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New Understanding and Achievements from Independent Injector Drive Experiments on HIT-SI

Author: David A Ennis
Requested Type: Consider for Invited
Submitted: 2009-12-04 19:23:27

Co-authors: B. S. Victor, J. S. Wrobel, C. Akcay, T. R. Jarboe, G. J. Marklin, B. A. Nelson, R. J. Smith

Contact Info:
University of Washington
430 AERB, Box 352250
Seattle, WA   98195-2
USA

Abstract Text:
Experiments in the Helicity Injected Torus-Steady Inductive (HIT-SI) spheromak have yielded improved current amplification and new understanding of the injector-spheromak interaction. HIT-SI investigates steady inductive helicity injection to form and sustain an equilibrium in a high-beta spheromak geometry using two semi-toroidal injectors. In each injector the toroidal flux and induced loop voltage are sinusoidally oscillated in phase at a frequency of 5.8 kHz. During standard operations the two HIT-SI injectors are 90° out of phase with each other, producing a constant rate of overall helicity injection with all applied power delivered into the plasma, rather than to the plasma facing surfaces. The lack of electrodes or other strong plasma-wall interactions make HIT-SI an ideal platform for clean studies of helicity injection current drive and current-profile relaxation. The HIT-SI diagnostic suite includes: FIR interferometry, IDS, bolometry, flux loops, internal and surface magnetic probes. Operations with equal helicity injection rates between the two injectors (i.e. equal injector flux and loop voltage amplitudes) yield constant helicity injection. Results of unequal helicity injection have produced the highest spheromak current (38 kA), current amplification (I_tor/I_inj_quad ~ 2), and poloidal flux amplification (ψ_pol/ψ_inj_quad > 6) to date. Single-injector operations established a preferred direction of the generated spheromak current for each injector determined by the sign of the injected helicity and the injector’s orientation relative to the confinement volume. Changing the sign of the helicity injection reverses the direction of preferred spheromak current. On HIT-SI, the oppositely mounted injectors with the same sign of helicity have a different preferred spheromak current direction. Operating the injectors with opposite signs of helicity gives the same preferred direction for both injectors but yields no current drive because the total injected helicity is zero. Observation of sustained spheromak current decreasing in time is thought to result from field errors due to the equilibrium field loss into the flux conserver, eventually leading to flipping of the spheromak current when operating the injectors with similar helicity injection rates. Future HIT-SI plans include installation of equilibrium flux control to reduce spheromak degradation and provide favorable boundary conditions. Additionally, mounting the injectors on the same side of the confinement volume will allow all injectors to operate more effectively in their preferred direction. Work supported by USDoE.

Characterization: A1,E3

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