Plasma rotation from momentum transport by neutrals in tokamaks

Abstract

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...

Edge rotation from momentum transport by neutrals

“…In Ref. [23] we found that the electric field increased with R n at high collisionality and decreased at low collisionality. The radius of the X-point decreases with increasing , so Fig.…”

“…Comparison of analytical and numerical results for the dependence of (a) toroidal flow velocity and (b) radial electric field at the outboard midplane on poloidal location of the neutrals. Dashed lines show the banana regime analytic results, (23) and (22), and dotted lines the Pfirsch-Schlüter analytic results, (20) and (19). Solid lines show numerical scans, with colours from blue to red corresponding to low to high collisionalityν, as shown in the legend.…”

“…In Ref. [23] we studied the effect on the electric field and rotation of varying the shaping parameters while keeping the neutrals localized at θ n = θ X , where θ X is the poloidal angle of the X-point, representing a divertor recycling source or private flux region fuelling. We concluded that the shaping parameters most relevant for machine operation, triangularity and X-point position, only affect the electric field and rotation by changing the major radius where the neutrals are localized, for a fixed collisionality.…”

“…The neutrals may be localized poloidally due to gas fuelling at a particular location, or divertor recycling causing the neutrals to be concentrated near the X-point. The resulting momentum transport and rotation is then controlled by the location of the neutrals [10,11,23]. In Ref.…”

Edge momentum transport by neutrals: an interpretive numerical framework

“…We have demonstrated here that charge-exchanging neutrals can carry a significant angular momentum flux in the tokamak edge. The formalism developed in [10,11] and first implemented numerically in [12] has been extended, to enable interpretive studies evaluating the momentum transport through neutrals for experimental profiles and equilibria. In this interpretive mode, the radial variation of the background profiles does not have to be neglected compared to the radial gradient of the neutral density.…”