TY - JOUR

T1 - Finite orbit width effects in large aspect ratio stellarators

AU - D'Herbemont, Vincent

AU - Parra, Felix I.

AU - Calvo, Iván

AU - Velasco, José Luis

N1 - Funding Information:
This work was supported by the U.S. Department of Energy under contract number DE-AC02-09CH11466. The United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Research and Training Programme (Grant Agreement No 101052200 – EUROfusion). Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them. This research was supported in part by Grant PGC2018-095307-B-I00, Ministerio de Ciencia, Innovación y Universidades, Spain.
Publisher Copyright:
Copyright © 2022 The Author(s).

PY - 2022/10/17

Y1 - 2022/10/17

N2 - New orbit-averaged equations for low collisionality neoclassical fluxes in large aspect ratio stellarators with mirror ratios close to unity are derived. The equations retain finite orbit width effects by employing the second adiabatic invariant J as a velocity-space coordinate and they have been implemented in the orbit-averaged neoclassical code KNOSOS (Velasco et al., J. Comput. Phys., vol. 418, 2020, 109512; Velasco et al., Nucl. Fusion, vol. 61, 2021, 116013). The equations are used to study the 1/ν regime and the lower collisionality regimes. For generic large aspect ratio stellarators with mirror ratios close to unity, as the collision frequency decreases, the 1/ν regime transitions directly into the ν regime, without passing through a √ν regime. An explicit formula for the neoclassical fluxes in the ν regime is obtained. The formula includes the effect of particles that transition between different types of wells. While these transitions produce stochastic scattering independent of the value of the collision frequency in velocity space, the diffusion in real space remains proportional to the collision frequency. The √ν regime is only recovered in large aspect ratio stellarators close to omnigeneity: large aspect ratio stellarators with large mirror ratios and optimized large aspect ratio stellarators with mirror ratios close to unity. Neoclassical transport in large aspect ratio stellarators with large mirror ratios can be calculated with the orbit-averaged equations derived by Calvo et al. (Plasma Phys. Control. Fusion, vol. 59, 2017, 055014). In these stellarators, the √ν regime exists in the collisionality interval (a/R) ln(R/a) ≪ νiiRa/ρivti ≪ R/a. In optimized large aspect ratio stellarators with mirror ratios close to unity, the √ν regime occurs in an interval of collisionality that depends on the deviation from omnigeneity δ: δ2| ln δ| ≪ νiiRa/ρivti ≪ 1.

AB - New orbit-averaged equations for low collisionality neoclassical fluxes in large aspect ratio stellarators with mirror ratios close to unity are derived. The equations retain finite orbit width effects by employing the second adiabatic invariant J as a velocity-space coordinate and they have been implemented in the orbit-averaged neoclassical code KNOSOS (Velasco et al., J. Comput. Phys., vol. 418, 2020, 109512; Velasco et al., Nucl. Fusion, vol. 61, 2021, 116013). The equations are used to study the 1/ν regime and the lower collisionality regimes. For generic large aspect ratio stellarators with mirror ratios close to unity, as the collision frequency decreases, the 1/ν regime transitions directly into the ν regime, without passing through a √ν regime. An explicit formula for the neoclassical fluxes in the ν regime is obtained. The formula includes the effect of particles that transition between different types of wells. While these transitions produce stochastic scattering independent of the value of the collision frequency in velocity space, the diffusion in real space remains proportional to the collision frequency. The √ν regime is only recovered in large aspect ratio stellarators close to omnigeneity: large aspect ratio stellarators with large mirror ratios and optimized large aspect ratio stellarators with mirror ratios close to unity. Neoclassical transport in large aspect ratio stellarators with large mirror ratios can be calculated with the orbit-averaged equations derived by Calvo et al. (Plasma Phys. Control. Fusion, vol. 59, 2017, 055014). In these stellarators, the √ν regime exists in the collisionality interval (a/R) ln(R/a) ≪ νiiRa/ρivti ≪ R/a. In optimized large aspect ratio stellarators with mirror ratios close to unity, the √ν regime occurs in an interval of collisionality that depends on the deviation from omnigeneity δ: δ2| ln δ| ≪ νiiRa/ρivti ≪ 1.

KW - fusion plasma

KW - plasma confinement

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U2 - 10.1017/S0022377822000897

DO - 10.1017/S0022377822000897

M3 - Article

AN - SCOPUS:85141528290

SN - 0022-3778

VL - 88

JO - Journal of Plasma Physics

JF - Journal of Plasma Physics

IS - 5

M1 - 905880507

ER -