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

The effects of three-dimensional magnetic perturbations and finite beta on
collissionless trapped electron mode and ion temperature gradient mode
instabilities in tokamak plasmas
Zhangsheng Huang, Weixin Guo, Lu Wang
Huazhong University of Science and Technology
e-mail: zhangshhuang@hust.edu.cnPlasmas with internal transport barriers(ITBs) have the benefit of better confinement. It has
been found that the internal kink modes with nonaxisymmetric flux surface have effects on theformation and evolution of ITBs[1,2]. In order tounderstand the corresponding mechanism, we havestudied the effects of three-dimensional magneticperturbations (3D MPs) and finite beta (𝛽, the ratio
between plasma kinetic pressure and magneticpressure) on the linear instabilities of collisionlesstrapped electron mode (CTEM) and ion temperature
gradient mode (ITG), respectively.
For the instability of CTEM, based on the local
3D equilibrium model[3], we have derived general
expressions for longitudinal invariant and thecorresponding precession drift frequency of trappedelectrons, which include the synergetic effects of 3DMPs and finite 𝛽. Then, the dispersion equation of
CTEM instability has been derived and solved byincluding the kinetic effects of trapped electrons[4].
The results show that the synergetic effects of 3DMPs and finite 𝛽 can either stabilize or destabilize
the CTEM instability due to the modification of theprecession resonance between trapped electrons andelectron drift wave. In the presence of MPs withdestabilizing phase, the synergetic effects can weakenor even reverse the reduction of precession drift
frequency by finite 𝛽 effects in two-dimensional(2D) case, and then there is more trapped electrons
resonant with the electron drift wave. This canenhance the excitation of CTEM instability, and thelinear growth rate of CTEM is thus increased. While,in the presence of MPs with stabilizing phase, thesynergetic effects can reduce the precession drift
frequency and the number of resonant trappedelectrons, and result in the suppression of CTEM
instability. Moreover, the required absolute value ofnegative magnetic shear to completely stabilize
CTEM instability could be also reduced as comparedto the case without 3D MPs.
For the ITG instability, we are studying theeffects of 3D MPs and finite beta with theconsideration of elongation for it’s impact on
turbulent transport[4]. It has been found that theimpact of 3D MPs on ITG instability which isinduced by the resonance between drift wave andmagnetic drift of ions is negligible for the position
localized in outer middle plane. While the 3D MPs
and finite beta have significant impact on the mode
structure and linear instability of ITG under thecondition of non-resonant and nonlocal position withconsideration of fluid limit, more results are underanalyzing. In conclusion, these results reveal thesynergetic effects of 3D MPs and finite 𝛽 on microinstability. This is possible to contribute for the designand control of operation scenario with highperformance plasmas in the future tokamak fusion
reactors.

References[1] B. Zhang, X. Gong, J. Qian, et al. Nuclear Fusion,
2022, 62(12): 126064[2] X. X. He, L. W. Yan, D. L. Yu, et al. Plasma Physics
and Controlled Fusion, 2021, 64(1): 015007[3] C. C. Hegna. Physics of Plasmas, 2000, 7(10): 3921
[4] Z. Huang, W. Guo, L. Wang. Nuclear Fusion, 2022,
62(6): 066044