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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. Zhu1)
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 Ip < 1.5 MA. Maximum toroidal and normalized beta have exceeded 31% and 5.5, respectively, with normalized beta reaching 7.4 li. 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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IAEA 2003