28D30P
ヘリカル装置LHDとトカマク装置HL-2Aにおける周辺不純物輸送の比較
Edge Impurity Transport Study in Stochastic Layer of LHD and Scrape-off
Layer of HL-2A
M. Kobayashi1, S. Morita1, C.F. Dong1, Y. Feng2, M. Goto1, S. Masuzaki1, the LHD Experimental group1
Z. Y. Cui3, Y. D. Pan3, Y. D. Gao3, H.Y. Zhou3, J. Cheng3, P. Sun3, Q. W. Yang3 and X. R. Duan31

National Institute for Fusion Science, Toki 509-5292, Japan
Max-Planck-Institute fuer Plasmaphysik, D-17491 Greifswald, Germany
3Southwestern Institute of Physics, P. O. Box 432, Chengdu 610041, China
2significant acceleration of background plasma evenat the midplane, the result might change the picture
obtained in the present modelling.
The carbon emission measured in the stochastic
layer of LHD indicates the screening effect at highdensity region. The result can be qualitativelyinterpreted by the present modeling, although themodeling shows a slight difference in thequantitative behavior of carbon in the stochastic
layer of LHD. On the other hand, comparison of thecarbon emission profile in HL-2A with themodelling is not straightforward. It is found that theimpurity distribution in the HL-2A SOL is very
sensitive to the impurity source location. In order tointerpret the experimental observations a further
study is necessary, in particular, on the impurity
source distribution in the divertor plate and the firstwall.
Divertor source
Wall source

10HL-2A

1
LHD

0.1
0.1

(a)

1  *ion

SOL

Screening

LHD and HL-2A have distinct edge magnetic
field structures in viewpoints of the connection
length and the flux tube topology. In order to study
the effect of magnetic field geometry on the edgeimpurity transport, a comparative transport analysis
has been conducted between LHD stochastic layerand HL-2A SOL. For this purpose, the 3D edgetransport code of EMC3-EIRENE has been
implemented and profile measurements of carbon
emissions have been performed using EUV andVUV spectrometers in the both devices.
Comparison of the simulation results shows clear
difference in the screening process between LHD
and HL-2A, as shown in Fig.1. HL-2A SOL has
strong screening effect against the divertor impurity
source at high density (collisionality). But almost
no screening is available against the first wall
source due to the residual thermal force at theupstream (above the X-point) even at the highest
density just before detachment onset. LHD
stochastic layer has screening effect against bothdivertor and first wall source. But the effect isweaker than the case of divertor impurity source of
HL-2A SOL.
The effective impurity screening against the firstwall source in LHD observed in the modelling isdue to the presence of the poloidally distributed
friction dominant region formed by a large numberof poloidal turns (several hundred turns) of fieldlines in the stochastic layer before they reach thedivertor plate. In HL-2A, on the other hand, one
poloidal turn flux tube creates a poloidally
asymmetric friction dominant region, i.e. only inthe vicinity of the divertor plate. The ionization
distribution of background plasma is also closely
related to these different magnetic structures. The
acceleration mechanism of background plasma andthus impurity flow along the magnetic field line isbasically determined by the ionization distribution
in the present modelling. However, further analysis
is necessary on the flow formation, especially in thetokamak SOL. Since the recent research reports a10

imp
impFig.1 Ratio of n LCFS
/ ndowm as a function ofcollisionality obtained by the 3D numericalimp
impand n dowm
are the impurity
simulation.. n LCFS

density at LCFS and near divertor plates,
respectively, summed up over all charge states(circles: impurity released at the divertor plate,triangles: impurity released at the first wall, blackfor LHD and grey for HL-2A).