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

Characteristics of global micro-instabilities in L-mode states consistent withJT-60U reversed magnetic shear plasmas
Rui ZHAO1, Akihiro ISHIZAWA1,2, Kenji IMADERA1,2, Yasuaki KISHIMOTO1,2,3
1
Graduate School of Energy Science, Kyoto University
2
Nonlinear/Non-equilibrium Plasma Science Research UNIT, Kyoto University
3
Institute of Advanced Energy, Kyoto University
e-mail (speaker): zhao.rui.27d@st.kyoto-u.ac.jp

The formation of internal transport barrier (ITB) is akey to achieving high performance fusion plasmas inmagnetically confined devices, such as tokamak. In JT60U experiments in reversed magnetic shear (RS)configuration using neutral beam (NB) injection, a heating
power threshold is found to exist for triggering ITBs,
under which plasmas show L-mode characteristics withstrong profile stiffness [1, 2]. To study such ITB formation,
we first need to understand the characteristic of L-mode
plasma in such RS configurations, especially the freeenergy source that drives linear instability. In this work,by using the 5D global gyro-kinetic code GKNET [3, 4],
we investigated the linear instabilities in JT-60U L-mode
RS plasmas. Here, we found two different branches ofdrift modes excited in different radial locations, which
regulate nonlinear dynamics causing profile stiffness.
The obtained linear dispersion relations, i.e. (a) real
frequency, (b) growth rate, and (c) mode radial location,with respect to the toroidal mode number 𝑛, are shown in
Fig.1. In Fig.1 (c), corresponding 𝑘𝜃 𝜌𝑖 values are shownto study the mode characteristics. Here, two different
branches which are excited in different radial locations,i.e., one is inside region with higher safety factor 𝑞values, the other is outside minimum 𝑞 (𝑞𝑚𝑖𝑛 ) region,are found to appear. Between the inside and outside
branch, there exists a “gap”. The modes in the twobranches all rotate in the ion diamagnetic direction (𝜔 <0). It is also found that the inside branch approximately
follows the line satisfying 𝑘𝜃 𝜌𝑖 = 0.2~1.0 , while theoutside branches weakly depend of 𝑘𝜃 𝜌𝑖 indicating thatthey are non-resonant infernal type modes. Note that theinside mode with lower toroidal mode number 𝑛 = 2with 𝑘𝜃 𝜌𝑖 = 0.4 exhibits the maximum growth rate,
which decreases with 𝑛 by shifting slowly outside withincreasing real frequency in the ion direction.
Fig. 2 shows corresponding mode structures to 𝑛 = 2
and 𝑛 = 30 mode in Fig. 1, which are the most unstablemode for the inside branch and outside branch,respectively. It is found that 𝑛 = 2 mode owes a large
poloidal mode number due to the high 𝑞 value, and 𝑛 =30 mode is highly localized at 𝑞𝑚𝑖𝑛 region.
In addition, we study the mode characteristics bychanging the temperature and density profiles
independently to identify the mode characteristics. To bespecific, the inside branches are considered to the pressure
driven modes, which are excited by both ion and density
gradient, while the outside branches are the ion
temperature gradient driven infernal type modes, whichshow a weak dependence with respect to the density
gradient. The special characters of the modes are strongly

related to the 𝑞 profile, especially the 𝑞𝑚𝑖𝑛 location.
Further study is carried out to investigate the two branches
nonlinear evolution and turbulence transport.
References[1] Shirai, H., et al. Nucl. Fusion 39, 1713.
[2] Kin, F., et al. Nucl. Fusion 63, 016015.
[3] Kishimoto, Y., et al. Philos. Trans. R. Soc. London,
Ser. A 381, 20210231.
[4] Imadera, K., et al. Plasma Phys. Controlled Fusion
65, 024003.
FiguresFig. 1. (a), (b) The relation between real frequency 𝜔,
growth rate 𝛾 to the toroidal mode number 𝑛 (c) The
center location 𝑟 of each toroidal mode.

Fig. 2. (a), (b) The 𝑛 = 2, 30 mode structures inpoloidal cross section, where the black dashed circle
represents 𝑞𝑚𝑖𝑛 location.