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

dc.contributor.authorWoodruff, S.
dc.contributor.authorCohen, B. I.
dc.contributor.authorHooper, E. B.
dc.contributor.authorMclean, H. S.
dc.contributor.authorStallard, B. W.
dc.contributor.authorHill, D. N.
dc.contributor.authorHolcomb, C. T.
dc.contributor.authorRomero-Talamás, Carlos
dc.contributor.authorWood, R. D.
dc.contributor.authorCone, G.
dc.contributor.authorSovinec, C. R.
dc.description.abstractIn 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 (⁠δB / B ~ 1% on the midplane edge) yields Tₑ profiles peaked at >200 eV⁠. Trends indicate a limiting beta (βₑ ⁠~ 4% - 6%), and so we have been motivated to increase Tₑ 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 δB / B ~ 2% and large voltage fluctuations δV ~ 1kV⁠, giving a 50% increase in current amplification, Itor / Igun⁠. (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 (⁠~0.7T 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.
dc.description.sponsorshipThe 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.
dc.format.extent14 pages
dc.genrejournal articles
dc.identifier.citationS. Woodruff, B. I. Cohen, E. B. Hooper, H. S. Mclean, B. W. Stallard, D. N. Hill, C. T. Holcomb, C. Romero-Talamas, R. D. Wood, G. Cone, C. R. Sovinec; Controlled and spontaneous magnetic field generation in a gun-driven spheromak. Phys. Plasmas 1 May 2005; 12 (5): 052502.
dc.relation.isAvailableAtThe University of Maryland, Baltimore County (UMBC)
dc.relation.ispartofUMBC Mechanical Engineering Department Collection
dc.rightsThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
dc.rightsPublic Domain Mark 1.0 en
dc.titleControlled and spontaneous magnetic field generation in a gun-driven spheromak


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