Speaker
Hiroshi Yamada
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1084.pdf
Characterization of Isotope Effect
on Confinement of NBI-Heated Plasmas on LHD
H. Yamada1,2, K.Tanaka1,3, T.Tokuzawa1, R.Seki1,4, C.Suzuki1, M.Yokoyama1,4,
K.Ida1,4, M.Yoshinuma1, K.Fujii5, S.Murakami5 and LHD Experiment Group
1
National Institute for Fusion Science/NINS, Toki, Japan
2
The Univ. Tokyo, Kashiwa, Japan, 3 Kyushu Univ., Kasuga, Japan,
4
SOKENDAI, Toki, Japan, 5Kyoto Univ., Kyoto, Japan
Energy confinement and thermal transport has been widely regarded as gyro-Bohm in
tokamak as well as stellarator-heliotron for a single kind of ion. However, this gyro-Bohm
model predicts confinement degradation in deuterium (D) plasmas because of larger
normalized gyro radius * than in hydrogen (H) plasmas, which conflicts with major
experimental observations. This study aims to quantify a peculiarity in dependence on * in
H and D plasmas in order to address this unresolved issue. The first deuterium plasma
campaign in LHD reveals characteristics of isotope effect from elaborated experiments on
NBI-heated plasmas. Thermal energy confinement time gives the regression expression
scaling with the isotope mass (A) as A0.15, which shows moderate improvement in D plasmas.
This positive isotope dependence contradicts with gyro-Bohm and is similar to the recent
result from L-mode plasmas in JET-ILW. Operational flexibility of magnetic field, density,
and heating power enables adjustment of three major non-dimensional parameters, those
being *, collisionality * and , and dimensionally similar plasmas of H and D in all these
three parameters can be obtained. Then TASK3D-a / FIT3D is used for analysis of heating
power deposition, power balance and local thermal transport. If gyro-Bohm nature
predominates in these plasmas, thermal diffusivity normalized by Bohm diffusion should be
the same in a pair of dimensionally similar plasmas of H and D. Different characteristics from
this conjecture have been found in electron and ion loss channels. Electron heat diffusivity
normalized by Bohm diffusion in H is lower than that in D and even lower by more than a
factor of 1/2 which means net improvement. This trend is robust and insensitive to
parameters such as *, *, and Ln. In contrast, ion thermal diffusivity shows same
characteristics in low collsionality regime while that in D compared with the case with H
degrades with the increase of collsionality. These results have shown definitively that the
gyro-Bohm nature is violated in the comparison of H and D plasmas in LHD.
