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(IC/P-05) Physics Issues for a Very-Low-Aspect-Ratio Quasi-Poloidal Stellarator (QPS)

J.F. Lyon¹⁾, L.A. Berry¹⁾, S.P. Hirshman¹⁾, D.A. Spong¹⁾, D.J. Strickler¹⁾, P.K. Mioduszewski¹⁾, B.E. Nelson¹⁾, D.E. Williamson¹⁾, A.S. Ware²⁾, E. Barcikowski²⁾, A.J. Deisher²⁾, A. Brooks³⁾, G.Y. Fu³⁾, D. Mikkelsen³⁾, D.A. Monticello³⁾, N. Pomphrey³⁾, R. Sanchez^4)
 
1) Oak Ridge National Laboratory, Oak Ridge, TN, USA
²⁾ University of Montana-Missoula, Missoula, Montana, USA
³⁾ Princeton Plasma Physics Laboratory, Princeton, NJ, USA
⁴⁾ Universidad Carlos III de Madrid, Madrid, Spain

Abstract.  A quasi-poloidal stellarator with very low plasma aspect ratio (R/a $\sim$ 2.7, 1/2-1/4 that of existing stellarators) is a new confinement approach that could ultimately lead to a high-beta compact stellarator reactor. The Quasi-Poloidal Stellarator (QPS) experiment is being developed to test key features of this approach. The QPS will study neoclassical and anomalous transport, stability limits at beta up to 2.5%, the configuration dependence of the bootstrap current, and equilibrium robustness. The quasi-poloidal symmetry leads to neoclassical transport that is much smaller than the anomalous transport. The reduced effective field ripple may also produce reduced poloidal viscosity, enhancing the ambipolar E x B poloidal drift and allowing larger poloidal flows for reduction of anomalous transport. A region of second stability exists in QPS at higher beta. Very-high-beta configurations with a tokamak-like transform profile have also been obtained with a bootstrap current 1/3-1/5 that in an equivalent tokamak. These configurations are stable to low-n ideal MHD kink and vertical instabilities for beta up to 11%. Ballooning-stable configurations are found for beta in the range 2% to 23%.

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