Turbulent transport of impurities

Impurity accumulation is considered to be a crucial issue in the path towards a fusion-grade stellarator reactor. The reason is that stellarator operation scenarios usually display a negative radial electric field that, according to standard neoclassical theory, should cause an inwards impurity pinch roughly proportional to the charge number Z. This impurity accumulation is observed in the experiment, with very few exceptions [Burhenn-09]. One of these exceptions is the so-called “impurity hole”, observed at low collisionalities in the Large Helical Device, in which a negative radial electric field coexists with a strong outwards- directed impurity pinch [Ida-09]; other well-known exception is the High Density H-mode of Wendelstein 7-AS [McCormick-02]. With the goal of understanding these scenarios, as a necessary step prior to trying to reproduce them in a future stellarator reactor, we have implemented several extensions to neoclassical theory in numerical codes [García-Regaña- 13, García-Regaña-17, Calvo-17, García-Regaña-18, Mollén-18, Velasco-18] (see also [Helander-17]). However, none of the simulations performed so far has been able to explain e.g. the impurity hole. In parallel, as mentioned before, first gyrokinetic simulations of impurity transport in stellarators have been performed [Mikkelsen-14, Nunami-16], also unsuccessfully. We note that in these works, the simulation domain was a flux tube, and the neoclassical background was not considered; however, for example, the low-collisionality plasmas that display impurity hole are in what we have called “deep √ν regime” [Velasco-17].

The objective of this task is:

  • Determine the sophistication in the treatment of the geometry and equilibrium distribution functions that is required to accurately model the transport of light and heavy impurities in low ion-collisionality stellarator plasmas. We will perform the first turbulent simulations that include these sophistications. A systematic comparison of the so-calculated turbulent fluxes with neoclassical ones is an expected outcome of this activity.

 

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