Parallel impurity dynamics in the TJ-II stellarator

Alonso, A.; Velasco, J. L.; Calvo, I.; Estrada, T.; Fontdecaba, J. M.; García-Regaña, J. M.; Geiger, J.; Landreman, Matt; McCarthy, K. J.; Medina, F.; Ochando, M. A.; Parra, Félix I.

doi:10.1088/0741-3335/58/7/074009

Cited by 10 publications

(10 citation statements)

References 31 publications

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“…It does not differ qualitatively from the standard neoclassical prediction, although quantitatively the changes observed in the flux are appreciable and tend to decrease the inward flux. Note the similar values of C 6+ normalised radial flux to those reported in [42]. The impact for carbon and tungsten is also comparable despite the different charge states.…”

Section: Tj-ii Resultssupporting

confidence: 78%

Electrostatic potential variation on the flux surface and its impact on impurity transport

et al. 2017

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The particle transport of impurities in magnetically confined plasmas under some conditions does not find, neither quantitatively nor qualitatively, a satisfactory theory-based explanation. This compromise the successful realization of thermo-nuclear fusion for energy production since its accumulation is known to be one of the causes that leads to the plasma breakdown. In standard reactor-relevant conditions this accumulation is in most stellarators intrinsic to the lack of toroidal symmetry, that leads to the neoclassical electric field to point radially inwards. This statement, that the standard theory allows to formulate, has been contradicted by some experiments that showed weaker or no accumulation under such conditions [1,2]. The charge state of the impurities makes its transport more sensitive to the electric fields. Thus, the short length scale turbulent electrostatic potential or its long wavelength variation on the flux surface Φ 1 -that the standard neoclassical approach usually neglects -might possibly shed some light on the experimental findings. In the present work the focus is put on the second of the two, and investigate its influence of the radial transport of C 6+ . We show that in LHD it is strongly modified by Φ 1 , both resulting in mitigated/enhanced accumulation at internal/external radial positions; for Wendelstein 7-X, on the contrary, Φ 1 is expected to be considerably smaller and the transport of C 6+ not affected up to an appreciable extent; and in TJ-II the potential shows a moderate impact despite of the large amplitude of Φ 1 for the parameters considered.

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Section: Tj-ii Resultssupporting

confidence: 78%

Electrostatic potential variation on the flux surface and its impact on impurity transport

et al. 2017

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“…Damping oscillation just after the pellet injection having m ¼ 1 mode structure was found in the TJ-II Stellarator. 29 This phenomenon was explained by the change of the friction force due to sudden change of plasma parameters induced by a pellet injection. The equilibrium of the LHD is also a three-dimensional equilibrium.…”

Section: Discussionmentioning

confidence: 99%

Simultaneous excitation of the snake-like oscillations and the m/n = 1/1 resistive interchange modes around the iota = 1 rational surface just after hydrogen pellet injections in LHD plasmas

et al. 2018

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Two types of oscillation phenomena are found just after hydrogen ice pellet injections in the Large Helical Device (LHD). Oscillation phenomena appear when the deposition profile of a hydrogen ice pellet is localized around the rotational transform i ¼ 1 rational surface. At first, damping oscillations (type-I) appear only in the soft X-ray (SX) emission. They are followed by the second type of oscillations (type-II) where the magnetic fluctuations and density fluctuations synchronized to the SX fluctuations are observed. Both oscillations have poloidal/toroidal mode number, m/n ¼ 1/1. Since the type-II oscillations appear when the local pressure is large and/or the local magnetic Reynold's number is small, it is reasonable that type-II oscillations are caused by the resistive interchange modes. Because both types of oscillations appear simultaneously at slightly different locations and with slightly different frequencies, it is certain that type-I oscillations are different from type-II oscillations, which we believe is the MHD instability. It is possible that type-I oscillations are caused by the asymmetric concentration of the impurities. The type-I oscillations are similar to the impurity snake phenomena observed in tokamaks though type-I oscillations survive only several tens of milliseconds in LHD.

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“…where the flux surface average operation and V ′ (ψ) have been defined in (23) and (24). A direct check shows that…”

Section: Appendix B Proof That the Adiabatic Response Does Not Contrmentioning

confidence: 99%

“…when δ = 0. From the start, it will be evident that the role of the electric field tangent to the flux surface is relevant when the collisionality is as low as in (4) (not to mention its importance for impurity transport, as pointed out, for example, in [22,23,24]). Furthermore, we will show that writing the contributions to the quasineutrality equation that gives the electric field tangent to the flux surface is a subtle issue.…”

Section: Introductionmentioning

confidence: 99%

The effect of tangential drifts on neoclassical transport in stellarators close to omnigeneity

Calvo¹,

Parra

Velasco³

et al. 2017

Plasma Phys. Control. Fusion

205

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Abstract. In general, the orbit-averaged radial magnetic drift of trapped particles in stellarators is non-zero due to the three-dimensional nature of the magnetic field. Stellarators in which the orbit-averaged radial magnetic drift vanishes are called omnigeneous, and they exhibit neoclassical transport levels comparable to those of axisymmetric tokamaks. However, the effect of deviations from omnigeneity cannot be neglected in practice, and it is more deleterious at small collisionalities. For sufficiently low collision frequencies (below the values that define the 1/ν regime), the components of the drifts tangential to the flux surface become relevant. This article focuses on the study of such collisionality regimes in stellarators close to omnigeneity when the gradient of the non-omnigeneous perturbation is small. First, it is proven that closeness to omnigeneity is required to actually preserve radial locality in the drift-kinetic equation for collisionalities below the 1/ν regime. Then, using the derived radially local equation, it is shown that neoclassical transport is determined by two layers located at different regions of phase space. One of the layers corresponds to the so-called √ ν regime and the other to the so-called superbanana-plateau regime. The importance of the superbanana-plateau layer for the calculation of the tangential electric field is emphasized, as well as the relevance of the latter for neoclassical transport in the collisionality regimes considered in this paper. In particular, the role of the tangential electric field is essential for the emergence of a new subregime of superbanana-plateau transport when the radial electric field is small. A formula for the ion energy flux that includes the √ ν regime and the superbanana-plateau regime is given. The energy flux scales with the square of the size of the deviation from omnigeneity. Finally, it is explained why below a certain collisionality value the formulation presented in this article ceases to be valid.

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Parallel impurity dynamics in the TJ-II stellarator

Cited by 10 publications

References 31 publications

Electrostatic potential variation on the flux surface and its impact on impurity transport

Electrostatic potential variation on the flux surface and its impact on impurity transport

Simultaneous excitation of the snake-like oscillations and the m/n = 1/1 resistive interchange modes around the iota = 1 rational surface just after hydrogen pellet injections in LHD plasmas

The effect of tangential drifts on neoclassical transport in stellarators close to omnigeneity

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