Pedestal transport in H-mode plasmas for fusion gain

Kotschenreuther, M.; Hatch, D. R.; Mahajan, S. M.; Valanju, P.; Zheng, Lin; Liu, X.

doi:10.1088/1741-4326/aa6416

Cited by 48 publications

(122 citation statements)

References 49 publications

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“…Our gyrokinetic analysis of the JET-C (78697) pedestal suggests that electron temperature gradient (ETG) driven modes and microtearing modes (MTM) are the major heat transport mechanisms. This is consistent with mounting evidence, spanning multiple machines [15,18], that the observed fluctuations display the expected characteristics of these modes. This combination of transport mechanisms has also been proposed and analyzed in the context of inter-ELM transport and transport at the pedestal top [19,20,21].…”

Section: Introductionsupporting

confidence: 90%

“…The contributors to pedestal transport must be consistent with the general picture outlined above. For example, each prospective transport mechanism has a distinctive transport fingerprint [18] defined by its relative impact on various transport channels. A minimum subset of pedestal transport mechanisms must include KBM (or similar MHD-like modes), ITG (or similar ion scale electrostatic modes), ETG, MTM, and neoclassical transport.…”

Section: Pedestal Transport Paradigmmentioning

confidence: 99%

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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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Section: Introductionsupporting

confidence: 90%

Section: Pedestal Transport Paradigmmentioning

confidence: 99%

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

Hatch

Kotschenreuther²,

Mahajan³

et al. 2019

Nucl. Fusion

Self Cite

104

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“…Reducing ρ * through this transition point would also lead to a degraded pedestal and reduced overall confinement [41]. It is clearly important in extrapolating to ITER that we identify which is the dominant effect and, if the effect of the oscillation on ELMs is key, we need to identify its origin and seek ways to eliminate it, or influence its ability to trigger ELMs and consequent pedestal collapse.…”

Section: Discussionmentioning

confidence: 99%

Pedestal evolution physics in low triangularity JET tokamak discharges with ITER-like wall

et al. 2017

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The pressure gradient of the high confinement pedestal region at the edge of tokamak plasmas rapidly collapses during plasma eruptions called edge localised modes (ELMs), and then re-builds over a longer time scale before the next ELM. The physics that controls the evolution of the JET pedestal between ELMs is analysed for 1.4 MA, 1.7 T, low triangularity, δ = 0.2, discharges with the ITER-like wall, finding that the pressure gradient typically tracks the ideal magneto-hydrodynamic ballooning limit, consistent with a role for the kinetic ballooning mode. Furthermore, the pedestal width is often influenced by the region of plasma that has second stability access to the ballooning mode, which can explain its sometimes complex evolution between ELMs. A local gyrokinetic analysis of a second stable flux surface reveals stability to kinetic ballooning modes; global effects are expected to provide a destabilising mechanism and need to be retained in such second stable situations. As well as an electronscale electron temperature gradient mode, ion scale instabilities associated with this flux surface include an electro-magnetic trapped electron branch and two electrostatic branches propagating in the ion direction, one with high radial wavenumber. In these second stability situations, the ELM is triggered by a peeling-ballooning mode; otherwise the pedestal is somewhat below the peeling-ballooning mode marginal stability boundary at ELM onset. In this latter situation, there is evidence that higher frequency ELMs are paced by an oscillation in the plasma, causing a crash in the pedestal before the peeling-ballooning boundary is reached. A model is proposed in which the oscillation is associated with hot plasma filaments that are pushed out towards the plasma edge by a ballooning mode, draining their free energy into the cooler plasma there, and then relaxing back to repeat the process. The results suggest

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“…Another class of ion scale modes that play a fundamental role in determining the core transport is the Ion temperature gradient instability (ITG). For pedestal parameters, however, the ITG modes, driven by ηi = d log Ti /d log n, are more slab like [19,20,63]. Instabilities usually require ηi ~ 1 or more.…”

Section: Itg/temmentioning

confidence: 99%

“…Using ω* ~ k θ ρi vi/L, k θ ∼ qn/a, cs k||/ω* ~ (L/R) (a/ρi) (1/nq 2 ), (a/ρi) ~ 300, 1/q 2 ~ 1/10, so this is ~ (L/R) (30/n). This is small, especially for n significantly above 1, such as is usually observed or simulated (n~ [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].…”

Section: Appendix: Details Of the Diffusivity Calculation From Fluid Mhdmentioning

confidence: 99%

Gyrokinetic analysis and simulation of pedestals to identify the culprits for energy losses using ‘fingerprints’

Kotschenreuther

Liu²,

Hatch³

et al. 2019

Nucl. Fusion

Self Cite

105

158

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Fusion performance in tokamaks hinges critically on the efficacy of the Edge Transport Barrier (ETB) at suppressing energy losses. The new concept of "fingerprints" is introduced to identify the instabilities that cause the transport losses in the ETB of many of today's experiments, from widely posited candidates. Analysis of the Gyrokinetic-Maxwell equations, and gyrokinetic simulations of experiments, find that each mode type produces characteristic ratios of transport in the various channels: density, heat and impurities. This, together with experimental observations of transport in some channel, or, of the relative size of the driving sources of channels, can identify or determine the dominant modes causing energy transport. In multiple ELMy H-mode cases that are examined, these fingerprints indicate that MHD-like modes are apparently not the dominant agent of energy transport; rather, this role is played by Micro-Tearing Modes (MTM) and Electron Temperature Gradient (ETG) modes, and in addition, possibly Ion Temperature Gradient (ITG)/Trapped Electron Modes (ITG/TEM) on JET. MHD-like modes may dominate the electron particle losses. Fluctuation frequency can also be an important means of identification, and is often closely related to the transport fingerprint. The analytical arguments unify and explain previously disparate experimental observations on multiple devices, including DIII-D, JET and ASDEX-U, and detailed simulations of two DIII-D ETBs also demonstrate and corroborate this.

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Pedestal transport in H-mode plasmas for fusion gain

Cited by 48 publications

References 49 publications

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

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

Pedestal evolution physics in low triangularity JET tokamak discharges with ITER-like wall

Gyrokinetic analysis and simulation of pedestals to identify the culprits for energy losses using ‘fingerprints’

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