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Controlled and spontaneous magnetic field generation in a gun-driven spheromak

Woodruff, S. and Cohen, B. I. and Hooper, E. B. and Mclean, H. S. and Stallard, B. W. and Hill, D. N. and Holcomb, C. T. and Romero-Talamás, C. and Wood, R. D. and Cone, G. and Sovinec, C. R. (2005) Controlled and spontaneous magnetic field generation in a gun-driven spheromak. Physics of Plasmas, 12 (5). Art. No. 052502. ISSN 1070-664X. doi:10.1063/1.1878772.

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In the Sustained Spheromak Physics Experiment, SSPX [E. B. Hooper, D. Pearlstein, and D. D. Ryutov, Nucl. Fusion 39, 863 (1999)], progress has been made in understanding the mechanisms that generate fields by helicity injection. SSPX injects helicity (linked magnetic flux) from 1 m diameter magnetized coaxial electrodes into a flux-conserving confinement region. Control of magnetic fluctuations (delta B/B similar to 1% on the midplane edge) yields T-e profiles peaked at > 200 eV. Trends indicate a limiting beta (beta(e)similar to 4%-6%), and so we have been motivated to increase T-e by operating with stronger magnetic field. Two new operating modes are observed to increase the magnetic field: (A) Operation with constant current and spontaneous gun voltage fluctuations. In this case, the gun is operated continuously at the threshold for ejection of plasma from the gun: stored magnetic energy of the spheromak increases gradually with delta B/B similar to 2% and large voltage fluctuations (delta V similar to 1 kV), giving a 50% increase in current amplification, I-tor/I-gun. (B) Operation with controlled current pulses. In this case, spheromak magnetic energy increases in a stepwise fashion by pulsing the gun, giving the highest magnetic fields observed for SSPX (similar to 0.7 T along the geometric axis). By increasing the time between pulses, a quasisteady sustainment is produced (with periodic good confinement), comparing well with resistive magnetohydrodynamic simulations. In each case, the processes that transport the helicity into the spheromak are inductive and exhibit a scaling of field with current that exceeds those previously obtained. We use our newly found scaling to suggest how to achieve higher temperatures with a series of pulses.

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Additional Information:Copyright © 2005 American Institute of Physics. Received 11 October 2004; accepted 27 January 2005; published online 13 April 2005. The authors would like to acknowledge useful discussions with Dmitri Ryutov and Ken Fowler, and other SSPX team members for keeping the machine operating. An earlier version of this paper was presented as an invited talk at the American Physical Society Division of Plasma Physics meeting, November, 2003. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Lab under Contract No. W-7405-ENG-48.
Subject Keywords:plasma guns; plasma confinement; plasma transport processes; plasma fluctuations; plasma instability; plasma simulation; plasma magnetohydrodynamics; plasma temperature
Issue or Number:5
Record Number:CaltechAUTHORS:WOOpop05
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2166
Deposited By: Tony Diaz
Deposited On:13 Mar 2006
Last Modified:08 Nov 2021 19:45

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