TY - JOUR
T1 - Beta-limiting instabilities and global mode stabilization in the National Spherical Torus Experiment
AU - Sabbagh, S. A.
AU - Bell, R. E.
AU - Bell, M. G.
AU - Bialek, J.
AU - Glasser, A. H.
AU - Leblanc, B.
AU - Menard, J. E.
AU - Paoletti, F.
AU - Stutman, D.
AU - Fredrickson, E.
AU - Garofalo, A. M.
AU - Gates, D.
AU - Kaye, S. M.
AU - Lao, L. L.
AU - Maingi, R.
AU - Mueller, D.
AU - Navratil, G.
AU - Ono, M.
AU - Peng, M.
AU - Synakowski, E.
AU - Zhu, W.
PY - 2002/5
Y1 - 2002/5
N2 - Research on the stability of spherical torus plasmas at and above the no-wall beta limit is being addressed on the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)], that has produced low aspect ratio plasmas, R/a∼1.27 at plasma current exceeding 1.4 MA with high energy confinement (TauE/TauE-ITER89P>2). Toroidal and normalized beta have exceeded 25% and 4.3, respectively, in q∼7 plasmas. The beta limit is observed to increase and then saturate with increasing l i. The stability factor β N/l i has reached 6, limited by sudden beta collapses. Increased pressure peaking leads to a decrease in β N. Ideal stability analysis of equilibria reconstructed with EFIT [L. L. Lao et al., Nucl. Fusion 25, 1611 (1985)] shows that the plasmas are at the no-wall beta limit for the n=1 kink/ballooning mode. Low aspect ratio and high edge q theoretically alter the plasma stability and mode structure compared to standard tokamak configurations. Below the no-wall limit, stability calculations show the perturbed radial field is maximized near the center column and mode stability is not highly effected by a nearby conducting wall due to the short poloidal wavelength in this region. In contrast, as beta reaches and exceeds the no-wall limit, the mode becomes strongly ballooning with long poloidal wavelength at large major radius and is highly wall stabilized. In this way, wall stabilization is more effective at higher beta in low aspect ratio geometry. The resistive wall mode has been observed in plasmas exceeding the ideal no-wall beta limit and leads to rapid toroidal rotation damping across the plasma core.
AB - Research on the stability of spherical torus plasmas at and above the no-wall beta limit is being addressed on the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)], that has produced low aspect ratio plasmas, R/a∼1.27 at plasma current exceeding 1.4 MA with high energy confinement (TauE/TauE-ITER89P>2). Toroidal and normalized beta have exceeded 25% and 4.3, respectively, in q∼7 plasmas. The beta limit is observed to increase and then saturate with increasing l i. The stability factor β N/l i has reached 6, limited by sudden beta collapses. Increased pressure peaking leads to a decrease in β N. Ideal stability analysis of equilibria reconstructed with EFIT [L. L. Lao et al., Nucl. Fusion 25, 1611 (1985)] shows that the plasmas are at the no-wall beta limit for the n=1 kink/ballooning mode. Low aspect ratio and high edge q theoretically alter the plasma stability and mode structure compared to standard tokamak configurations. Below the no-wall limit, stability calculations show the perturbed radial field is maximized near the center column and mode stability is not highly effected by a nearby conducting wall due to the short poloidal wavelength in this region. In contrast, as beta reaches and exceeds the no-wall limit, the mode becomes strongly ballooning with long poloidal wavelength at large major radius and is highly wall stabilized. In this way, wall stabilization is more effective at higher beta in low aspect ratio geometry. The resistive wall mode has been observed in plasmas exceeding the ideal no-wall beta limit and leads to rapid toroidal rotation damping across the plasma core.
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U2 - 10.1063/1.1468230
DO - 10.1063/1.1468230
M3 - Article
AN - SCOPUS:0001235383
SN - 1070-664X
VL - 9
SP - 2085
EP - 2092
JO - Physics of Plasmas
JF - Physics of Plasmas
IS - 5
ER -