Impurity transport and bulk ion flow in a mixed collisionality stellarator plasma

Newton, Sarah; Helander, P.; Mollén, A.; Smith, H. M.

doi:10.1017/s0022377817000745

Cited by 23 publications

(27 citation statements)

References 46 publications

(172 reference statements)

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“…The same argument applies to F − i1(−1) , unless there is a parallel impurity flow in (A 9) to orderν −1 to act as a source in (A 6). Such orderν −1 flows cannot arise in the mixed-collisionality regime, so F − i1(−1) = 0 (Newton et al 2017). However, to make the F i1 formulas in this section apply for any impurity collisionality, we will nevertheless allow for F − i1(−1) = 0 below, as it turns out to not be inconvenient to calculate F − i1(−1) together with F − i1(0) .…”

Section: Discussionmentioning

confidence: 99%

“…2017 a ; Newton et al. 2017). This is due to a cancellation between the flux driven by impurity parallel flow and the ion thermodynamic forces.…”

Section: Introductionmentioning

confidence: 99%

“…In this work, we generalize the analytical calculation in (Newton et al 2017) to account for flux-surface variation of the impurity density in stellarators, using a fluid description for the impurities and solving for the ion distribution function in the 1/ν regime. Our expression for the impurity flux agrees with that in Calvo et al (2018), where the same problem is treated fully kinetically.…”

Section: Introductionmentioning

confidence: 99%

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Collisional transport of impurities with flux-surface varying density in stellarators

et al. 2018

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High-Z impurities in magnetic confinement devices are prone to develop density variations on the flux-surface, which can significantly affect their transport. In this paper, we generalize earlier analytic stellarator calculations of the neoclassical radial impurity flux in the mixed-collisionality regime (collisional impurities and low-collisionality bulk ions) to include the effect of such flux-surface variations. We find that only in the homogeneous density case is the transport of highly collisional impurities (in the Pfirsch-Schlüter regime) independent of the radial electric field. We study these effects for a Wendelstein 7-X (W7-X) vacuum field, with simple analytic models for the potential perturbation, under the assumption that the impurity density is given by a Boltzmann response to a perturbed potential. In the W7-X case studied, we find that larger amplitude potential perturbations cause the radial electric field to dominate the transport of the impurities. In addition, we find that classical impurity transport can be larger than the neoclassical transport in W7-X. † Email address for correspondence: bstefan@chalmers.se

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Section: Discussionmentioning

confidence: 99%

“…2017 a ; Newton et al. 2017). This is due to a cancellation between the flux driven by impurity parallel flow and the ion thermodynamic forces.…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Collisional transport of impurities with flux-surface varying density in stellarators

et al. 2018

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“…Temperature gradients are known to affect the impurity pinch in different ways, depending on the details of the system [19,20,36,42]. This is an area of active research; for instance, there has been recent progress on the possibility of using temperature gradients to mitigate the impurity pinch in stellarators [43,44].…”

Section: Discussionmentioning

confidence: 99%

Strategies for advantageous differential transport of ions in magnetic fusion devices

2018

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In a variety of magnetized plasma geometries, it has long been known that highly charged impurities tend to accumulate in regions of higher density. Here, we examine how this "collisional pinch" changes in the presence of additional forces, such as might be found in systems with gravity, fast rotation, or non-negligible space charge. In the case of a rotating, cylindrical plasma, we describe a regime in which the radially outermost ion species is intermediate in both mass and charge. We discuss implications for fusion devices and plasma mass filters.

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“…To understand why the classical transport is relevant in an optimized stellarator, we employ recent analytical results on neoclassical transport for a collisional impurity (Braun & Helander 2010;Helander et al 2017;Newton et al 2017) to show that the ratio of classical to neoclassical fluxes is proportional to a geometrical factor (2.1), which turns out to be larger than one in W7-X.…”

Section: Introductionmentioning

confidence: 99%

The importance of the classical channel in the impurity transport of optimized stellarators

et al. 2019

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In toroidal magnetic confinement devices, such as tokamaks and stellarators, neoclassical transport is usually an order of magnitude larger than its classical counterpart. However, when a high-collisionality species is present in a stellarator optimized for low Pfirsch-Schlüter current, its classical transport can be comparable to the neoclassical transport. In this letter, we compare neoclassical and classical fluxes and transport coefficients calculated for Wendelstein 7-X (W7-X) and Large Helical Device (LHD) cases. In W7-X, we find that the classical transport of a collisional impurity is comparable to the neoclassical transport for all radii, while it is negligible in the LHD cases, except in the vicinity of radii where the neoclassical transport changes sign. In the LHD case, electrostatic potential variations on the flux-surface significantly enhance the neoclassical impurity transport, while the classical transport is largely insensitive to this effect in the cases studied.

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Impurity transport and bulk ion flow in a mixed collisionality stellarator plasma

Cited by 23 publications

References 46 publications

Collisional transport of impurities with flux-surface varying density in stellarators

Collisional transport of impurities with flux-surface varying density in stellarators

Strategies for advantageous differential transport of ions in magnetic fusion devices

The importance of the classical channel in the impurity transport of optimized stellarators

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