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

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/ν-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 Z eff of order unity can affect the bootstrap current enough to cause a deviation in the divertor strike point locations.

Impurity transport and bulk ion flow in a mixed collisionality stellarator plasma

“…Note that numerical indications of temperature screening were already seen in Mollén et al. (2015), and the analysis presented here and summarised in Helander et al. (2017 a ) provides an explanation of those results.…”

“…where the contribution from the drift term in eq. (2.11) gave rise to the known geometry function (Nakajima et al 1989;Helander et al 2011)…”

Parallel impurity dynamics in the TJ-II stellarator

“…In this condition, the impurity density peaking (i.e. the normalised inverse impurity density gradient) expected from the neoclassical balance is found to be considerably large for relevant conditions and impurity species of next generation stellarator devices like W7-X [1,2]. In present devices the accumulation can limit the discharge duration [3], but operational conditions have been found in which the plasma impurity content is kept down to affordable levels to maintain a stable plasma discharge [4,5].…”

The importance of the classical channel in the impurity transport of optimized stellarators

“…Specifically, we will look at a simulated W7-X standard configuration case at the radial location , with and impurity parameters , , studied by Mollén et al. (2015). The normalized radius is defined as , with the toroidal flux and its value at the last-closed flux surface.…”

“…For this study, we will look at two stellarator configurations, where the neoclassical transport coefficients have been calculated across a wide range of collisionalities. Specifically, we will look at a simulated W7-X standard configuration case at the radial location r N = 0.88, with T = 1 keV and impurity parameters Z = 6, Z eff = 2.0, studied by Mollén et al (2015). The normalized radius is defined as r N = √ ψ t /ψ t,LCFS , with ψ t the toroidal flux and ψ t,LCFS its value at the last-closed flux surface.…”