7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Impurity neoclassical transport analysis in LHD Plasma
S. Satake1, M. Nishiura1, A. Shimizu1 and LHD Experiment group1
1National Institute for Fusion Science, Toki, Gifu, Japane-mail (speaker):satake.shinsuke@nifs.ac.jp
In LHD, spontaneous expel of impurity ions such as C6+from the center of the plasma is repeatedly observed inNBI-heated, relatively high-Ti discharges, which iscalled “impurity hole” plasma [1]. In conventional
neoclassical transport theory, which is based on theradially-local approximation, could not explain theoutward impurity flux. Gyrokinetic turbulencesimulation predicted inward impurity flux either, when
the impurity density profile becomes hollow. Therefore,
some other mechanisms are expected to cancel the
inward turbulent flux to make and sustain the hollow
impurity density profile.
In recent years, we have developed a new neoclassicalcode FORTEC3D-MPS [2,3], which is non-local andapplicable to multi-ion-species plasmas in 3-dimensional
magnetic configurations. It also includes the effect of thepotential variation on a flux surface, or the 𝜙𝜙1 -effect,
which is important to calculate the neoclassical transportof high-Z ions. Applying the simulation code to
impurity-hole plasma analysis, some of new features
have been found [2,4] as follows:
1: The ambipolar radial electric field (𝐸𝐸𝑟𝑟 ) profile isnegative near the magnetic axis but changes signs topositive at around r=0.2-0.3a, though conventional local
calculation predicted all negative 𝐸𝐸𝑟𝑟 profile.
2: The negative-to-positive 𝐸𝐸𝑟𝑟 profile agrees with theobservation by HIBP [5].
3: The carbon impurity flux directs outward even in thenegative-𝐸𝐸𝑟𝑟 region. The ion temperature gradient isfound to act as outward driving force of impurity flux.
4: The variation of 𝜙𝜙1 potential enhances the outward
impurity neoclassical flux, and this enhanced outward
flux balances with inward turbulent flux in theimpurity-hole state.
These findings by the precise neoclassical simulation
can explain the mechanism of the impurity hole.
However, there are still several things that have notexplained by the numerical simulation. For example, it isobserved that the impurity hole is more easily created in
the magnetic configuration in which magnetic axis isshifted outward, for example 𝑅𝑅𝑎𝑎𝑎𝑎 = 3.75m case than
3.60m case. Since neoclassical transport strongly
depends on the magnetic configuration, it is expectedthat the formation of impurity hole is correlated with theneoclassical transport dependence on the magnetic axis
position. Also, experimentally it is found that heavier ion

species, such as neon, creates deeper and more clear
hollow impurity density hole than lighter species ascarbon. This fact also suggests the correlation between
impurity hole and the ion-species dependence ofneoclassical transport.
To confirm these dependences found in the impurity
hole experiments by the neoclassical transport simulation,
we carry out several simulations by FORTEC3D-MPS in
different magnetic configurations and particle species. To
validate the simulations, we also carried out
impurity-hole experiments in different conditions in
2021 and 2022. In the previous impurity hole
experiments, radial electric field in the core region havebeen measured only in a few shots. Therefore, in theseries of experiments we measured the core radial
electric field profile by HIBP system. We found that thenegative-to-positive 𝐸𝐸𝑟𝑟 profile found in 𝑅𝑅𝑎𝑎𝑎𝑎 = 3.60m
case also appeared in other configuration cases.
Comparing the neoclassical carbon ion fluxes simulated
in different magnetic configurations, it is found that thecarbon outward flux is larger for outward-shiftedconfiguration. It is consistent with the fact that thecarbon hollow density profile develops faster in theoutward-shifted configuration experimentally. Also, wefind that the particle flux amplitude estimated from theobserved time change in density profile is the order as
the simulated neoclassical flux. These findings support
that the neoclassical impurity flux acts the key role in theformation of impurity hole.
In the presentation, more detailed analysis andcomparisons between simulations and observation will
be reported.
References[1] K. Ida et al., Physics of Plasmas 16, 56111(2009)[2] K. Fujita et al., Plasma and Fusion Research 14,3403102 (2019)[3] S. Satake et al., Comp. Phys. Comm. 255 107249(2020)[4] K. Fujita et al., Nuclear Fusion vol. 61, 086025(2021)[5] T. Ido et al., Plasma Phys. Control. Fusion 52124025 (2010)