Confinement degradation and enhanced microturbulence as long-time precursors to high-density-limit tokamak disruptions

Brower, D. L.; Yu, Cheng‐Wei; Bravenec, R. V.; Lin, Hong; Luhmann, Neville C.; Peebles, W. A.; Ritz, Ch. P.; Smith, B.A.; Wootton, A. J.; Zhang, Z. M.; Zhao, Shourong

doi:10.1103/physrevlett.67.200

Cited by 28 publications

(16 citation statements)

References 15 publications

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“…A large increase in both D and V were found as the density limit was approached while the edge temperature and gradient dropped precipitously. Similar results were obtained on TEXT, which used the propagation of sawtooth perturbations to probe thermal and particle diffusivities as a function of plasma density [247]. Both e χ and D increased strongly as the density was raised, even well before a density limit disruption.…”

Section: Experimental Evidencesupporting

confidence: 73%

Density limits in toroidal plasmas

Greenwald

2002

Plasma Phys. Control. Fusion

467

477

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In addition to the operational limits imposed by MHD stability on plasma current and pressure, an independent limit on plasma density is observed in confined toroidal plasmas. This review attempts to summarize recent work on the phenomenology and physics of the density limit. Perhaps the most surprising result is that all of the toroidal confinement devices considered operate in similar ranges of (suitably normalized) densities. The empirical scalings derived independently for tokamaks and reversed field pinches (RFP) are essentially identical, while stellarators appear to operate at somewhat higher densities with a different scaling. Dedicated density limit experiments have not been carried out for spheromaks and field-reversed configurations (FRC), however "optimized" discharges in these devices are also well characterized by the same empirical law. In tokamaks, where the most extensive studies have been conducted, there is strong evidence linking the limit to physics near the plasma boundary thus it is possible to extend the operational range for line-averaged density by operating with peaked density profiles. Additional particles in the plasma core apparently have no effect on density limit physics. While there is no widely accepted, first principles model for the density limit, research in this area has focussed on mechanisms which lead to strong edge cooling. Theoretical work has concentrated on the consequences of increased impurity radiation which may dominate power balance at high densities and low temperatures. These theories are not entirely satisfactory as they require assumptions about edge transport and make predictions for power and impurity scaling that may not be consistent with experimental results. A separate thread of research looks for the cause in collisionality enhanced turbulent transport. While there is experimental and theoretical support for this approach, understanding of the underlying mechanisms is only at a rudimentary stage and no predictive capability is yet available..

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Section: Experimental Evidencesupporting

confidence: 73%

Density limits in toroidal plasmas

Greenwald

2002

Plasma Phys. Control. Fusion

467

477

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“…Using this apparatus, a variety of physical phenomena has been observed including a strong up-down asymmetry in fluctuation amplitude, 213 a quasicoherent mode~Dv0v Յ 0.2, Dk 4 0k 4 Ӎ 0.7! in the inner equator, 214,215 experimental evidence for ion pressure gradient driven turbulence or h i -modes, 216,217 experimental evidence for coupling of plasma particle and heat transport, 218 observation of confinement degradation and enhanced microturbulence as long-time precursors to high-density-limit disruptions, 219 and the effect of tearing modes on the spectrum and magnitude of microturbulence and experimental evidence for electron drift turbulence. 220,221 The multichannel FIR scattering system on the DIII-D tokamak 197,222 utilized a special radial geometry to overcome the problem of poor access to the plasma.…”

Section: Via2 Representative Collective Scattering Systemsmentioning

confidence: 99%

Chapter 3: Microwave Diagnostics

Luhmann

Bindslev

Park

et al. 2008

Fusion Science and Technology

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“…[1]). On the other hand, the density limit is preceded by a significant reduction of the particle confinement and an increase in the cross-field convective power losses at the plasma edge [1,3,4]. This evolution is accompanied by a principle change in the edge turbulence, and, in particular, the amplitude of lowfrequency perturbations attributed to drift resistive ballooning (DRB) instability [5] increases significantly [1].…”

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confidence: 99%

“…As an example, Fig. 1 shows T versus n core computed for the parameters of Texas Experimental Tokamak (TEXT) [4]: R 1 m, B T 2:8 T, q core 3:5 W=cm 2 , q 3. First, consider the curve 1 found without impurity radiation.…”

mentioning

confidence: 99%

Synergy of Anomalous Transport and Radiation in the Density Limit

Tokar

2003

Phys. Rev. Lett.

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The critical plasma density n(cr) above which the edge anomalous transport in tokamaks is dominated by drift resistive ballooning instability is found analytically. In this transport regime, the drastic increase of particle losses and drop of the edge temperature provoke a strong increase in impurity radiation, and thermal equilibrium does not exist if the density is ramped up above the ultimate limit n(max). Because of the nonlinear character of impurity radiation, this density limit n(max) is very close to n(cr) and practically does not change with the ion effective charge. The importance of the synergy between the anomalous transport and impurity radiation for the density limit phenomenon is confirmed by the results of numerical simulations.

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Confinement degradation and enhanced microturbulence as long-time precursors to high-density-limit tokamak disruptions

Cited by 28 publications

References 15 publications

Density limits in toroidal plasmas

Density limits in toroidal plasmas

Chapter 3: Microwave Diagnostics

Synergy of Anomalous Transport and Radiation in the Density Limit

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