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(EX/S2-2) The Resistive Wall Mode and Feedback Control Physics Design in NSTX

S.A. Sabbagh¹⁾, J. Bialek¹⁾, R.E. Bell²⁾, A.H. Glasser³⁾, B. LeBlanc²⁾, J.E. Menard²⁾, F. Paoletti¹⁾, M.G. Bell²⁾, R. Fitzpatrick⁴⁾, E. Fredrickson²⁾, A.M. Garofalo¹⁾, D. Gates²⁾, S.M. Kaye²⁾, L.L. Lao⁵⁾, R. Maingi⁶⁾, D. Mueller²⁾, G.A. Navratil¹⁾, D. Stutman⁷⁾, W. Zhu^1)
 
1) Columbia University, New York, USA
²⁾ Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ, USA
³⁾ Los Alamos National Laboratory, Los Alamos, NM, USA
⁴⁾ University of Texas at Austin, Austin, TX, USA
⁵⁾ General Atomics, San Diego, CA, USA
⁶⁾ Oak Ridge National Laboratory, Oak Ridge, TN, USA
⁷⁾ Johns Hopkins University, Baltimore, MD, USA

Abstract.  The National Spherical Torus Experiment has been designed to investigate the physics of ST global mode stabilization. NSTX has R = 0.86 m, a < 0.7 m, B₀ < 0.45 T, and I_p < 1.5 MA. Maximum toroidal and normalized beta have exceeded 31% and 5.5, respectively, with normalized beta reaching 7.4 l_i. Sudden beta collapses have been correlated to violation of the n = 1 ideal MHD beta limit using time-evolving EFIT reconstructions of experimental discharges. The resistive wall mode (RWM) was observed in experiments maximizing plasma coupling to the stabilizing conducting plates. A large rotation damping rate of -300 kHz/s was observed in RWM discharges in contrast to -75 kHz/s for plasmas exhibiting n = 2 and 3 rotating modes. The computed RWM perturbed field structure from experimental equilibria was input to the VALEN code and the computed n = 1 mode growth time of 4.6 ms agrees well with the experimental value of 5 ms. Increased beta improves wall coupling, and passive stabilization of an equilibrium with normalized beta of 5.2 and pressure peaking factor of 2.2 yields a growth time of 23.5 ms. This plasma would be completely stabilized by a proposed global instability feedback system.

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