Recent results from the HIT-SI experiment

Abstract

New understanding and improved parameters have been achieved on the Helicity Injected Torus with Steady Inductive helicity injection current drive (HIT-SI) experiment. The experiment has a bowtie-shaped spheromak confinement region with two helicity injectors. The inductive injectors are 180° segments of a small, oval cross section toroidal pinch. Spheromaks with currents up to 38 kA and current amplification of 2 have been achieved with only 6 MW of injector power. The Taylor-state model is shown to agree with HIT-SI surface and internal magnetic profile measurements. Helicity balance predicts the peak magnitude of toroidal spheromak current and the threshold for spheromak formation. The model also accurately predicts the division of the applied loop voltage between the injector and spheromak regions. Single injector operation shows that the two injectors have opposing, preferred spheromak current directions. An electron locking relaxation model is consistent with the preferred direction, with ion Doppler data and with bolometric data. Results from higher frequency operation are given. The impact of the new understanding on the future direction of the HIT programme is discussed.

Validation of single-fluid and two-fluid magnetohydrodynamic models of the helicity injected torus spheromak experiment with the NIMROD code

“…Comprehensive descriptions of these diagnostics can be found in various papers about the experiment. 13,25,39 We only provide a brief description of each diagnostic here. The IMP is a three-stem array of 3-axis probes spanning approximately 20 cm, covering the region between the magnetic axis at 33 cm and the vicinity of the outboard side wall (R % 55 cm).…”

“…Other methods include (electrostatic) coaxial helicity injection (CHI) used on spheromaks [4][5][6][7] and spherical tori, [8][9][10] and inductive methods like the oscillating field current drive (OFCD) used on the Madison symmetric torus (MST) 11 reversed-field pinch (RFP 12 ), and steady inductive helicity injection (SIHI) on the Helicity Injected Torus experiment (HIT-SI). 13 It is not clear how these complex processes, which are often difficult to measure and quantify, drive the plasma to a relaxed state. This is where validated models can play a vital role: illuminating plasma dynamics that are difficult to measure.…”

Evidence for Separatrix Formation and Sustainment with Steady Inductive Helicity Injection

“…Electrode based helicity injection has been used for spheromak formation [4] and is an effective method for the startup of toroidal devices [5][6][7]. Steady inductive helicity injection (SIHI) requires less density injection [8] and should have less impurity generation than electrode based methods. SIHI involves the nonlinear plasma phenomena of self-organization, relaxation, and reconnection that are of general interest in the study of planetary and solar magnetic fields.…”

“…However, the walls quickly become saturated with hydrogen isotopes, increasing wall recycling and radiated power. Past experimental results on HIT-SI have used helium as the operating gas [8,[11][12][13]. Earlier experiments performed in hydrogen and deuterium without helium conditioning had achieved low toroidal currents.…”

“…This implies the decrease in the measured n ¼ 1 Fourier mode is the result of less energy in the n ¼ 1 eigenmode of the confinement volume. It is proposed that as the toroidal current increases the injectors begin to transfer helicity directly to the toroidal current (perhaps through electron locking [8] correlation of toroidal current with measured n ¼ 1 Fourier mode for six deuterium shots is summarized in Fig. 7.…”