Electrostatic potential variation on the flux surface and its impact on impurity transport

TL;DR

This paper investigates how electrostatic potential variations on flux surfaces influence impurity transport in magnetically confined plasmas, revealing significant effects in some stellarators and minimal impact in others, challenging standard neoclassical theories.

Contribution

It demonstrates the impact of flux surface electrostatic potential variations on impurity transport, providing insights that could explain experimental discrepancies in stellarator devices.

Findings

In LHD, $ ext{ extPhi}_1$ strongly modifies impurity transport.

In Wendelstein 7-X, $ ext{ extPhi}_1$ has negligible effect.

In TJ-II, $ ext{ extPhi}_1$ moderately influences impurity behavior.

Abstract

The particle transport of impurities in magnetically confined plasmas under some conditions does not find, neither quantitatively nor qualitatively, a satisfactory theory-based explanation. This compromise the successful realization of thermo-nuclear fusion for energy production since its accumulation is known to be one of the causes that leads to the plasma breakdown. In standard reactor-relevant conditions this accumulation is in most stellarators intrinsic to the lack of toroidal symmetry, that leads to the neoclassical electric field to point radially inwards. This statement, that the standard theory allows to formulate, has been contradicted by some experiments that showed weaker or no accumulation under such conditions \cite{Ida_pop_16_056111_2009, Yoshinuma_nf_49_062002_2009}. The charge state of the impurities makes its transport more sensitive to the electric fields. Thus, the short length scale turbulent electrostatic potential or its long wave-length variation on the flux surface $\Phi_{1}$ -- that the standard neoclassical approach usually neglects -- might possibly shed some light on the experimental findings. In the present work the focus is put on the second of the two, and investigate its influence of the radial transport of C$^{6+}$. We show that in LHD it is strongly modified by $\Phi_{1}$, both resulting in mitigated/enhanced accumulation at internal/external radial positions; for Wendelstein 7-X, on the contrary, $\Phi_{1}$ is expected to be considerably smaller and the transport of C$^{6+}$ not affected up to an appreciable extent; and in TJ-II the potential shows a moderate impact despite of the large amplitude of $\Phi_1$ for the parameters considered.