Edge rotation from momentum transport by neutrals

Omotani, John; Newton, Sarah; Pusztai, István; Fülöp, Tünde

doi:10.1088/1742-6596/775/1/012011

Cited by 3 publications

(3 citation statements)

References 36 publications

(76 reference statements)

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“…There are other momentum damping and driving mechanisms which can influence the toroidal rotation profile that have not been included. Among other possible contributors are resonant and thermal NTV torques [12,41,46], changes in turbulence [46] and intrinsic rotation driven torques [61][62][63][64][65], friction with neutrals at the plasma edge [66,67] and ambipolarity restoring torques [67][68][69][70] due to edge stochastization [71,72] studied the resonant and thermal NTV torque in the low q 95 discharge, and showed that the thermal NTV torque is negligible for this case. Note that the study presented in [72] relied on beam torque calculations from TRANSP (in a 2D equilibrium, which does not include the impact of the perturbation on non-ambipolar fastion transport), and the resonant torques were adjusted manually to match the experimental rotation braking.…”

Section: Discussion On Other Damping and Driving Momentum Mechanismsmentioning

confidence: 97%

Fast-ion transport and toroidal rotation response to externally applied magnetic perturbations at the ASDEX Upgrade tokamak

Cano-Megias¹,

Viezzer²,

Galdón-Quiroga³

et al. 2022

Nucl. Fusion

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This paper studies the effect of 3D magnetic perturbations on fast-ion confinement, and its impact on the toroidal rotation velocity profile. Two low collisionality H-mode experiments carried out at the ASDEX Upgrade tokamak have been analyzed. The two discharges feature different magnetic field helicity (q95), and differences in the velocity-space and level of fast-ion losses are observed. A new analysis technique has been developed that sheds light on the dependencies between fast-ion losses and toroidal rotation, providing for the first time correlation patterns resolved in radius and velocity space of the lost fast-ions. The correlation intensifies towards the plasma edge and is strongly dependent on the orbit topology of the lost fast-ions. The ASCOT orbit following code has been used to characterize the fast-ion resonant transport and beam driven torques, using the vacuum approach and including plasma response. The change of the toroidal canonical momentum, which serves as figure of merit for resonant fast-ion transport, has been calculated with ASCOT. The beam geometry and q95 are found to have a strong impact on the fast-ion transport and losses. The fast-ion transport induced by the magnetic perturbations affects the beam driven torques. The effect of the changes of the jxB and collisional torques on plasma rotation is analyzed using the torques simulated by ASCOT and simple momentum balance calculations. For the low q95 = 3.8 discharge, which benefits from a resonant amplification, we find excellent agreement with the measured variation of the toroidal velocity. For the high q95 = 5.5 discharge, the inclusion of the plasma response improves the comparison with experimental data with respect to the vacuum estimation, but still some differences with experiments are observed. This suggests that other non-resonant effects could play a role for the determination of the toroidal rotation profile.

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Section: Discussion On Other Damping and Driving Momentum Mechanismsmentioning

confidence: 97%

Fast-ion transport and toroidal rotation response to externally applied magnetic perturbations at the ASDEX Upgrade tokamak

Cano-Megias¹,

Viezzer²,

Galdón-Quiroga³

et al. 2022

Nucl. Fusion

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“…As noted in the introduction, this system was used to explore the effects of magnetic geometry and collisionality on intrinsic rotation driven by momentum transport through neutrals [12,27]. In the next section we introduce the interpretive framework which can be used to diagnose experimental results.…”

Section: Neutral Momentum Transportmentioning

confidence: 99%

Edge momentum transport by neutrals: an interpretive numerical framework

Omotani¹,

Newton²,

Pusztai³

et al. 2017

Nucl. Fusion

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Due to their high cross-field mobility, neutrals can contribute to momentum transport even at the low relative densities found inside the separatrix and they can generate intrinsic rotation. We use a charge-exchange dominated solution to the neutral kinetic equation, coupled to neoclassical ions, to evaluate the momentum transport due to neutrals. Numerical solutions to the drift-kinetic equation allow us to cover the full range of collisionality, including the intermediate levels typical of the tokamak edge. In the edge there are several processes likely to contribute to momentum transport in addition to neutrals. Therefore, we present here an interpretive framework that can evaluate the momentum transport through neutrals based on radial plasma profiles. We demonstrate its application by analysing the neutral angular momentum flux for an L-mode discharge in the ASDEX Upgrade tokamak. The magnitudes of the angular momentum fluxes we find here due to neutrals of up to 1−2 N m are comparable to the net torque on the plasma from neutral beam injection, indicating the importance of neutrals for rotation in the edge.

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“…, assuming standard, radially-local neoclassical ions, in the Pfirsch-Schlüter [79][80][81][82], plateau [83], or banana [82] regime, or evaluated trans-collisionally using numerically simulated neoclassical f i [84][85][86]. The predicted rotation depends strongly on the poloidal distribution of neutrals, which, combined with effects of the neoclassical parallel ion heat flux, can contribute a P n driving a nonzero (usually countercurrent) edge toroidal rotation in the zero-torque case [81,82,84,85].…”

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confidence: 99%

Intrinsic rotation in axisymmetric devices

Stoltzfus-Dueck

2019

Plasma Phys. Control. Fusion

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Toroidal rotation is critical for fusion in tokamaks, since it stabilizes instabilities that can otherwise cause disruptions or degrade confinement. Unlike present-day devices, ITER might not have enough neutral-beam torque to easily avoid these instabilities. We must therefore understand how the plasma rotates 'intrinsically,' that is, without applied torque. Experimentally, torque-free plasmas indeed rotate, with profiles that are often non-flat and even non-monotonic. The rotation depends on many plasma parameters including collisionality and plasma current, and exhibits sudden bifurcations ('rotation reversals') at critical parameter values.Since toroidal angular momentum is conserved in axisymmetric systems, and since experimentally inferred momentum transport is much too large to be neoclassical, theoretical work has focused on rotation drive by nondiffusive turbulent momentum fluxes. In the edge, intrinsic rotation relaxes to a steady state in which the total momentum outflux from the plasma vanishes. Ion drift orbits, scrape-off-layer flows, separatrix geometry, and turbulence intensity gradient all play a role. In the core, nondiffusive and viscous momentum fluxes balance to set the rotation gradient at each flux surface. Although many mechanisms have been proposed for the nondiffusive fluxes, most are treated in one of two distinct but related gyrokinetic formulations. In a radially local fluxtube, appropriate for  r * 1, the lowest-order gyrokinetic formulations exhibit a symmetry that prohibits nondiffusive momentum flux for nonrotating plasmas in an updown symmetric magnetic geometry with no É B shear. Many symmetry-breaking mechanisms have been identified, but none have yet been conclusively demonstrated to drive a strong enough flux to explain commonly observed experimental rotation profiles. Radially global gyrokinetic simulations naturally include many symmetry-breaking mechanisms, and have shown cases with experimentally relevant levels of nondiffusive flux. These promising early results motivate further work to analyze, verify, and validate.This article provides a pedagogical introduction to intrinsic rotation in axisymmetric devices. Intended for both newcomers to the topic and experienced practitioners, the article reviews a broad range of topics including experimental and theoretical results for both edge and core rotation, while maintaining a focus on the underlying concepts.

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Edge rotation from momentum transport by neutrals

Cited by 3 publications

References 36 publications

Fast-ion transport and toroidal rotation response to externally applied magnetic perturbations at the ASDEX Upgrade tokamak

Fast-ion transport and toroidal rotation response to externally applied magnetic perturbations at the ASDEX Upgrade tokamak

Edge momentum transport by neutrals: an interpretive numerical framework

Intrinsic rotation in axisymmetric devices

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