Controlling edge plasma rotation through poloidally localized refueling

Helander, P.; Fülöp, Tünde; Catto, Peter J.

doi:10.1063/1.1616014

Cited by 34 publications

(74 citation statements)

References 25 publications

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“…Our numerical tool allows these effects to be taken into account simultaneously. We also find good agreement with analytical results [18,19] in the appropriate limits.…”

supporting

confidence: 74%

“…This is due to the fact that toroidal flow is needed to give a radial momentum flux balancing that due to the toroidal heat flux, which is present because of the radial temperature gradient [19]. In the limits R n → ∞ in Fig.…”

mentioning

confidence: 99%

“…Other experimental observations correlate the edge intrinsic toroidal rotation with CX dynamics [14] or the major radius of the Xpoint [15]. These recent observations triggered renewed interest in the role of the neutrals in the edge region of tokamaks.Previous analytical work based on neoclassical theory has shown that if the neutral contribution to the toroidal angular momentum is dominant, neutrals can modify or even determine the edge plasma rotation and radial electric field [13,[16][17][18][19][20][21]. It has been shown that even if there is no input of external momentum into the plasma, the neutrals drive intrinsic momentum transport and hence rotation.…”

mentioning

confidence: 99%

“…Previous analytical work based on neoclassical theory has shown that if the neutral contribution to the toroidal angular momentum is dominant, neutrals can modify or even determine the edge plasma rotation and radial electric field [13,[16][17][18][19][20][21]. It has been shown that even if there is no input of external momentum into the plasma, the neutrals drive intrinsic momentum transport and hence rotation.…”

mentioning

confidence: 99%

“…However, it is plausible to think that the neutral and turbulent momentum fluxes should be at least comparable, given the fact that in steady state the particle losses are balanced by fueling and recycling. The neutral particle transport is then equal to ion particle transport due to both collisions and turbulence, since every recycling ion that leaves the plasma comes back as a neutral [19]. Therefore considering the neutral momentum transport in isolation is an important step in understanding plasma rotation in regions where neutrals are present.…”

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confidence: 99%

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Plasma rotation from momentum transport by neutrals in tokamaks

et al. 2016

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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...

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“…Our numerical tool allows these effects to be taken into account simultaneously. We also find good agreement with analytical results [18,19] in the appropriate limits.…”

supporting

confidence: 74%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Plasma rotation from momentum transport by neutrals in tokamaks

et al. 2016

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Localized Gas Puffing Control of Edge Rotation and Electric Field

Catto¹,

Fülöp²,

Helander³

2004

Contrib. Plasma Phys.

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Neutrals at the edge of tokamaks can influence confinement by altering the radial electric field and toroidal plasma flow velocity through charge exchange and ionization interactions when the high diffusivity of the neutrals results in neutral viscosity dominating over ion neoclassical and turbulent viscosities [1][2][3][4][5]. We find that the flow and electric field are very sensitive to the poloidal location of the neutrals and the ion collisionality [6,7]. These predictions are consistent with differences observed on MAST between measured toroidal flows for inboard and outboard gas puffing, and may also explain observations indicating that inboard puffing allows easier H mode access [8]. In particular, we find that the inward directed radial electric field and counter-current directed outboard toroidal flow velocity in a collisional or Pfirsch-Schlüter edge plasma tend to be larger if the atoms are concentrated on the inboard side rather than on the outboard side [6,7] -consistent with the MAST results and suggesting that the flow shear introduced by the limited penetration of the neutrals and the radial variation of the ion temperature gradient suppresses edge turbulence and plays a role in forming the edge transport barrier. The ion temperature gradient terms enter because ion heat flow modifies the neutral viscosity via charge exchange.The results are found to be relatively insensitive to neutral collisionality [4], which tends to cause up to order unity changes in amplitudes but not changes in direction. More importantly, the results are found to be sensitive to plasma collisionality [7] because of the important role played by ion temperature gradient terms -as in standard neoclassical theory. In the case of banana regime ions and collisional neutrals [7], inboard puffing reduces the counter-current rotation, while the radial electric field always remains inward. Consequently, if there is a narrow collisional layer separating the separatrix from the pedestal as in Alcator C-Mod, then this effect will further shear the flow. However, for banana regime ions the largest outboard toroidal flow is obtained for outboard puffing and is in the counter-current direction -suggesting that outboard puffing might be more effective in suppressing edge turbulence and generating an edge transport barrier in tokamaks which remain in the banana regime all the way to the separatrix (assuming no other source of flow shear). Finally, impurities affect the size of the flow and electric field; increasing the effect in the Pfirsch-Schlüter regime and decreasing it when the ions are in the banana regime [7]. These results suggest that flexible neutral fueling is desirable since it may allow some external control over the tokamak edge and overall performance.In spite of any obvious source of momentum input, axisymmetric tokamak plasmas rotate toroidally. One mechanism causing the edge plasma to rotate can be summarized as follows. The magnetic field introduces a preferred direction. It breaks toroidal symmetry by requiring a Pfirs...

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Modelling of the Edge Plasma with Account of Self-Consistent Electric Fields

Rozhansky

2006

Contrib. Plasma Phys.

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Recent results on the simulation of the edge plasma parameters are discussed with emphasis on the role of self-consistent electric fields. It is demonstrated that simulation with B2SOLPS5.0 code as well as with the other codes is consistent with the neoclassical nature of the radial electric field in the core region 1-2 cm inside the separatrix. Near the separatrix the viscous layer exists, where the impact of the parallel fluxes of the SOL is important and electric field deviates from the neoclassical value. It is shown that the experimental dependence of the L-H transition threshold on the local and global plasma parameters might be explained on the basis of the simulations. The impact of the geometrical factors on the radial electric field structure is discussed. It is shown that the change of the parallel fluxes in the scrape-off layer, which are transported through the separatrix due to turbulent viscosity and diffusivity, should result in variation of the radial electric field. The parallel (toroidal) fluxes in the SOL and mechanisms of their formation are analyzed for different geometry and directions of the magnetic field. Currents in the SOL and in the core region are analyzed and compared with the analytical predictions.

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Controlling edge plasma rotation through poloidally localized refueling

Cited by 34 publications

References 25 publications

Plasma rotation from momentum transport by neutrals in tokamaks

Plasma rotation from momentum transport by neutrals in tokamaks

Localized Gas Puffing Control of Edge Rotation and Electric Field

Modelling of the Edge Plasma with Account of Self-Consistent Electric Fields

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