Analysis of pedestal plasma transport

Callen, J. D.; Groebner, R. J.; Osborne, T.H.; Canik, J.; Owen, L.W.; Pankin, A.Y.; Rafiq, T.; Rognlien, T. D.; Stacey, Weston M.

doi:10.1088/0029-5515/50/6/064004

Cited by 81 publications

(132 citation statements)

References 63 publications

Supporting

Mentioning

114

Contrasting

Unclassified

Order By: Relevance

“…Already important simulation and theoretical work are forthcoming, focused on including the non-local transport effects caused by large gradients in the density and temperature 17,23,[44][45][46] .…”

Section: Discussionmentioning

confidence: 99%

“…While the radial transport timescale we've shown in Figure 17 is based on a flux-surface averaged diffusion and convection coefficient, in reality the radial transport flux varies strongly with poloidal position 17,29,39 . In regions of low radial transport compared to parallel or poloidal transport, such as the core, this poloidal variation of radial transport doesn't matter, since the parallel transport is so fast that impurities will sample all of the flux surface, averaging out the poloidal variation in radial transport.…”

Section: Timescale Examplementioning

confidence: 92%

“…The assumption that T e is constant on a flux surface 7,[17][18][19] is based on the fact that electron parallel heat conductivity is rapid: greater than ion parallel conductivity by a factor of approximately m D /m e ∼ 60. The electron profiles are aligned such that the electron temperature at the separatrix satisfies constraints set by parallel heat conduction to the divertor 13,14 .…”

Section: Electron Profile Alignmentmentioning

confidence: 99%

See 2 more Smart Citations

Poloidal asymmetries in edge transport barriers

et al. 2015

View full text Add to dashboard Cite

Measurements of impurities in Alcator C-Mod indicate that in the pedestal region, significant poloidal asymmetries can exist in the impurity density, ion temperature, and main ion density. In light of the observation that ion temperature and electrostatic potential are not constant on a flux surface [Theiler et al., Nucl. Fusion 54, 083017 (2014)], a technique based on total pressure conservation to align profiles measured at separate poloidal locations is presented and applied. Gyrokinetic neoclassical simulations with XGCa support the observed large poloidal variations in ion temperature and density, and that the total pressure is approximately constant on a flux surface. With the updated alignment technique, the observed in-out asymmetry in impurity density is reduced from previous publishing [Churchill et al., Nucl. Fusion 53, 122002 (2013)], but remains substantial (nz,H/nz,L∼6). Candidate asymmetry drivers are explored, showing that neither non-uniform impurity sources nor localized fluctuation-driven transport are able to explain satisfactorily the impurity density asymmetry. Since impurity density asymmetries are only present in plasmas with strong electron density gradients, and radial transport timescales become comparable to parallel transport timescales in the pedestal region, it is suggested that global transport effects relating to the strong electron density gradients in the pedestal are the main driver for the pedestal in-out impurity density asymmetry.

show abstract

Section: Discussionmentioning

confidence: 99%

Section: Timescale Examplementioning

confidence: 92%

Section: Electron Profile Alignmentmentioning

confidence: 99%

See 1 more Smart Citation

Poloidal asymmetries in edge transport barriers

et al. 2015

View full text Add to dashboard Cite

show abstract

“…The neutrals may be localized at a particular poloidal position, representing the location of a gas puff. They may also be concentrated near the X-point when the recycling from the targets is strong [14,24], or if the plasma is gas fueled from the private flux region. Therefore we consider two scenarios.…”

mentioning

confidence: 99%

Plasma rotation from momentum transport by neutrals in tokamaks

et al. 2016

View full text Add to dashboard Cite

Neutral atoms can strongly influence the intrinsic rotation and radial electric field at the tokamak edge. Here, we present a framework to investigate these effects when the neutrals dominate the momentum transport. We explore the parameter space numerically, using highly flexible model geometries and a state of the art kinetic solver. We find that the most important parameters controlling the toroidal rotation and electric field are the major radius where the neutrals are localized and the plasma collisionality. This offers a means to influence the rotation and electric field by, for example, varying the radial position of the X-point to change the major radius of the neutral peak.The level of plasma rotation has a fundamental impact on the performance of magnetically confined plasmas. Establishing what determines plasma rotation is important both from a theoretical and practical point of view, since rotation has a strong effect on the confinement and stabilizes magnetohydrodynamic instabilities, such as resistive wall modes.In future magnetic fusion devices, where the effect of alpha-particle heating will be dominant, the external torque from auxiliary heating will be considerably lower than in current devices and the moment of inertia will be higher. It is therefore important to understand the intrinsic toroidal rotation that arises independently of externally applied momentum sources; momentum transport by neutral atoms is a mechanism that generates intrinsic rotation.There is a wealth of experimental evidence that shows that neutrals have a substantial influence on tokamak edge processes. They are observed to influence global confinement [1,2] and the transition from low (L) to high (H) confinement mode [3][4][5][6][7][8][9][10][11], which are critical to the performance of tokamak fusion reactors. While the physics of the transition to H-mode is far from fully understood, it is clear that rotational flow shear plays an important role [12]. It is therefore important to be able to model the effect of neutral viscosity on the flow shear in the edge plasma.Neutrals influence the ion dynamics in plasmas through atomic processes, mainly through chargeexchange (CX), ionization, and recombination. Due to their high cross-field mobility they can be the most significant momentum transport channel even at low relative densities. The effect of the neutrals is typically significant if the neutral fraction in the plasma exceeds about 10 −4 [13], which is usually the case in the tokamak edge region not too far inside the separatrix; the neutrals can penetrate even to the pedestal top in the JET tokamak [14] and may be expected to penetrate further in an Lmode plasma due to the lower edge density.Recent experimental results at JET have demonstrated that changes in divertor strike point positions are correlated with strong modification of the global energy confinement [1, 2]. It was speculated that the reason for this may be that neutrals are directly affecting the edge ion flow and electric potential [1]. Other experiment...

show abstract

“…An extensive study of plasma transport in a DIII-D H-mode pedestal [3] indicated paleoclassical plasma transport [4,5] may play a significant role there. This paper develops and tests the fundamental predictions of a paleoclassical-based model for the "pedestal structure" in the edge of H-mode plasmas.…”

mentioning

confidence: 99%