Observation of anomalous impurity transport during low-density experiments in W7-X with laser blow-off injections of iron

Geiger, B.; Wegner, Thomas; Beidler, C. D.; Burhenn, R.; Buttenschön, B.; Dux, R.; Langenberg, A.; Pablant, N.; Pütterich, T.; Turkin, Y.; Windisch, T.; Winters, V.; Beurskens, M.; Biedermann, C.; Brunner, K. J.; Cseh, G.; Damm, H.; Effenberg, F.; Fuchert, G.; Grulke, O.; Harris, J. H.; Killer, C.; Knauer, J.; Kocsis, G.; Krämer‐Flecken, A.; Kremeyer, T.; Krychowiak, M.; Marchuk, O.; Nicolai, D.; Rahbarnia, K.; Satheeswaran, G.; Schilling, J.; Schmitz, O.; Schröder, T.; Szepesi, T.; Thomsen, H.; Mora, H. Trimiño; Traverso, P.; Zhang, D.

doi:10.1088/1741-4326/aaff71

Cited by 50 publications

(71 citation statements)

References 47 publications

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“…Even in the non-ideal scenario of η −1 = 0.5, the majority of configurations show neoclassical impurity particle fluxes that do not exceed 10 % of the respective turbulent flux, even for highly charged impurities. However, a complete understanding of the implications of a relatively large turbulent particle flux will require further work, since determining sign of the particle flux may be more complicated than taking the sign of the largest transport channel (Geiger 2019). In other words, while neoclassical fluxes may potentially be small, they cannot be considered irrelevant.…”

Section: Discussionmentioning

confidence: 99%

Impurity temperature screening in stellarators close to quasisymmetry

Martin

Landreman

2020

J. Plasma Phys.

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Impurity temperature screening is a favourable neoclassical phenomenon involving an outward radial flux of impurity ions from the core of fusion devices. Quasisymmetric magnetic fields lead to intrinsically ambipolar neoclassical fluxes that give rise to temperature screening for low enough $\unicode[STIX]{x1D702}^{-1}\equiv d\ln n/d\ln T$ . In contrast, neoclassical fluxes in generic stellarators will depend on the radial electric field, which is predicted to be inward for ion-root plasmas, potentially leading to impurity accumulation. Here, we examine the impurity particle flux in a number of approximately quasisymmetric stellarator configurations and parameter regimes while varying the amount of symmetry breaking in the magnetic field. For the majority of this work, neoclassical fluxes have been obtained using the SFINCS drift-kinetic equation solver with electrostatic potential $\unicode[STIX]{x1D6F7}=\unicode[STIX]{x1D6F7}(r)$ , where $r$ is a flux-surface label. Results indicate that achieving temperature screening is possible, but unlikely, at low-collisionality reactor-relevant conditions in the core. Thus, the small departures from symmetry in nominally quasisymmetric stellarators are large enough to significantly alter the neoclassical impurity transport. A further look at the magnitude of these fluxes when compared to a gyro-Bohm turbulence estimate suggests that neoclassical fluxes are small in configurations optimized for quasisymmetry when compared to turbulent fluxes.

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Section: Discussionmentioning

confidence: 99%

Impurity temperature screening in stellarators close to quasisymmetry

Martin

Landreman

2020

J. Plasma Phys.

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“…A significant fraction of the transport in this type of discharge is anomalous (Geiger et al. 2019; Klinger et al. 2019).…”

Section: Introductionmentioning

confidence: 99%

Phase contrast imaging measurements and numerical simulations of turbulent density fluctuations in gas-fuelled ECRH discharges in Wendelstein 7-X

et al. 2021

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The fundamental nature of turbulent density fluctuations in standard Wendelstein 7-X (W7-X) stellarator discharges is investigated experimentally via phase contrast imaging (PCI) in combination with gyrokinetic simulations with the code GENE. We find that density fluctuations are ion-temperature-gradient-driven and radially localised in the outer half of the plasma. It is shown that the line-integrated PCI measurements cover the right range of wavenumbers and a favourable toroidal and poloidal location to capture some of the strongest density fluctuations in W7-X. Due to the radial localisation of fluctuations, measured wavenumber–frequency spectra exhibit a dominant phase velocity, which can be related to the $\boldsymbol {E\times B}$ rotation velocity at the radial position of a well in the neoclassical radial electric field. The match is robust against variations of heating power and line-integrated density, which is partly due to the localisation of fluctuations and partly due to effects of the radial gradient in the $\boldsymbol {E\times B}$ velocity profile on the wavenumber–frequency spectrum. The latter effect is studied with a newly built synthetic PCI diagnostic and global gyrokinetic simulations with GENE-3D.

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“…However, for highly charged impurities, even if the diffusive part of the transport is dominated by turbulence (as found in Ref. 17), the convective part of the transport (i.e. the one independent of the impurity density gradient) -that determines the sign of the peaking factor -can still be dominated by collisional transport, as it increases with Z.…”

Section: Introductionmentioning

confidence: 75%

Optimization of flux-surface density variation in stellarator plasmas with respect to the transport of collisional impurities

et al. 2019

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Avoiding impurity accumulation is a requirement for steady-state stellarator operation. The accumulation of impurities can be heavily affected by variations in their density on the flux-surface. Using recently derived semi-analytic expressions for the transport of a collisional impurity species with high-Z and flux-surface densityvariation in the presence of a low-collisionality bulk ion species, we numerically optimize the impurity density-variation on the flux-surface to minimize the radial peaking factor of the impurities. These optimized density-variations can reduce the core impurity density by 0.75 Z (with Z the impurity charge number) in the Large Helical Device case considered here, and by 0.89 Z in a Wendelstein 7-X standard configuration case. On the other hand, when the same procedure is used to find density-variations that maximize the peaking factor, it is notably increased compared to the case with no density-variation. This highlights the potential importance of measuring and controlling these variations in experiments.

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Observation of anomalous impurity transport during low-density experiments in W7-X with laser blow-off injections of iron

Cited by 50 publications

References 47 publications

Impurity temperature screening in stellarators close to quasisymmetry

Impurity temperature screening in stellarators close to quasisymmetry

Phase contrast imaging measurements and numerical simulations of turbulent density fluctuations in gas-fuelled ECRH discharges in Wendelstein 7-X

Optimization of flux-surface density variation in stellarator plasmas with respect to the transport of collisional impurities

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