TY - JOUR

T1 - Optimizing stellarators for large flows

AU - Calvo, Iván

AU - Parra, Felix I.

AU - Alonso, J. Arturo

AU - Velasco, José Luis

N1 - Funding Information:
The authors are grateful to Mrs. Binkowski, Mrs. Benemann and Mrs. Lange of BAM, for optical microscopy, SEM analyses and ATR-FTIR spectroscopy, respectively. This work was funded by the German Research Foundation (DFG), under the project-no. GR1002/10.

PY - 2014/9/1

Y1 - 2014/9/1

N2 - Plasma flow is damped in stellarators because they are not intrinsically ambipolar, unlike tokamaks, in which the flux-surface averaged radial electric current vanishes for any value of the radial electric field. Only quasisymmetric stellarators are intrinsically ambipolar, but exact quasisymmetry is impossible to achieve in non-axisymmetric toroidal configurations. By calculating the violation of intrinsic ambipolarity due to deviations from quasisymmetry, one can derive criteria to assess when a stellarator can be considered quasisymmetric in practice, i.e. when the flow damping is weak enough. Let us denote by α a small parameter that controls the size of a perturbation to an exactly quasisymmetric magnetic field. Recently, it has been shown that if the gradient of the perturbation is sufficiently small, the flux-surface averaged radial electric current scales as α2 for any value of the collisionality. It was also argued that when the gradient of the perturbation is large, the quadratic scaling is replaced by a more unfavorable one. In this paper, perturbations with large gradients are rigorously treated. In particular, it is proven that for low collisionality a perturbation with large gradient yields, at best, an O(|α|) deviation from quasisymmetry. Heuristic estimations in the literature incorrectly predicted an O(|α|3/2) deviation.

AB - Plasma flow is damped in stellarators because they are not intrinsically ambipolar, unlike tokamaks, in which the flux-surface averaged radial electric current vanishes for any value of the radial electric field. Only quasisymmetric stellarators are intrinsically ambipolar, but exact quasisymmetry is impossible to achieve in non-axisymmetric toroidal configurations. By calculating the violation of intrinsic ambipolarity due to deviations from quasisymmetry, one can derive criteria to assess when a stellarator can be considered quasisymmetric in practice, i.e. when the flow damping is weak enough. Let us denote by α a small parameter that controls the size of a perturbation to an exactly quasisymmetric magnetic field. Recently, it has been shown that if the gradient of the perturbation is sufficiently small, the flux-surface averaged radial electric current scales as α2 for any value of the collisionality. It was also argued that when the gradient of the perturbation is large, the quadratic scaling is replaced by a more unfavorable one. In this paper, perturbations with large gradients are rigorously treated. In particular, it is proven that for low collisionality a perturbation with large gradient yields, at best, an O(|α|) deviation from quasisymmetry. Heuristic estimations in the literature incorrectly predicted an O(|α|3/2) deviation.

KW - magnetic confinement fusion

KW - plasma rotation

KW - quasisymmetry

KW - stellarator

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U2 - 10.1088/0741-3335/56/9/094003

DO - 10.1088/0741-3335/56/9/094003

M3 - Article

AN - SCOPUS:84906542979

SN - 0741-3335

VL - 56

JO - Plasma Physics and Controlled Fusion

JF - Plasma Physics and Controlled Fusion

IS - 9

M1 - 094003

ER -