasmas for the purpose of a steady-state fusion reactor. The Helicity Injected Torus with Steady Inductive helicity injection (HIT-SI) experiment uses this novel approach to both form and sustain spheromak plasmas. The HIT-SI program supports fusion energy research through the development of this new current drive technique and through efforts to advance the understanding of spheromak physics. The results presented here increase our understanding in the areas of spheromak formation, sustainment, relaxation current drive, and stability. Further, comparison is made of experiment data to numerical equilibrium and magnetohydrodynamic (MHD) simulations through the PSITET and the NIMROD codes under study by the Plasma Science and Innovation (PSI) Center.;An enabling diagnostic for pursuit of these research goals is the array of surface magnetic probes, embedded in the HIT-SI spheromak flux conserver, and that now resolves plasma dynamics from 10 Hz -- 200 kHz. The surface magnetic probe array provides an extensive and non-perturbative set of measurements to better understand the plasma dynamics in HIT-SI. At the very start, measurements from the surface magnetic probes uncovered a flaw in the HIT-SI flux conserver design that limited spheromak performance, and thus initiated a set of improvements in the flux conserver that resulted in higher toroidal currents. Amperian loops formed by sub-arrays of the surface magnetic probes at toroidal angles of 0°, 45°, 180°, and 225° allow measurement of non-axisymmetric toroidal plasma currents, capturing both injector-induced and spheromak currents. These toroidal current measurements indicate that toroidal-current-reversal events maintain a plasma object, as the rise time and decay time of the symmetric toroidal current is much longer than the current-reversal time. Single-injector operations show a rise in the toroidally-symmetric toroidal current at pauses in the helicity injection that occur at each injector-current zero crossing. Concurrent with these rises in the toroidal current, the surface fields quickly become very Taylor-like near these times of injector quiescence. A time-dependent helicity balance model provides a comparison to instantaneous relaxation behavior, and the ability to calculate relaxation times. An injector-driven n=1 mode is seen to form and precede n=0 formation, and an observed exchange of n=1 to n=0 mode energy is explained in the context of a picture of the pathways for relaxation of helicity and dissipation of magnetic energy in HIT-SI.