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

Impact of Safety Factor Curvature on Mode Characteristics andTransport in Reversed Shear Plasmas with Internal Transport Barrier
LIU Jianfu1, ISHIZAWA Akihiro1,2,3, IMADERA Kenji1,2,3, KISHIMOTO Yasuaki1,2,3
1

GSES, Kyoto Univ., 2Plasma Science UNIT, Kyoto Univ., 3IAE, Kyoto Univ.
e-mail (speaker): riu.jianfu.28n@st.kyoto-u.ac.jp

In tokamak operations, the bootstrap current might
have an off-center maximum leading to a reversed shear
configuration in which safety factor (𝑞) profile has alocal minimum. This reversal fundamentally modifies theplasma behavior by breaking the ballooning coupling,promoting the formation of a strong zonal flow, andinfluencing momentum transport [1]. Furthermore, it isintimately related to the internal transport barrier (ITB)
formation which significantly enhances tokamak
confinement. In this study, using δf electromagnetic
version of the GKNET code with kinetic electrons, we
simulated 3 plasmas with weak reversal, moderate
reversal, and strong reversal in safety factor profile bothin the linear and nonlinear regimes [2,3], to investigate
the impact of curvature on the mode characteristics andthe associated turbulent transport in reversed shear
plasmas with a strong ITB established in the zero-shear
region.
In linear analyses, we have identified the infernal
type of MHD mode is induced in the bad curvature
region. As shown in figure 1, we found that withincreasing curvature, mode location approaches theminimum 𝑞 surface from the outside and the mode
width shrinks. It is also evident that curvature effectively
suppresses fluctuations, making the mode structure morepredictable. In the contrast, the impact on the dispersion
relation is either indirect or minute. Fluctuation observedin real frequency is shown to be strongly correlated withfluctuation in mode location.
The nonlinear evolution of mode could be roughly
divided into 3 phases. In phase 1, modes growexponentially; in phase 2, temporary saturation isestablished in terms of energy and this phase is ourprimary focus; in phase 3, saturation is broken as a resultof temperature relaxation and the mode nature changes.
As in Figure 2, we showed that saturation potential ofturbulence decreases with curvature. Also, weinvestigated the expansion of mode in phase 2 bydefining the inner and other turbulent fronts, within
which 70% of turbulence energy is confined. we foundthe difference in expansion rate is almost entirely due tothe expansion of the outer front which is more rapid for a
configuration with a less degree of curvature. In addition,
we illustrated transports are to a large extent via burst
which is periodic for the moderately reversed andstrongly reversed cases and intermittent for the weakly
reversed configuration. Finally, the degrees oftemperature relaxation and density steepening decrease
with increasing curvature.

Figure 1: Toroidal number spectra of (a) mode
location, (b) mode width, (c) real frequency, and (d)growth rate for plasmas with weak reversal,moderate reversal, and strong reversal.

Figure 2: Time evolutions of (a) space-averaged electric
potential of zonal flow (dashed line) and turbulence (solidline), and (b) locations of inner and outer turbulent fronts
for plasmas with weak reversal, moderate reversal, andstrong reversal.

References[1] Y. Ishida, et al., PoP 27, 092302 (2020)[2] A. Ishizawa, et al., PoP 26, 082301(2019)[3] K. Imadera, et al., in IAEA-FEC(Kyoto, 2016), p.
TH/P3-3.