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