Low-n ideal MHD stability of tokamaks: Current and beta limits

Turnbull, A. D.; Yasseen, F.; Roy, Anirban; Sauter, O.; Cooper, W. A.; Nicli, S.; Troyon, F.

doi:10.1088/0029-5515/29/4/008

Cited by 20 publications

(30 citation statements)

References 19 publications

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“…The observation of an ideal kink mode at q 95 ϭ5 is unusual by tokamak standards, where typically q a ϭ2 -3 is required for instability to the ideal kink. This observation is in accordance with predictions 3,4 that unstable values of q 95 increase as A decreases. At the time of the instability the internal inductance is relatively low (l i Ϸ0.5) and toroidal beta is considerable (␤ t Ϸ18%).…”

Section: B External Kink Modessupporting

confidence: 93%

“…Pegasus is designed to expand the operational space of the ST to the region where AϽ1. 3 and I p /I t f Ͼ2. This includes the study of magnetohydrodynamic ͑MHD͒ stability limits at very-low aspect ratio.…”

Section: Introductionmentioning

confidence: 98%

“…The external kink boundary itself may change as A approaches 1, with unstable values of edge safety factor q increasing above the conventional limits of 2-3. 3,4 The Pegasus Toroidal Experiment 5 ͑Pegasus͒ is an extremely low aspect ratio ST developed to explore quasispherical high-pressure plasmas with the goal of minimizing the central column while maintaining good confinement and stability. Pegasus is designed to expand the operational space of the ST to the region where AϽ1.…”

Section: Introductionmentioning

confidence: 99%

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Performance and stability of near-unity aspect ratio plasmas in the Pegasus Toroidal Experiment

et al. 2003

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The Pegasus Toroidal Experiment ͓R. Fonck et al., Bull. Am. Phys. Soc. 41, 1400 ͑1996͔͒ is a spherical torus designed to study the limits of plasma behavior as the aspect ratio A approaches unity. Access to near-unity A is achieved through the use of a novel high-stress reinforced solenoid magnet. High toroidal beta ␤ t is obtained in ohmically-heated plasmas by operation at low field with densities up to the Greenwald limit. Values of ␤ t up to 20% and normalized beta up to 5 have been obtained. The ratio of plasma current to toroidal field rod current, known as the toroidal field utilization, reaches values as large as 1 but appears to approach a ''soft'' boundary at that level related to both ohmic flux limitations and the onset of resistive magnetohydrodynamic ͑MHD͒ activity. The m/nϭ2/1 and 3/2 modes are most frequently observed, in agreement with the inferred safety factor profiles. Experiments are beginning to access the external kink stability boundary at edge safety factor q 95 ϭ5, which is significantly higher than that observed in conventional tokamaks. Calculations using the DCON code ͓A. H. Glasser and M. S. Chance, Bull. Am. Phys. Soc. 42, 1848 ͑1997͔͒ confirm instability to the ideal kink.

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Section: B External Kink Modessupporting

confidence: 93%

Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

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Performance and stability of near-unity aspect ratio plasmas in the Pegasus Toroidal Experiment

et al. 2003

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“…Previous p optimization studies (Turnbull 1989) in conjunctions with the straight tokamak model (Friedberg 1987, Wesson 1978. suggest this promising approach to improving stability.…”

Section: The Second Stable Core Vh-modementioning

confidence: 99%

Optimized profiles for improved confinement and stability in the DIII-D tokamak

Taylor¹,

John²,

Turnbull³

et al. 1994

Plasma Phys. Control. Fusion

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Simultaneous achievement of high energy confinement, 'ZE, and high plasma beta, P. leads to an economically attractive compact tokamak fusion reactor. High confinement enhancement, H = 'CF/'ZE.ITER~~P = 4, and high normalized beta PN = P/(UaB) = 6%-m-T/MA. have been obtained in DIU-D experimental discharges.These improved confinement and/or improved stability limis are observed in several Dll-D high performance operational regimes: VH-mode, high 4 H-mode, second stable core, and high beta poloidal. We have identified several important features of the improved performance in these discharges: details of the plasma shape, toroidal rotation or ErB flow profile, q profile and current density profile, and pressure profile. From our improved physics understanding of these enhanced performance regimes, we have developed operational scenarios which maintain the essential features of the improved confinement and which increase the stability limits using localized current profile control. The stability limit is increased by modifying the interior safety factor profile to be nonmonotonic with high central q. while maintaining the edge current density consistent with the improved transport regimes and the high edge bootstrap current. We have calculated high beta equilibria with BN = 6.5, stable to ideal n=l kinks and stable to ideal ballooning modes.The safety factor at the 95% flux surface is 6. the central q value is 3.9 and the minimum in q is 2.6. The current density profile is maintained by the natural profile of the bootstrap current, and a modest amount of electron cyclotmn current drive. ' T E R ~R -S ~~)are observed in several operational regimes; VH-mode (Jackson 1991), high internal inductance (&) H-mode (Lao 1993a). and high poloidal @ (Politzer 1994). The performance in the high confinenent regimes observed in the JET, JT4OU, TFTR, and DIU-D tokamaks is no longer limited by transport or heating power, but instead are limited by stability limits at~high @T ( E T Team 1993. Mauel 1993, Zamstorff 1993. Taylor 1993. Strait 1993. Perfomance in present day tokamaks can be improved if the @-limit can be increased while maintaining the observed high confinement. Our ssategy for identifying 3 self-consistent high confinement high beta steady-state discharge scenario is to identify and maintain those features that are favorable for the high confinement and then modify the profiles to increase the stability limit, without adversely affecting the confinement. The features that we have identified that are favorable for high confinement are: (1) strong plasma shaping, high triangularity (6) and high elongation (K):(2) high plasma rotation. large shear in the ExB flow;(3) finite current density near the edge; (4) negative central shear; and (5) high q(0). We intend to show are compatible with steady-state high p.In the next section, we review a number of high performance regimes that have been identified in DlI-D experimental discharges, and discuss the features that we believe are important for achieving the high

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“…8 All of these designs are based on several predicted advantages for steady-state operation at high q min . These include (i) a high ideal-wall kink mode b limit when the current density is sufficiently broad or hollow to produce strong coupling to a nearby conducting wall, 9 bootstrap current fraction, which scales as b-poloidal (b P ) or qb N , 10 (iii), avoidance of low-order tearing modes by exclusion of rational surfaces from the plasma, e.g., the m=n ¼ 2=1 tearing mode when q min > 2, 11 and (iv) thermal confinement exceeding typical H-mode levels enabled by weak or negative magnetic shear. 12 Present experiments can test physics that goes into predictive models for next-step steady-state devices.…”

Section: Introductionmentioning

confidence: 99%

Fast-ion transport in qmin>2, high-β steady-state scenarios on DIII-D

et al. 2015

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Results from experiments on DIII-D [J. L. Luxon, Fusion Sci. Technol. 48, 828 (2005)] aimed at developing high b steady-state operating scenarios with high-q min confirm that fast-ion transport is a critical issue for advanced tokamak development using neutral beam injection current drive. In DIII-D, greater than 11 MW of neutral beam heating power is applied with the intent of maximizing b N and the noninductive current drive. However, in scenarios with q min > 2 that target the typical range of q 95 ¼ 5-7 used in next-step steady-state reactor models, Alfv en eigenmodes cause greater fast-ion transport than classical models predict. This enhanced transport reduces the absorbed neutral beam heating power and current drive and limits the achievable b N . In contrast, similar plasmas except with q min just above 1 have approximately classical fast-ion transport. Experiments that take q min > 3 plasmas to higher b P with q 95 ¼ 11-12 for testing long pulse operation exhibit regimes of better than expected thermal confinement. Compared to the standard high-q min scenario, the high b P cases have shorter slowing-down time and lower rb fast , and this reduces the drive for Alfv enic modes, yielding nearly classical fast-ion transport, high values of normalized confinement, b N , and noninductive current fraction. These results suggest DIII-D might obtain better performance in lower-q 95 , high-q min plasmas using broader neutral beam heating profiles and increased direct electron heating power to lower the drive for Alfv en eigenmodes. V C 2015 AIP Publishing LLC. [http://dx.

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Low-n ideal MHD stability of tokamaks: Current and beta limits

Cited by 20 publications

References 19 publications

Performance and stability of near-unity aspect ratio plasmas in the Pegasus Toroidal Experiment

Performance and stability of near-unity aspect ratio plasmas in the Pegasus Toroidal Experiment

Optimized profiles for improved confinement and stability in the DIII-D tokamak

Fast-ion transport in qmin>2, high-β steady-state scenarios on DIII-D

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Low-n ideal MHD stability of tokamaks: Current and beta limits

Cited by 20 publications

References 19 publications

Performance and stability of near-unity aspect ratio plasmas in the Pegasus Toroidal Experiment

Performance and stability of near-unity aspect ratio plasmas in the Pegasus Toroidal Experiment

Optimized profiles for improved confinement and stability in the DIII-D tokamak

Fast-ion transport in qmin&gt;2, high-β steady-state scenarios on DIII-D

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Product

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Fast-ion transport in qmin>2, high-β steady-state scenarios on DIII-D