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Presentation:submitted:by:
jsw_icc_paper.pdf2011-09-07 17:49:53Aaron Hossack
ach_icc_poster.pdf2011-08-26 18:32:40Aaron Hossack
ach_icc_talk_v2.pdf2011-08-26 18:32:10Aaron Hossack

Measurements of Energy and Helicity Cascades and Relaxation Rate in HIT-SI

Author: Aaron C. Hossack
Requested Type: Consider for Invited
Submitted: 2011-06-10 16:00:15

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

Contact Info:
University of Washington
AERB Rm. 120
Seattle, WA   98195
USA

Abstract Text:
Inverse cascades of magnetic helicity to lower toroidal modes and cascades of free energy to higher modes are observed in the HIT-SI device. An array of surface magnetic probes embedded in the HIT-SI flux conserver resolve plasma dynamics from 10 Hz to 200 kHz. Amperian loops formed by probe sub-arrays at 0, 45, 180 and 225 degrees toroidal allow non-axisymmetric plasma currents to be measured. Two 16-position toroidal arrays allow the study of the toroidal mode structure up to n = 7. The HIT-SI injectors have odd symmetry and inject helicity at a higher lambda than the relaxed axisymmetric spheromak. Measurements from the surface probes show that the injectors initially couple to an n = 1 eigenmode of the confinement volume and the helicity in this mode subsequently decays to the n = 0 minimum energy spheromak state. Thus helicity follows an inverse-cascade to lower lambda and toroidal modes. At the same time, magnetic energy released in the relaxation process follows a normal cascade to higher toroidal modes which are more dissipative.
The relaxation time in HIT-SI is also calculated using surface probe measurements. During single injector operation, a helicity balance model predicts peaks in toroidal current when the injector voltage and flux change polarity because the injector helicity content transfers to the spheromak. Using measured injector currents and helicity decay time as inputs, helicity balance calculations predict the time of peaks in the toroidal current assuming instantaneous relaxation. The time delay to the actual toroidal current peaks as measured by the surface probes is a measure of the relaxation time. The relaxation time is measured to be 4.2 +/- 2.8 microseconds- much faster than the Sweet-Parker relaxation time of 60 to 100 microseconds. Work supported by USDoE.

Characterization: D6

Comments:
Please group with HIT-SI presentations

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011