Effect of the relative shift between the electron density and temperature pedestal position on the pedestal stability in JET-ILW and comparison with JET-C

Štefániková, E.; Frassinetti, L.; Saarelma, S.; Loarte, A.; Nunes, I.; Garzotti, L.; Lomas, P. J.; Rimini, F.; Drewelow, P.; Kruezi, U.; Lomanowski, B.; Luna, E. de la; Meneses, L.; Peterka, M.; Viola, B.; Giroud, C.; Maggi, C. F.; Contributors, Jet

doi:10.1088/1741-4326/aab216

Cited by 43 publications

(94 citation statements)

References 41 publications

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“…Given the role of the pedestal position in the pedestal performance, it is important to identify the parameters that lead to the variation of ne pos and Te pos . So far, the main message presented in the literature is that the key parameter is the gas fuelling rate [28,39,72]. This section shows that also the power has a major impact on the pedestal position.…”

Section: Pedestal Widthmentioning

confidence: 90%

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Pedestal structure, stability and scalings in JET-ILW: the EUROfusion JET-ILW pedestal database

et al. 2020

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The EUROfusion JET-ILW pedestal database is described, with emphasis on three main issues. First, the technical aspects are introduced, including a description of the data selection, the datasets, the diagnostics used, the experimental and theoretical methods implemented and the main definitions. Second, the JET-ILW pedestal structure and stability are described. In particular, the work describes the links between the engineering parameters (power, gas and divertor configuration) and the disagreement with the peeling-ballooning (PB) model implemented with ideal MHD equations. Specifically, the work clarifies why the JET-ILW pedestal tends to be far from the PB boundary at high gas and high power, showing that a universal threshold in power and gas cannot be found but that the relative shift (the distance between the position of the pedestal density and of the pedestal temperature) plays a key role. These links are then used to achieve an empirical explanation of the behavior of the JET-ILW pedestal pressure with gas, power and divertor configuration. Third, the pedestal database is used to revise the scaling law of the pedestal stored energy. The work shows a reasonable agreement with the earlier Cordey scaling in terms of plasma current and triangularity dependence, but highlights some differences in terms of power and isotope mass dependence.

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Section: Pedestal Widthmentioning

confidence: 90%

“…(ii) However, a non-negligible region of the JET-ILW pedestal density is often located outside the separatrix. See for example figure 1(b) or references [39,72] or, later, figure 16(b).…”

Section: Definitions Using the Mtanh Functionmentioning

confidence: 99%

Pedestal structure, stability and scalings in JET-ILW: the EUROfusion JET-ILW pedestal database

et al. 2020

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“…No shift in density pedestal position without a significant change in other pedestal parameters was observed. So a direct study of the impact of the pedestal position on pedestal height, as in AUG [7] and JET [9], was not possible. The reduction in pedestal height seen with an outward shift is qualitatively consistent with measurements in AUG and JET.…”

Section: Effects Of Fuelling and Seeding On The Pedestalmentioning

confidence: 99%

“…AUG explains this through a change in the high field side high density (HFSHD) [5,6] region, leading to an outward shift of the pedestal position and a consequent degradation in pedestal stability [7]. There is no definitive explanation for pedestal degradation on JET but a clear correlation was observed for a relative shift between the temperature and density pedestal positions [8,9]. Impurity seeding has led to increases in pedestal performance in metal walled machines.…”

Section: Introductionmentioning

confidence: 99%

Pedestal structure and energy confinement studies on TCV

Sheikh

Dunne

Frassinetti

et al. 2018

Plasma Phys. Control. Fusion

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High external gas injection rates are foreseen for future devices to reduce divertor heat loads and this can influence pedestal stability. Fusion yield has been estimated to vary as strongly as T 2 e,ped so an understanding of the underlying pedestal physics in the presence of additional fuelling and seeding is required. To address this, a database scanning plasma triangularity, fuelling and nitrogen seeding rates in neutral beam (NBH) heated ELM-y H-mode plasmas was constructed on TCV. Low nitrogen seeding was observed to increase pedestal top pressure but all other gas injection rates led to a decrease. Lower triangularity discharges were found to be less sensitive to variations in gas injection rates. No clear trend was measured between plasma top Pe and stored energy which is attributed to the non-stiffness of core plasma pressure profiles. Peeling ballooning stability analysis put these discharges close to the ideal MHD stability boundary. A constant for D in the relation pedestal width w = D β P ed θ , was not found. Experimentally inferred values of D were used in EPED1 simulations and gave good agreement for pedestal width. Pedestal height agreed well for high triangularity but was overestimated for low triangularity. IPED simulations showed that relative shifts in pedestal position were contributing significantly to the pedestal height and were able to reproduce the measured profiles more accurately.

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“…A major cause of these changes is likely the effects of a metal wall on the density profile (both direct and indirect via the need for gas puffing) [22]. Often this change in the density profile is manifest as an outward shift [23], which effectively reduces the density gradient in the upper pedestal. Higher η translates into more robust ETG and ion temperature gradient (ITG) instabilities; in combination, demand additional heating power and limit accessible pedestal top temperatures.…”

Section: Introductionmentioning

confidence: 99%

Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls

Hatch

Kotschenreuther²,

Mahajan³

et al. 2019

Nucl. Fusion

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104

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This paper compares the gyrokinetic instabilities and transport in two representative JET pedestals, one (pulse 78697) from the JET configuration with a carbon wall (C) and another (pulse 92432) from after the installation of JET's ITERlike Wall (ILW). The discharges were selected for a comparison of JET-ILW and JET-C discharges with good confinement at high current (3 MA, corresponding also to low ρ * ) and retain the distinguishing features of JET-C and JET-ILW, notably, decreased pedestal top temperature for JET-ILW. A comparison of the profiles and heating power reveals a stark qualitative difference between the discharges: the JET-ILW pulse (92432) requires twice the heating power, at a gas rate of 1.9×10 22 e/s, to sustain roughly half the temperature gradient of the JET-C pulse (78697), operated at zero gas rate. This points to heat transport as a central component of the dynamics limiting the JET-ILW pedestal and reinforces the following emerging JET-ILW pedestal transport paradigm, which is proposed for further examination by both theory and experiment. ILW conditions modify the density pedestal in ways that decrease the normalized pedestal density gradient a/L n , often via an outward shift of the density pedestal. This is attributable to some combination of direct metal wall effects and the need for increased fueling to mitigate tungsten contamination. The modification to the density profile increases η = L n /L T , thereby producing more robust ion temperature gradient (ITG) and electron temperature gradient driven instability. The decreased pedestal gradients for JET-ILW (92432) also result in a strongly reduced E × B shear rate, further enhancing the ion scale turbulence. Collectively, these effects limit the pedestal temperature and demand more heating power to achieve good pedestal performance. Our simulations, consistent with basic theoretical arguments, find higher ITG turbulence, stronger stiffness, and higher pedestal transport in the ILW plasma at lower ρ * .

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Effect of the relative shift between the electron density and temperature pedestal position on the pedestal stability in JET-ILW and comparison with JET-C

Cited by 43 publications

References 41 publications

Pedestal structure, stability and scalings in JET-ILW: the EUROfusion JET-ILW pedestal database

Pedestal structure, stability and scalings in JET-ILW: the EUROfusion JET-ILW pedestal database

Pedestal structure and energy confinement studies on TCV

Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls

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