(EX/P1-3) Progress on NSTX towards a stable steady state at low aspect ratio

D.A. Gates1), J. Menard1), R. Maingi2), S. Kaye1), S.A. Sabbagh3), S. Diem1), J.R. Wilson1), M.G. Bell1), R.E. Bell1), J. Ferron4), E.D. Fredrickson1), C.E. Kessel1), B.P. LeBlanc1), F. Levinton5), D. Mueller1), R. Raman6), T. Stevenson1), D. Stutman7), G. Taylor1), K. Tritz7), H. Yu5)
1) Princeton Plasma Physics Lab., Princeton, United States of America
2) Oak Ridge National Laboratory, Oak Ridge, TN, USA
3) Dept. of Applied Physics, Columbia Univ., NYC, NY, USA
4) General Atomics, San Diego, CA, USA
5) Nova Photonics, Princeton, NJ, USA
6) University of Washington, Seattle, WA, USA
7) Johns Hopkins University, Baltimore, MD, USA

Abstract.  Modifications to the NSTX poloidal field coils have led to a significant enhancement in shaping capability and has led to has lead to the achievement of a record shape factor (S = q95(Ip/aBt)) of 37 [MA/mTesla]. Achieving high shape factor is an important result for future ST burning plasma experiments as exemplified by studies for future ST reactor concepts, such as ARIES-ST [S. Jardin, et al., J. Comput. Phys. 66 481 (1986)], as well as neutron producing devices such as the Component Test Facility (CTF) [F. Najmabadi, et al., Fusion Engineering and Design 65 143 (2003)], which rely on achieving even higher shape factors in order to achieve steady-state operation while maintaining MHD stability at high βt. Plasmas with high shape factor have been sustained for pulse lengths which correspond to τpulse = 1.6 s∼50 τE∼5τCR, where τCR is the current relaxation time and τE is the energy confinement time. The non-inductive current fraction in the longest pulse discharges has reached 65%, with 55% pressure driven current and 10% neutral beam driven current. An interesting feature of these discharges is the observation that the central value of the safety factor q0 remains elevated for several current diffusion times. Use of the ``early H-mode'' scenario has been further optimized during the 2005, exhibiting a substantial reduction in the frequency and size of ELMs. The reduction in ELM magnitude and frequency has improved energy confinement time. NSTX operates with peak divertor heat fluxes which are in the same range as those expected for the ITER device, i.e. with Pheatmax∼10 MW/m2. High triangularity, high elongation plasmas on NSTX have been demonstrated to have reduced peak heat flux to the divertor plates to < 3 MW/m2.
* This research was supported by U.S. DOE contract DE-AC02-76-CH03073.

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