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

Role of 𝑬 × 𝑩 velocity shear for triggering the I-mode and ion ITB on HL-2A
A.S. Liang1, *, X.L. Zou2, W.L. Zhong1, G.L. Xiao1, R. Ke1, X.X. He1, Z.J. Li1, M. Jiang1, Z.C.
Yang1, P.W. Shi1, J. Wen1, G.Q. Xue1,3, Y.R. Zhu1, R.H. Tong1, B.D. Yuan1, J. Yin1, L.Z. Liu1, Z.Y.
Yang1, B. Li1, W. Chen1, D.L. Yu1, Z.B. Shi1, M. Xu1 and X.R. Duan1
1Southwestern Institute of Physics
2CEA, IRFM
3
Dalian University of Technology
E-mail (speaker): liangas@swip.ac.cn
The improved energy confinement mode (I-mode),which shows high energy confinement comparable toH-mode, and particle confinement comparable to
L-mode, is one of the promising operation regimes
achieved in tokamaks. The I-mode features an edge
temperature transport barrier without a particle transportbarrier. It is characterized by a steep temperature
gradient comparable to the H-mode, while the density
gradient remains similar to the L-mode. Comparing toH-mode, I-mode can avoid the accumulation ofimpurities and Helium ’ash’ due to the absence of anedge particle transport barrier, which is beneficial for
fusion reactors. Moreover, it has been found that theI-mode pedestal is ideal peeling-ballooning stable, i.e.,
no ELMs are expected. Therefore, I-mode is being
investigated as an alternative operation scenario for
future fusion devices.
The I-mode plasmas were observed for the first timeon the HL-2A tokamak [2]. As shown in Figure 1(a), theI-mode is obtained in NBI-heated plasmas and theplasma density is basically unchanged after L-mode toI-mode (L-I) transition. However, the edge electron
temperature gradient increased dramatically, forming anedge electron temperature transport barrier, as shown in
Figure 1(b). The weakly coherent mode (WCM) can beobserved in the spectrogram of density fluctuation with afrequency range of 𝑓~60 − 160 𝑘𝐻𝑧 (Figure 1(c)). It
has been found that there is a critical value of the 𝐸 × 𝐵
velocity shear for triggering the I-mode, as shown in
Figure 1(d), and this value (~80 𝑘𝐻𝑧) is much lowerthan that for triggering the H-mode (~120 𝑘𝐻𝑧) [3].
In addition, an ion internal transport barrier (ITB) has
been observed in the I-mode plasmas, as shown in Figure1(e). The formation of this ion ITB is due to theturbulence suppression (Figure 1(g)) by the 𝐸 × 𝐵
velocity shear (Figure 1(f)). Transport analysis confirms
the existence of electron ETB (I-mode) and ion ITB.
The plasma confinement is significantly improved in
I-mode with ITB regime, which is comparable to that in
H-mode, suggesting that the I-mode with ion ITB could
be an interesting operation scenario for future fusion
devices.

Figure 1. Waveform of an I-mode discharge with ion ITB
on HL-2A: (a) NBI power (black) and central
line-averaged electron density (blue), (b) electron
temperature gradient 𝛻𝑇𝑒 at edge region (𝑟/𝑎 = 0.9), (c)spectrogram of density fluctuation measured by BES at
𝑟/𝑎 = 0.90, (d) 𝐸 × 𝐵 velocity shear at 𝑟/𝑎 = 0.90,(e) ion temperature gradient in the plasma core (𝑟/𝑎 =0.4 ), (f) 𝐸 × 𝐵 velocity shear at 𝑟/𝑎 = 0.45 , (g)turbulence spectrogram measured by Doppler
reflectometry in plasma core (𝑟/𝑎 = 0.45), and (h) theturbulence intensity
References[1] A.S. Liang et al 2023 Nucl. Fusion 63 056017[2] A.S. Liang et al 2020 Nucl. Fusion 60 092002