Optimization of DIII-D advanced tokamak discharges with respect to the β limit

Abstract

Results are presented from comparisons of modeling and experiment in studies to assess the best choices of safety factor q profile, pressure profile, and discharge shape for high β, steady-state, noninductive advanced tokamak operation in the DIII-D device [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. These studies are motivated by the need for high qminβN to maximize the self-driven bootstrap current while maintaining high toroidal β to increase fusion gain. Modeling shows that increases in the normalized beta βN stable to ideal, low toroidal mode number (n=1,2), instabilities can be obtained through broadening of the pressure profile and use of a symmetric double-null divertor shape. Experimental results are in agreement with this prediction. The general trend is for qminβN to increase with the minimum q value (qmin) although βN decreases as qmin increases. By broadening the pressure profile, βN≈4 is obtained with qmin≈2. Modeling of equilibria with near 100% bootstrap current indicates that operation with βN≈5 should be possible with a sufficiently broad pressure profile.

Progress towards high-performance steady-state operation on DIII-D

“…Further analysis at the time indicated on the figure (2.05 s) finds that the ideal-wall stability limits to the n = 1, 2 and 3 kink modes increase with q min above 2. The no-wall limit does not increase at higher q min , consistent with the previous observations [15]. These stability calculations indicate that considerable increases in β may still be available in these discharges if RWM control is maintained.…”

“…As β N gradually increases, the pressure profile becomes more peaked, primarily due to changes in the density profile. As β N crosses 3.3, the calculated wall position for n = 1 marginal stability reaches the DIII-D conformal wall position (1.45a) [15]. Soon afterward, we observe an n = 1 fast growing mode that in turn triggers n = 1 neoclassical tearing modes.…”

“…Stability limits usually appear at β N ≤ 3 in discharges with ITBs [19] due to the associated highly peaked pressure profile. In AT experiments, as described in Section 2, high β is usually difficult to obtain with high q min > 2 [15].…”

“…Future development of this tool is expected to facilitate operation with β N approaching the ideal wall limit to the n = 1 kink mode. Strong plasma shaping has also been shown to be beneficial for a high β operation limit [15]. Since ECCD is most efficient at low density and high electron temperatures (I CD ∝ (T e /n e )P CD ), the plasma shape used for AT experiments must be matched with the cryopump used to control the density.…”

Progress toward fully noninductive, high beta conditions in DIII-D

“…These experiments take advantage of recent improvements on DIII-D, including resistive wall mode ͑RWM͒ stabilization via plasma rotation and active feedback control with external, and more recently additional internal nonaxisymmetric coils, 8 which allows routine operation above the no-wall beta limit. 9 In these experiments, off-axis electron cyclotron current drive ͑ECCD͒ has been shown to be effective for modifying and/or controlling the current profile in high ␤ plasmas. 6,7 Density control of AT discharges 7 with edge localized modes ͑ELMs͒ using divertor cryopumps facilitates high current drive efficiency at reactor relevant collisionality.…”