High density operation close to Greenwald limit and H mode limit in ASDEX upgrade

Mertens, V.; Kaufmann, Michael; Neuhäuser, J.; Schweinzer, J.; Stöber, J.; Büchl, K.; Gruber, O.; Haas, G.; Herrmann, A.; Kallenbach, A.; Weinlich, M.

doi:10.1088/0029-5515/37/11/i10

Cited by 66 publications

(80 citation statements)

References 20 publications

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“…ASDEX has found much lower radiated power fractions at the density limit, in the range of 30-40% from the main chamber [8,9] and not exceeding 60-70% when radiation from the divertor is included [7]. ASDEX Upgrade reports density limits with the P RAD /P IN between 60% and 80% [60,41]. These differences are perhaps due to different interpretations of the transient behavior which occurs as a discharge proceeds to the limit.…”

Section: General Observationsmentioning

confidence: 99%

“…The relation between the H/L and disruptive limits can be seen clearly when the threshold power is plotted against the normalized density as in figure 4 [68,41,92,93]. The required power increases dramatically as the density limit is approached, deviating from the linear density scaling usually seen for the power threshold [94,95].…”

Section: Effects On H-mode Confinement and Charactermentioning

confidence: 99%

“…The power dependence was stronger at q = 2, though these discharges are near the current limit as well and could not attain densities as high as plasmas with larger q. ASDEX-Upgrade investigated the , was seen on the L-mode disruptive limit resulting in a greater separation between the two limits at high power [68,41]. Overall, it must be concluded that compared to the expectations from arguments based on power balance, the density limit is not found to increase strongly with input power.…”

Section: Power Dependencementioning

confidence: 99%

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Density limits in toroidal plasmas

Greenwald

2002

Plasma Phys. Control. Fusion

454

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: General Observationsmentioning

confidence: 99%

Section: Effects On H-mode Confinement and Charactermentioning

confidence: 99%

Section: Power Dependencementioning

confidence: 99%

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Density limits in toroidal plasmas

Greenwald

2002

Plasma Phys. Control. Fusion

454

477

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“…The HDL has to be discriminated against an H-L transition, which is caused by the reduction of the heating power below the threshold, since it is reported to be independent of the heating power 15,25 . Studies at ASDEX Upgrade (AUG) and JET, both with a fully carbon covered wall, attribute the HDL to the full detachment of the divertor 5,7 .…”

Section: Introductionmentioning

confidence: 99%

The H-mode density limit in the full tungsten ASDEX Upgrade tokamak

Bernert

Eich

Kallenbach

et al. 2014

Plasma Phys. Control. Fusion

Self Cite

120

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The high confinement mode (H-mode) is the operational scenario foreseen for ITER, DEMO and future fusion power plants. At high densities, which are favorable in order to maximize the fusion power, a back transition from the H-mode to the low confinement mode (L-mode) is observed. In present tokamaks, this H-mode density limit (HDL) occurs at densities on the order of, but below, the Greenwald density.In gas ramp discharges at the fully tungsten covered ASDEX Upgrade tokamak (AUG), four distinct operational phases are identified in the approach towards the HDL. These phases are a stable H-mode, a degrading H-mode, the breakdown of the H-mode and an L-mode. They are reproducible, quasi-stable plasma regimes and provide a framework in which the HDL can be further analyzed. During the evolution, energy losses are increased and a fueling limit is encountered. The latter is correlated to a plateau of electron density in the scrape-off layer (SOL). The well-known extension of the good confinement at high density with high triangularity is reflected in this scheme by extending the first phase to higher densities.In this work, two mechanisms are proposed, which can explain the experimental observations. The fueling limit is most likely correlated to an outward shift of the ionization profile. The additional energy loss channel is presumably linked to a regime of increased radial filament transport in the SOL. The SOL and divertor plasmas play a key role for both mechanisms, in line with the previous hypothesis that the HDL is edgedetermined.The four phases are also observed in carbon covered AUG, although the HDL density exhibits a different dependency on the heating power and plasma current. This can be attributed to a changed energy loss channel in the presented scheme.

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“…Database values for P LH have been used to extrapolate to ITER [6,7]. Some experiments indicate that there is hysteresis in the transition, with P HL being considerably lower than P LH [8,9].…”

Section: Plasmas Studiedmentioning

confidence: 99%

Comparisons of Predicted Plasma Performance in ITER H-mode Plasmas with Various Mixes of External He

Budny¹

2009

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Performance in H-mode DT plasmas in ITER with various choices of heating systems are predicted and compared. Combinations of external heating by Negative Ion Neutral Beam Injection (NNBI), Ion Cyclotron Range of Frequencies (ICRF), and Electron Cyclotron Heating (ECRH) are assumed. Scans with a range of physics assumptions about boundary temperatures in the edge pedestal, alpha ash transport, and toroidal momentum transport are used to indicate effects of uncertainties. Time-dependent integrated modeling with the PTRANSP code is used to predict profiles of heating, beam torque, and plasma profiles. The GLF23 model is used to predict temperature profiles. Either GLF23 or the assumption of a constant ratio for χ φ /χi is used to predict toroidal rotation profiles driven by the beam torques. Large differences for the core temperatures are predicted with different mixes of the external heating during the density and current ramp-up phase, but the profiles are similar during the flattop phase. With χ φ /χi = 0.5, the predicted toroidal rotation is relatively slow and the flow shear implied by the pressure, toroidal rotation, and neoclassical poloidal rotation are not sufficient to cause significant changes in the energy transport or steady state temperature profiles. The GLF23-predicted toroidal rotation is faster by a factor of six, and significant flow shear effects are predicted. Heating mixes with more NNBI power are predicted to have up to 20% higher fusion power during steady state phases. This advantage is decisive in some cases where the physics assumptions are close to marginal or critical values. L-mode plasmas are predicted having QDT ≃ 2-4.

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High density operation close to Greenwald limit and H mode limit in ASDEX upgrade

Cited by 66 publications

References 20 publications

Density limits in toroidal plasmas

Density limits in toroidal plasmas

The H-mode density limit in the full tungsten ASDEX Upgrade tokamak

Comparisons of Predicted Plasma Performance in ITER H-mode Plasmas with Various Mixes of External He

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