Bounds on edge shear layer persistence while approaching the density limit

Singh, Rameswar; Diamond, P. H.

doi:10.1088/1741-4326/abfadb

Cited by 15 publications

(30 citation statements)

References 68 publications

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“…The key point is that higher current strengthens zonal flow shear, for fixed drive. Since the edge is of primary interest in the context of DLs, we comment here that Singh & Diamond [33] also have shown that favourable Ip scaling of the asymptotic temporal response of the zonal potential persists in the plateau regime. In particular, the current scaling is the same, though the response is numerically smaller.…”

Section: Density Limit Scaling Theorymentioning

confidence: 87%

“…These lead one to a new predator-prey model for turbulence-zonal flow evolution. Such a model was developed by Singh and Diamond in 2021 (hereafter SD1) by extending previous versions of the predator-prey model [33]. This model has features summarized in figure 14.…”

Section: Density Limit Scaling Theorymentioning

confidence: 99%

“…To extract a quantitative DL scaling, one must combine: fluctuation and zonal flow evolution,neoclassical zonal flow screening,zonal flow damping (collisional),incoherent mode coupling and modulational instability. These lead one to a new predator–prey model for turbulence-zonal flow evolution. Such a model was developed by Singh and Diamond in 2021 (hereafter SD1) by extending previous versions of the predator–prey model [33]. This model has features summarized in figure 14.…”

Section: Density Limit Scaling Theorymentioning

confidence: 99%

“…Inclusion of neoclassical zonal flow screening in the vein of the SD1 analysis recovers Greenwald scaling within the shear layer collapse paradigm.

Figure 14Extended predator–prey model, yielding current scaling of the limiting density [33]. (Online version in colour.…”

Section: Density Limit Scaling Theorymentioning

confidence: 99%

“…Theoretical work focuses on linking microphysics to macroscopic scalings. Greenwald scaling is shown to follow from neoclassical dielectric screening effects on the zonal flow response [33]. A predator–prey model is developed to predict zonal flow collapse.…”

Section: Introductionmentioning

confidence: 99%

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How the birth and death of shear layers determine confinement evolution: from the L → H transition to the density limit

Diamond

Singh

Long

et al. 2023

Phil. Trans. R. Soc. A.

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Electric field profile structure—especially its shear—is a natural order parameter for the edge plasma, and characterizes confinement regimes ranging from the H-mode (Wagner et al. 1982 Phys. Rev. Lett. 49 , 1408–1412 ( doi:10.1103/PhysRevLett.49.1408 )) to the density limit (DL) (Greenwald et al. 1988 Nucl. Fusion 28 , 2199–2207 ( doi:10.1088/0029-5515/28/12/009 )). The theoretical developments and lessons learned during 40 years of H-mode studies (Connor & Wilson 1999 Plasma Phys. Control. Fusion 42 , R1–R74 ( doi:10.1088/0741-3335/42/1/201 ); Wagner 2007 Plasma Phys. Control. Fusion 49 , B1–B33 ( doi:10.1088/0741-3335/49/12b/s01 )) are applied to the shear layer collapse paradigm (Hong et al. 2017 Nucl. Fusion 58 , 016041 ( doi:10.1088/1741-4326/aa9626 )) for the onset of DL phenomena. Results from recent experiments on edge shear layers and DL phenomenology are summarized and discussed in the light of L → H transition physics. The theory of shear layer collapse is then developed. We demonstrate that shear layer physics captures both the well known current (Greenwald) scaling of the DL (Greenwald 2002 Plasma Phys. Control. Fusion 44 , R27–R53 ( doi:10.1088/0741-3335/44/8/201 ); Greenwald et al. 2014 Phys. Plasmas 21 , 110501 ( doi:10.1063/1.4901920 )), as well as the emerging power scaling (Zanca, Sattin, JET Contributors 2019 Nucl. Fusion 59 , 126011 ( doi:10.1088/1741-4326/ab3b31 )). The derivation of the power scaling theory exploits an existing model, originally developed for the L → H transition (Diamond, Liang, Carreras, Terry 1994 Phys. Rev. Lett. 72 , 2565–2568 ( doi:10.1103/PhysRevLett.72.2565 ); Kim & Diamond 2003 Phys. Rev. Lett. 90 , 185006 ( doi:10.1103/PhysRevLett.90.185006 )). We describe the enhanced particle transport events that occur following shear layer collapse. Open problems and future directions are discussed. This article is part of a discussion meeting issue ‘H-mode transition and pedestal studies in fusion plasmas’.

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Section: Density Limit Scaling Theorymentioning

confidence: 87%

Section: Density Limit Scaling Theorymentioning

confidence: 99%

Section: Density Limit Scaling Theorymentioning

confidence: 99%

“…Inclusion of neoclassical zonal flow screening in the vein of the SD1 analysis recovers Greenwald scaling within the shear layer collapse paradigm.

Figure 14Extended predator–prey model, yielding current scaling of the limiting density [33]. (Online version in colour.…”

Section: Density Limit Scaling Theorymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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How the birth and death of shear layers determine confinement evolution: from the L → H transition to the density limit

Diamond

Singh

Long

et al. 2023

Phil. Trans. R. Soc. A.

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Theory of self-generated vortex flows in a tokamak magnetic island

Choi

2024

Rev. Mod. Plasma Phys.

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We present a gyrokinetic theory of an E$$\times$$ × B vortex flow in a magnetic island self-generated from turbulence in a collisionless tokamak plasma. We have found that after fast collisionless damping, the self-generated vortex flow arrives at a residual level higher than the Rosenbluth-Hinton level predicted in the tokamak geometry. In the longer term, the residual vortex flow undergoes further damping with a deformation toward a zonal-vortex mixture, making open streamlines near the X-points that reduce the isolated region of the island. It is due to the helical symmetry breaking by the toroidal precession, expected to be stronger in higher-temperature plasmas and in lower aspect ratio tokamaks. A strong enough background vortex flow can significantly suppress the toroidicity-induced damping and deformation of the self-generated vortex flow, proposing that the island boundary is a key location for the bifurcation of the confinement state.

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Development of compact tokamak fusion reactor use cases to inform future transport studies

Holland,

Bass,

Orlov

et al. 2023

J. Plasma Phys.

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The OMFIT STEP (Meneghini et al., Nucl. Fusion, vol. 10, 2020, p. 1088) workflow has been used to develop inductive and steady-state H-mode core plasma scenario use cases for a $B_0 = 8 \, {\rm T}$ , $R_0 = 4 \, {\rm m}$ machine to help guide and inform future higher-fidelity studies of core transport and confinement in compact tokamak reactors. Both use cases are designed to produce 200 MW or more of net electric power in an up-down symmetric plasma with minor radius $a = 1.4 \, {\rm m}$ , elongation $\kappa = 2.0$ , triangularity $\delta = 0.5$ and effective charge $Z_{{\rm eff}} \simeq 2$ . Additional considerations based on the need for compatibility of the core with reactor-relevant power exhaust solutions and external actuators were used to guide and constrain the use case development. An extensive characterization of core transport in both scenarios is presented, the most important feature of which is the extreme sensitivity of the results to the quantitative stiffness level of the transport model used as well as the predicted critical gradients. This sensitivity is shown to arise from different levels of transport stiffness exhibited by the models, combined with the gyroBohm-normalized fluxes of the predictions being an order of magnitude larger than other H-mode plasmas. Additionally, it is shown that although heating in both plasmas is predominantly to the electrons and collisionality is low, the plasmas remain sufficiently well coupled for the ions to carry a significant fraction of the thermal transport. As neoclassical transport is negligible in these conditions, this situation inherently requires long-wavelength ion gyroradius-scale turbulence to be the dominant transport mechanism in both plasmas. These results are combined with other basic considerations to propose a simple heuristic model of transport in reactor-relevant plasmas, along with simple metrics to quantify coupling and core transport properties across burning and non-burning plasmas.

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Bounds on edge shear layer persistence while approaching the density limit

Cited by 15 publications

References 68 publications

How the birth and death of shear layers determine confinement evolution: from the L → H transition to the density limit

How the birth and death of shear layers determine confinement evolution: from the L → H transition to the density limit

Theory of self-generated vortex flows in a tokamak magnetic island

Development of compact tokamak fusion reactor use cases to inform future transport studies

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