Impurities in a non-axisymmetric plasma: transport and effect on bootstrap current

TL;DR

This paper uses a new kinetic solver to analyze impurity transport and its impact on bootstrap current in stellarator plasmas, revealing key scaling behaviors and effects of impurity content.

Contribution

It introduces the SFINCS code for detailed impurity transport calculations and compares results with theoretical asymptotes, highlighting new impurity transport scalings.

Findings

1/nu-scaling of inter-species radial transport at low collisionality

Radial electric field suppresses certain impurity transport scalings

Impurity content significantly affects bootstrap current and divertor strike points

Abstract

Impurities cause radiation losses and plasma dilution, and in stellarator plasmas the neoclassical ambipolar radial electric field is often unfavorable for avoiding strong impurity peaking. In this work we use a new continuum drift-kinetic solver, the SFINCS code (the Stellarator Fokker-Planck Iterative Neoclassical Conservative Solver) [M. Landreman et al., Phys. Plasmas 21 (2014) 042503] which employs the full linearized Fokker-Planck-Landau operator, to calculate neoclassical impurity transport coefficients for a Wendelstein 7-X (W7-X) magnetic configuration. We compare SFINCS calculations with theoretical asymptotes in the high collisionality limit. We observe and explain a 1/nu-scaling of the inter-species radial transport coefficient at low collisionality, arising due to the field term in the inter-species collision operator, and which is not found with simplified collision models even when momentum correction is applied. However, this type of scaling disappears if a radial electric field is present. We also use SFINCS to analyze how the impurity content affects the neoclassical impurity dynamics and the bootstrap current. We show that a change in plasma effective charge Zeff of order unity can affect the bootstrap current enough to cause a deviation in the divertor strike point locations.