Speaker
Stefan Buller
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1085.pdf
Collisional transport of heavy impurities with flux-surface density
variation in stellarators
S. Buller1 , H.M. Smith2 , P. Helander2 , A. Mollén2 , S.L. Newton3 , I. Pusztai1
1 Department of Physics, Chalmers University of Technology, SE-41295, Göteborg, Sweden
2 Max-Planck-Institut für Plasmaphysik, 17491 Greifswald, Germany
3 CCFE, Culham Science Centre, Abingdon, Oxon OX14 3DB, UK
Due to their ability to cause intolerable radiation losses, heavy impurities cannot be allowed in
the core of a magnetic fusion reactor. In addition, heavy impurities can have observable density
variations on flux-surfaces due to their sensitivity to any flux-surface variations in electrostatic
potential. Such density variations have been analytically shown to significantly affect the colli-
sional impurity transport in tokamaks [1, 2], while their importance in stellarators have mainly
been investigated numerically [3].
We have included impurity density flux-surface variations in an analytic calculation for a
stellarator in the mixed-collisionality regime, building on recent analytical results in stellarator
impurity transport [4, 5] – and have thus generalized the calculation in Ref. [2] to stellarator
geometry. Specifically, we have derived an expression for the radial transport of a heavy impu-
rity, using a mass-ratio expanded ion-impurity collision operator. In contrast to both the general
tokamak case, and the stellarator case with flux-function impurity density, the neoclassical flux
includes an electrostatic flux term [6], which must be retained.
We have found the neoclassical transport to be sensitive to the distribution of impurities
over the flux-surface in simple test cases with a W7-X vacuum field – even changing sign
compared to the homogeneous case when the impurity density becomes sufficiently localized
around extrema in the magnetic field strength. Interestingly, for these test cases, this effect
on the neoclassical transport is overshadowed by classical transport, which appears to be the
dominant collisional transport mechanism for a collisional species in W7-X. Work is currently
being carried out to investigate in which scenarios classical transport is relevant.
References
[1] P. Helander, et al., Physics of plasmas 5 3999 (1998).
[2] C. Angioni & P. Helander, Plasma Physics and Controlled Fusion 56 124001 (2014).
[3] J. M. García-Regaña et al., Nuclear Fusion 57 056004 (2017).
[4] P. Helander, et al., Physical Review Letters 118 155002 (2017).
[5] S.L. Newton, et al., Journal of Plasma Physics 83 5 (2017).
[6] S.P. Hirshman & D.J. Sigmar, Nuclear Fusion 21 1079 (1981), page 1098.
