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10-15 May 2021
Virtual Event
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

Impurity Transport in Ion- and Electron-Root Plasmas of Wendelstein 7-X

13 May 2021, 14:00
4h 45m
Virtual Event

Virtual Event

Regular Poster Magnetic Fusion Experiments P6 Posters 6

Speaker

Andreas Langenberg (Max-Planck-Institut für Plasmaphysik,17491 Greifswald, Germany)

Description

In non axis-symmetric, magnetic confinement fusion devices like the optimized stellarator Wendelstein 7-W (W7-X), recent theoretically predicted aspects on impurity transport, as the existence of a mixed collisionality regime $^1$ or the build up of a radial electric field $E_r$ $^2$, have been addressed in several initial experimental studies. Based on measurements of impurity transport times $^{3,4}$ or radial impurity diffusivity profiles $^{5,6}$, these studies are hinting for a strong anomalous impurity transport mechanism in W7-X.
In this work, a possible suppression of the anomalous impurity transport in a so-called ion-root plasma scenario is investigated, as in those scenarios already a significant improve of the energy confinement has been observed $^7$. Using X-ray imaging spectrometer (XICS) data, impurity density profiles of neighboring Ar charge states, namely $n_{Ar}^{15+}$, $n_{Ar}^{16+}$, and $n_{Ar}^{17+}$ (see Fig.1), are used to derive impurity fluxes of $Ar^{16+}$ as described in detail elsewhere $^5$.
In Fig.2, experimentally derived, radial fluxes of $Ar^{16+}$ are shown comparatively for an experiment program during a pure ion-root confinement time interval (Fig.3 a) and a central electron root confinement (CERC) phase (Fig.3 b).
As evident from Fig.2, in the CERC scenario a positive, radially outwards directed Ar flux from the plasma center up to half of the plasma radius $\rho$ = 0-0.5 (see dashed line) is observed, being dominant over a negative, radially inwards directed Ar flux from half of the plasma radius to the plasma edge. In the ion-root scenario, this positive Ar flux is restricted to the plasma center, now with a dominant negative radially inward directed Ar flux in the entire bulk plasma region of $\rho$ > 0.2.
Finally, Fig.3 c)+d) show measured diffusion and velocity profiles, derived from the above shown Ar flux measurements for the ion-root plasma scenario. Compared to typical diffusivities of D ~ 1.5-3 m$^2$/s measured in CERC plasmas (see shaded area in Fig.3c) $^{5,6}$, one finds a significantly reduced impurity diffusivity D in the plasma bulk region $\rho$ = 0-0.6, accompanied by a strong negative convection velocity v for the ion-root confinement scenario.
As the the ion- and electron-root plasma scenarios exist at different values of $n_e$, $T_e$, and $T_i$, additional neoclassical STRAHL simulations are performed to disentangle possible $n_e$, $T_e$, and $T_i$ contributions to the observed changes in the Ar fluxes, by comparing STRAHL simulated and measured Ar density profiles, given the measured diffusion and velocity profiles.

References:

$^1$ P. Helander, S.L. Newton, A. Mollén et al. Phys. Rev. Lett. 118, 155002 (2017)
$^2$ N.A. Pablant, A. Langenberg, A. Alonso et al. Physics of Plasmas 25, 022508 (2018)
$^3$ A. Langenberg, F. Warmer, G. Fuchert et al. Plasma Physics and Controlled Fusion 61, 014030 (2019)
$^4$ Th. Wegner, B. Geiger, F. Kunkel et al. Review of Scientific Instruments 89 073505 (2018)
$^5$ A. Langenberg, N.A. Pablant, O. Marchuk et al. Nuclear Fusion 57 086013 (2017)
$^6$ B. Geiger, Th. Wegner, C.D. Beidler et al. Nuclear Fusion 59 046009 (2019)
$^7$ R. Wolf, A. Alonso, S. Akkäslompolo et al. Physics of Plasmas 26 082504 (2019)

Argon XVI - XVIII impurity density profiles as measured by XICS in a W7-X ion-root plasma scenario.

Experimentally derived Ar16+ fluxes for an ion-root and a central electron-root plasma scenario. Positive fluxes (radially outward directed) are shown in blue, negative fluxes (radially inward directed) are shown in red.

a)+b) Experimentally observed radial electric fields for a) ion- and b) electron-root confinement. c)+d) Ar diffusion and convection profiles in an ion-root plasma scenario. Shaded area denotes typical observed maximum D values in CERC plasma scenario.

Country or International Organization Germany
Affiliation Max-Planck-Institute for Plasma Physics

Primary authors

Andreas Langenberg (Max-Planck-Institut für Plasmaphysik,17491 Greifswald, Germany) Dr Novimir Pablant (Princeton Plasma Physics Laboratory) Dr Oleksandr Marchuk (Institute for Energy and Climate Research Jülich) Dr Peter Traverso (Auburn University) Dr Golo Fuchert (Max-Planck Institut für Plasmaphysik) Dr Sergey Bozhenkov (Max-Planck Institut für Plasmaphysik) Dr Hannes Damm (Max-Planck Institut für Plasmaphysik) Dr Ekkehard Pasch (Max-Planck-Institute for Plasma Physics) Dr Kai Jakob Brunner (Max-Planck Institut für Plasmaphysik) Dr Jens Knauer (Max-Planck Institut für Plasmaphysik) Dr Marc Beurskens (Max-Planck Institut für Plasmaphysik) Dr Rainer Burhenn (Max-Planck-Institute for Plasma Physics) Robert Wolf (Max-Planck-Institute for Plasma Physics)

Presentation Materials