A Study of Plasma Dynamics in HIT-SI using Ion Doppler Spectroscopy

Abstract

The HIT-SI device is a unique experiment which uses inductive helicity injectors to form and sustain a spheromak plasma. The n = 1 symmetry of the injectors enables stable spheromak sustainment by imposed-dynamo current drive (IDCD). The experiment is diagnosed with spectroscopy, interferometry, imaging, and internal and surface magnetic probes. Two methods of density reduction and control are presented. A helicon preionization source enables plasma breakdown and operations at densities an order of magnitude lower than previously possible. The system is critical for all operations at injector frequencies greater than 14.5 kHz and single-injector operations. Additionally, a high-speed piezoelectric gas injection valve was developed to enable dynamic injector fueling adjustable on a timescale of ~0.5 ms. The focus of this work is on results from an ion Doppler spectrometer (IDS) which was upgraded to multi-chord capability. Two coherent, linear fiber optic cables with small, wide-angle lenses simultaneously collect light from 30 - 40 chords. Additionally, biorthogonal decomposition is used as a novel filtering method for raw data. Impurity radiation measurements of high power plasmas show no toroidal flow associated with toroidal current and temperature evolution which rises during toroidal current ramp-up and falls during current sustainment. Coherent velocity fluctuations show rigid, oscillatory motion of the spheromak plasma driven by the helicity injectors. The coherent motion combined with a lack of magnetic instabilities indicates that the spheromak is stable. Comparisons with NIMROD and PSI-TET simulations show similar chord-averaged velocity oscillations but fail to show the observed coherent, rigid motion of the spheromak. Additionally, strong flows and reconnection events in simulations which are not observed in the experiment indicate that agreement may improve with higher viscosity. The measured C III temperatures lie between the two codes' estimates. Line-radiation from high-frequency operations (53.5 and 68.5 kHz) was too weak for the camera so an image intensifier had to be used, yielding a sparse data set. Composite measurements from multiple discharges show higher ion temperatures and lower intensity fluctuations than at 14.5 kHz. Negligible toroidal flow is measured at 68.5 kHz, supporting the conclusion that an outward Shafranov shift is due to pressure confinement rather than rotation. Measured beta values assuming T_i = T_e and n_i = n_e are in agreement with Grad-Shafranov fitting to magnetic equilibria supporting the claim that high-beta pressure confinement was achieved in high frequency operations.