PPPL-3815 is available in <a href="PPPL-3815.pdf">pdf</a> format (2.6 MB).

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<b>Beta-limiting MHD Instabilities in Improved-performance NSTX Spherical Torus Plasmas</b>
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<b>Authors:</b> J.E. Menard, M.G. Bell, R.E. Bell, E.D. Fredrickson, D.A. Gates, S.M. Kaye, B.P. LeBlanc, R. Maingi, D. Mueller, S.A. Sabbagh, D. Stutman, C.E. Bush, D.W. Johnson, R. Kaita, H.W. Kugel, R.J. Maqueda, F. Paoletti, S.F Paul, M. Ono, Y.-K.M. Peng, C.H. Skinner,
E.J. Synakowski, and the NSTX Research Team
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<b>Date of PPPL Report:</b> May 2003
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<b>Submitted to:</b> Nuclear Fusion.</p>


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Global magnetohydrodynamic stability limits in the National Spherical Torus Experiment (NSTX) have increased significantly recently due to a combination of device and operational improvements. First, more routine H-mode operation with broadened pressure profiles allows access to higher normalized beta and lower internal inductance. Second, the correction of a poloidal field coil induced error-field has largely eliminated locked tearing modes during normal operation and increased the maximum achievable beta. As a result of these improvements, peak beta values have reached (not simultaneously) <font face ="symbol">b</font><sub>t</sub> = 35%, <font face ="symbol">b</font><sub>N</sub> = 6.4, [<font face ="symbol">b</font><sub>N</sub>] = 4.5, <font face ="symbol">b</font><sub>N</sub> /l<sub>i</sub> = 10, and <font face ="symbol"> b</font><sub>P</sub> = 1.4. High <font face ="symbol"> b</font><sub>P</sub>  operation with reduced tearing activity has allowed a doubling of discharge pulse-length to just over 1 second with sustained periods of <font face ="symbol">b</font><sub>N</sub> approximately equal to 6 above the ideal no-wall limit and near the with-wall limit. Details of the <font face ="symbol">b</font> limit scalings and <font face ="symbol">b</font>-limiting instabilities in various operating regimes are described.




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