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Observations Supporting Electron Hyper-Viscosity Current Drive in the HIT-SI Spheromak

Author: Aaron C Hossack
Requested Type: Consider for Invited
Submitted: 2009-12-08 13:38:17

Co-authors: M.A. Chilenski, D.A. Ennis, T.R. Jarboe, M. Nagata, B.A. Nelson, B.S. Victor, J.S. Wrobel

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

Abstract Text:
The velocity profiles measured by Ion Doppler Spectroscopy (IDS) on HIT-SI are consistent with an electron hyper-viscosity as the relaxation mechanism. The HIT-SI experiment uses inductive helicity injection to form and sustain a spheromak. Oscillating magnetic flux and electric fields are induced in phase by each injector which drives helicity into the confinement region. Relaxation towards a minimum energy state forms and sustains the spheromak. An important question is the relaxation mechanism: either reconnection events or electron hyper-viscosity. In the latter, dynamo action, derived from the generalized Ohm’s law, including the hall term, can drive current across closed flux [1]. A simple picture of how magnetic fluctuations produce hyper-viscosity will be given. With hyper-viscosity-drive, in the injector-driven region, the electric field is stronger than needed to overcome resistive drag and the ions are accelerated parallel to the current. In the hyper viscosity-driven spheromak region, the electrons are driven against resistive drag by a magnetic force. Ions experience only the opposite resistive drag and are accelerated anti-parallel to the current [2]. These regions are distinguishable on HIT-SI because the injector driven regions have a different geometry, frequency, and temperature than the hyper-viscosity-driven spheromak.

Measurements from optical diagnostics of the interactions of the injector-driven plasma with the sustained-spheromak plasma are presented and confirm the electron hyper-viscosity current drive mechanism. IDS measures the temperature and velocity of the bulk ion species, He II, at 468.57 nm. Ion velocities out of the injector are much higher in the direction parallel to the current and the toroidal flow, time-averaged over many cycles, is opposed to the spheromak toroidal current, both consistent with the hyper-viscosity mechanism. The time dependent velocity flow pattern in the confinement region is also in agreement with the ions flowing parallel to the injector-driven current and anti-parallel to the sustained-spheromak currents. The temperature of the injected plasma is 2-3 times greater than the plasma in the confinement region due to stronger heating in the injector and slow perpendicular thermalization times. Bolometry measurements show increased radiation at the injector opening when the electron flow is out of the injector. Thus, the preferred heat flow is with the electrons. Work supported by US DoE.

[1] H. Ji, Phys. Rev. Lett. 83, 3198 (1999).
[2] K.J. McCollam, T. R. Jarboe, Plasma Phys. Control. Fusion 44, 493-517 (2002).

Characterization: A1,E3

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Please group with other HIT-SI presentations.