Speaker
Håkan Smith
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1092.pdf
Neoclassical transport in the High density H-mode in Wendelstein 7-AS –
revisited with new tools
H. M. Smith, A. Mollén, C. D. Beidler
Max Planck Institute for Plasma Physics, Greifswald, Germany
In view of the aim for long-pulse operation of Wendelstein 7-X, it is important to understand
under which circumstances one can expect to avoid impurity accumulation. Unless the impu-
rity sources at the edge were kept small in Wendelstein 7-AS (W7-AS) normal confinement
(NC) plasmas, impurities often accumulated in the centre which resulted in a radiation collapse
that terminated the discharge [1]. However, this was avoided in so-called high-density H-mode
(HDH) plasmas, which were NBI heated and characterised by a density exceeding a certain
heating-power-dependent threshold (1.5 − 2.1 · 1020 m−3 ). The transport in the HDH regime
was analysed in Ref. [2], but the experimentally observed efficient flush-out of impurities could
not be explained by neoclassical transport, and there was no definite experimental evidence for
turbulent mode activity at the plasma edge. Recently, analytical work [3, 4] has shown that when
the impurities are in the highly collisional Pfirsch-Schlüter regime and the main ions in the long
mean free path regime, neoclassical “temperature screening” (outward flux of impurities driven
by the temperature gradient) can prevent accumulation in stellarators, even when the radial
electric field points inwards. To include this effect in a numerical analysis of the neoclassical
impurity transport in the W7-AS HDH mode, one needs a more detailed physics model than was
used in previous investigations. In this work, we therefore use the SFINCS code [5, 6], which
enables us to include the full linearised Fokker-Planck collision operator as well as the variation
of the electrostatic potential on the flux surface. The SFINCS results are compared with results
from the DKES code [7], which employs a pitch-angle scattering collision operator.
References
[1] R. Burhenn, Y. Feng, K. Ida, H. Maassberg et al., Nucl. Fusion 49, 065005 (2009).
[2] R. Burhenn, J. Baldzuhn, R. Brakel, H. Ehmler et al., Fusion Science and Technology 46, 115 (2004).
[3] P. Helander, S. L. Newton, A. Mollén and H. M. Smith, PRL 118, 155002 (2017).
[4] S. L. Newton, P. Helander, A. Mollén and H. M. Smith, J. Plasma Phys. 83 905830505 (2017).
[5] M. Landreman, H. M. Smith, A. Mollén and P. Helander, Phys. Plasmas 21 042503 (2014).
[6] A. Mollén, M. Landreman, H. M. Smith, S. Braun and P. Helander, Phys. Plasmas 22 112508 (2015).
[7] S. P. Hirshman, K. C. Shaing, W. I. van Rij, C. O. Beasley, Jr., and E. C. Crume, Jr., Phys. Fluids 29, 2951
(1986).
