MHD and gyro-kinetic stability of JET pedestals

Saarelma, S.; Beurskens, M.; Dickinson, David; Frassinetti, L.; Leyland, Matthew; Roach, C. M.; Contributors, Efda‐Jet

doi:10.1088/0029-5515/53/12/123012

Cited by 59 publications

(85 citation statements)

References 39 publications

(84 reference statements)

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“…Consequently it may not be appropriate to perform an EPED comparison for all pulses within the new fuelling database. We note that if resistive effects or an effect associated with Type II ELMs were able to hold the pressure gradient slightly below the KB critical value, this would result in a prediction of a somewhat wider and higher pedestal [52,53], which would be qualitatively consistent with observations. …”

Section: Eped Comparisonsupporting

confidence: 74%

“…On average there is good agreement between the model results and the experimental pre-ELM measurements where the average ratio of predicted pedestal height to observed pedestal height is 1.04±0.22 and the average ratio of predicted pedestal width to observed pedestal width is 1.01±0.23. This spread is within the range of EPED predictive accuracy as observed in a wider multi machine comparison [20,52,53,60]. However the JET results do show a number of differences in comparison to studies on other machines.…”

Section: Discussionsupporting

confidence: 56%

“…This behaviour is still consistent with the EPED premise as long as KB modes do eventually limit the pressure gradient towards the end of the ELM cycle. This is a topic of further research as discussed in [53] which presents a detailed gyrokinetic analysis of the pure Type I ELM and mixed Type I/II ELM pulses presented in this paper. Nevertheless EPED predicts the JET pedestal height within a 20% error.…”

Section: Discussionmentioning

confidence: 93%

“…The difference in the dynamics between MAST and JET is further discussed in [53]. Figure 15 shows the result of the PB stability analysis performed by MISHKA-1; an ideal MHD eigenvalue solver [30].…”

Section: Identical To Figure 10 There Are Three Isobars Indicated Bymentioning

confidence: 99%

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Pedestal study across a deuterium fuelling scan for highδELMy H-mode plasmas on JET with the carbon wall

Leyland¹,

Beurskens²,

Frassinetti³

et al. 2013

Nucl. Fusion

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We present the results from a new fuelling scan database consisting of 14 high triangularity ( ~ 0.41), Type I ELMy H-mode JET plasmas. As the fuelling level is increased from low, ( D ~ 0.2x10 22 el/s, n e,ped /n gw =0.7), to high dosing ( D ~ 2.6x10 22 el/s, n e,ped /n gw =1.0) the variation in ELM behaviour is consistent with a transition from 'pure Type I' to 'mixed Type I/II' ELMs [1]. However, the pulses in Scattering (HRTS) system. We continue by presenting, for the first time, the role of pedestal structure, as quantified by a least squares mtanh fit to the HRTS profiles, on the performance across the fuelling scan. A key result is that the pedestal width narrows and peak pressure gradient increases during the ELM cycle for low fuelling plasmas, whereas at high fuelling the pedestal width and peak pressure gradient saturates towards the latter half of the ELM cycle. An ideal MHD stability analysis shows that both low and high fuelling plasmas move from stable to unstable approaching the ideal ballooning limit of the finite peeling ballooning stability boundary. Comparison to EPED predictions show on average good agreement with experimental measurements for both pedestal height and width however when presented as a function of pedestal density, experiment and model show opposing trends. The measured pre-ELM pressure pedestal height increases by ~ 20% whereas EPED predicts a decrease of 25% from low to high fuelling. Similarly the measured pressure pedestal width widens by ~ 55%, in poloidal flux space, whereas EPED predicts a decrease of 20% from low to high fuelling. We give two possible explanations for the disagreement. First, it may be that EPED under-predicts the critical density, which marks the transition from kink-peeling to ballooning limited plasmas. Second, the stronger broadening of the experimental pedestal width than predicted by EPED is an indication that other transport related processes contribute to defining the pedestal width such as enhanced inter-ELM transport as observed at high fuelling, for mixed Type I/II ELMy pulses.M. Leyland

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Section: Eped Comparisonsupporting

confidence: 74%

Section: Discussionsupporting

confidence: 56%

Section: Discussionmentioning

confidence: 93%

Section: Identical To Figure 10 There Are Three Isobars Indicated Bymentioning

confidence: 99%

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Pedestal study across a deuterium fuelling scan for highδELMy H-mode plasmas on JET with the carbon wall

Leyland¹,

Beurskens²,

Frassinetti³

et al. 2013

Nucl. Fusion

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“…In the local linear gyrokinetic analysis of experimental MAST and JET plasmas we have found that, like the n=∞ ideal MHD ballooning modes, the KBMs can access locally so called second stability if the magnetic shear becomes low enough [2,3]. However, in the pedestal region the local assumption that the equilibrium can be considered radially constant for the investigated modes is no longer justified.…”

Section: Introductionmentioning

confidence: 99%

Non-local effects on pedestal kinetic ballooning mode stability

Saarelma

Martin-Collar

Dickinson

et al. 2017

Plasma Phys. Control. Fusion

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The H-mode pedestal height plays an important role in determining the global confinement of the tokamak plasma. In type I ELMy H-mode the ultimate limit for the pedestal pressure at constant width is set by the ideal MHD peeling-ballooning modes that are thought to be the trigger for the ELMs. However, the peeling-ballooning mode criterion does not uniquely determine the pedestal. Increasing the width of the pedestal, the marginally peeling-ballooning stable pedestal height increases as well. The second criterion for the pedestal is set by the transport processes in the pedestal that limit the gradient between the ELMs.One candidate for driving this transport is the kinetic ballooning mode (KBM) that is driven by the pressure gradient Non-Local Effects on Pedestal Kinetic Ballooning Mode Stability 2 with the pressure gradient clamped near to the stability limit. In the local linear gyrokinetic analysis of experimental MAST and JET plasmas we have found that, like the n=∞ ideal MHD ballooning modes, the KBMs can access locally so called second stability if the magnetic shear becomes low enough [2,3]. However, in the pedestal region the local assumption that the equilibrium can be considered radially constant for the investigated modes is no longer justified. In this paper we revisit the KBM analysis using a global code ORB5 to investigate whether second stability access exists for KBMs.We find that counter to the local analysis, the global KBM stability is not sensitive to the magnetic shear in the pedestal region. At sufficiently high β (but still below the ideal peeling-ballooning limit) the pedestal region becomes KBM unstable regardless of the amount of bootstrap current assumed in the equilibrium reconstruction. However, just as in local analysis, the mode is stabilised by reducing the pressure gradient. This suggests that KBMs can regulate the pedestal pressure gradient during the ELM cycle even when local analysis finds them stable due to high bootstrap current.

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