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

Investigation of turbulence transition and its threshold in LHDToshiki Kinoshita1, Kenji Tanaka2,3, Tatsuya Yokoyama4, Akihiro Ishizawa5, Hikona Sakai3
1
Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan
2
National Institute for Fusion Science, National Institutes of Natural Sciences, Toki, Gifu 5095292, Japan
3
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka
816-8580, Japan
4
National Institutes for Quantum and Radiological Science and Technology, Naka, Ibaraki, 3110193, Japan
5
Graduate School of Energy Science, Kyoto University, Uji, Kyoto, 611-0011, Japane-mail (speaker):t.kinoshita@triam.kyushu-u.ac.jp
Ion-scale turbulences play a key role in transport
processes in magnetically confined plasmas. In tokamaks,
trapped electron mode (TEM) and ion temperature
gradient mode (ITG), have been reported to realize
different confinement properties such as linear-ohmic
confinement
andsaturated-ohmic
confinement,
respectively[1]. Therefore, it is important to understandturbulence mode to improve plasma confinement.
In order to investigate a comprehensive nature of ionscale turbulence in the Large Helical Device (LHD),
firstly, the systematic density scan experiments inhydrogen (H) and deuterium(D) plasmas were conducted.
Figure 1 (a) and (b) show the line-averaged electron
density dependence of ion-scale turbulence level𝑛𝑛�𝑒𝑒 ⁄𝑛𝑛𝑒𝑒 (i.e., the local turbulence amplitude normalized by
the local electron density) and its phase velocity on aplasma frame 𝑉𝑉𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 (i.e., subtract E × B poloidal
rotation velocity from phase velocity in the laboratoryframe). In this study, the transition density 𝑛𝑛𝑡𝑡𝑡𝑡 isdefined to be the density when the turbulence level
become a minimum, as indicated by the dashedlines in Fig. 1. The transition densities in the H and D~1.6 × 1019 m−3
and~2.6 ×
plasmas are
19 −3
10 m , respectively. In terms of isotope effects,
there are negligible and clear isotope effects in lowdensity (𝑛𝑛�𝑒𝑒 < 𝑛𝑛𝑡𝑡𝑡𝑡 ) and high-density (𝑛𝑛�𝑒𝑒 > 𝑛𝑛𝑡𝑡𝑡𝑡 ) regimes,respectively. Simultaneously, as shown in Fig. 1(b),the turbulence in the low- and high-density regime
propagates to the ion-diamagnetic (i-dia) and theelectron-diamagnetic(e-dia)
direction.
Asmentioned above, the opposite density dependences,different isotope effects and propagation direction
indicate different turbulence modes were excited ineach density regime. To identify turbulence modes, thedependence of measured turbulence on plasma parameters
and the density dependence of the linear growth rateevaluated in two-types of simulations were investigated.
As a result, it was found that the turbulence propagatingin the i-dia direction in the low-density regime is ITG, andin the e-dia direction in the high-density regime is resistive
interchange (RI) turbulence.
Secondly, a transition threshold between ITG and RI
turbulences has been evaluated. This is because thetransition threshold is essential information not only forknowing the current turbulence mode, but also for
conducting experiments with suppressed anomaloustransport. As shown in Fig.1 (a), the higher transition
density in the D plasma indicates the turbulence transition
cannot be detected with only electron density. Figure 1(c)shows the density dependence of the normalized density
gradient, which shows a similar trend to that of theturbulence level, indicating that it contributes toturbulence destabilization regardless of ITG or RI
turbulence. On the other hand, Fig. 1(d) shows the density
dependence of the ion-electron temperature ratio, whichshows different dependence on the turbulence level in the
low-density ITG and high-density RI regimes,respectively. That is, the turbulence transitions aredescribed by a complex combination of plasma
parameters, including ion masses.
In this presentation, turbulent transitions in LHD will
be introduced including identification studies and theirthresholds will be discussed.
[1] J. Rice et al., Nuclear Fusion 60, 105001 (2020).

Figure 1 Line-averaged electron density dependence of(a) the ion-scale turbulence level, (b) phase velocity inthe plasma frame, (c) normalized density gradient and(d) ion-electron temperature ratio. These values areaveraged values at normalized radii ρ = 0.5– 0.7.