Strong suppression of impurity accumulation in steady-state hydrogen discharges with high power NBI heating on LHD

Nakamura, Yukio; Tamura, N.; Yoshinuma, M.; Suzuki, C.; Yoshimura, S.; Kobayashi, M.; Yokoyama, M.; Nunami, M.; Nakata, M.; Nagaoka, K.; Tanaka, K.; Peterson, B.J.; Ida, K.; Osakabe, M.; Morisaki, T.

doi:10.1088/1741-4326/aa6187

Cited by 15 publications

(15 citation statements)

References 37 publications

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“…In LHD, the impurity transport is quite sensitive to the collisionality of the plasma. 27 Moreover, a recent LHD experiment revealed that the strong gradient of the electron density causes the outward transport of impurities. 28 Then it is possible that impurity ions in the core plasma are transported to the outward direction by the formation of the steep electron density gradient by the ablation of the pellet.…”

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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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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“…In contrast, recently observed discrepancies between modeled and experimental light impurity radial profiles at the ASDEX Upgrade (AUG) and JET tokamaks are challenging the current theoretical understanding [11][12][13]. Indeed, in regimes where the normalised ion temperature and toroidal rotation gradients are large and where transient transport mechanisms such as sawteeth and edge localised modes (ELM) are negligible, theory does not quantitatively predict the experimental negative boron/carbon peaking factors (hollow profiles) and in some cases even predict peaked profiles instead [11,13,14]. A more detailed experimental analysis of the diffusive and convective parts of the boron flux performed at AUG for low NBI heating cases (≤ 5 MW) [15] showed good agreement with the modelling results [16].…”

Section: Introductionmentioning

confidence: 99%

Light impurity transport in tokamaks: on the impact of neutral beam fast ions

et al. 2020

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Previous studies (e.g. [Kappatou et al Nuclear Fusion, 2019]) have shown that discrepancies exist between experimental and modelled light impurity peaking in specific regimes. In particular, in NBI heated plasmas, the strong hollowness of boron profiles are not captured by the modelling. In this context, a dedicated ASDEX Upgrade discharge, including boron and helium density measurements, is studied where such discrepancies appear. Emphasis is given to the impact of fast ions on the turbulent and neoclassical impurity transport computed with the codes GKW and NEO respectively. It is shown that fast ions increase the turbulent outward impurity convection via an increase of both thermo-and roto-diffusion. This increase is correlated with the stabilisation of the Ion Temperature Gradient driven mode (ITG). Additionally, the ratio of the turbulent impurity diffusion over the ion heat diffusion is reduced with fast ions, which in turn, increases the neoclassical contributions to the impurity peaking. Finally, neoclassical convection is also enhanced through changes in the ion density gradient due to the modification of the quasi-neutrality condition in the presence of peaked fast ion profiles. The combination of these three mechanisms induced by the fast ion population has a significant impact on the development of hollow boron profiles, up to experimental levels.

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“…The situation is completely different in stellarators. Experimentally, stellarator impurity accumulation in the plasma core is consistently observed [11] (although there are remarkable, not completely understood, exceptions like the "impurity hole" [12,13] in the Large Helical Device (LHD) and the "High Density H mode" in Wendelstein 7-AS [14]; see also [15] for another exception in LHD involving plasmas with higher density than those typical of the impurity hole). Impurity accumulation leads to fuel dilution and even to plasma termination by radiative collapse.…”

Section: Introductionmentioning

confidence: 99%

Stellarator impurity flux driven by electric fields tangent to magnetic surfaces

Calvo¹,

Parra

Velasco³

et al. 2018

Nucl. Fusion

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The control of impurity accumulation is one of the main challenges for future stellarator fusion reactors. The standard argument to explain this accumulation relies on the, in principle, large inward pinch in the neoclassical impurity flux caused by the typically negative radial electric field in stellarators. This simplified interpretation was proven to be flawed by Helander et al. [Phys. Rev. Lett. 118, 155002 (2017)], who showed that in a relevant regime (low-collisionality main ions and collisional impurities) the radial electric field does not drive impurity transport. In that reference, the effect of the component of the electric field that is tangent to the magnetic surface was not included. In this letter, an analytical calculation of the neoclassical radial impurity flux incorporating such effect is given, showing that it can be very strong for highly charged impurities and that, once it is taken into account, the dependence of the impurity flux on the radial electric field reappears. Realistic examples are provided in which the inclusion of the tangential electric field leads to impurity expulsion.

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Strong suppression of impurity accumulation in steady-state hydrogen discharges with high power NBI heating on LHD

Cited by 15 publications

References 37 publications

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

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

Light impurity transport in tokamaks: on the impact of neutral beam fast ions

Stellarator impurity flux driven by electric fields tangent to magnetic surfaces

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