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Verification of Energetic-Particle-Induced Geodesic Acoustic Mode in Gyrokinetic Particle Simulations
Yang Chen, Wenlu Zhang, Jian Bao, Zhihong Lin, Chao Dong, Jintao Cao, and Ding Li
Chin. Phys. Lett.    2020, 37 (9): 095201 .   DOI: 10.1088/0256-307X/37/9/095201
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The energetic-particle-induced geodesic acoustic mode (EGAM) is studied using gyrokinetic particle simulations in tokamak plasmas. In our simulations, exponentially growing EGAMs are excited by energetic particles with a slowing-down distribution. The frequencies of EGAMs are always below the frequencies of GAMs, which is due to the non-perturbative contribution of energetic particles (EPs). The mode structures of EGAMs are similar to the corresponding mode structures of GAMs. Our gyrokinetic simulations show that a high EP density can enhance the EGAM growth rate, due to high EP free energy, and that EPs' temperature and the pitch angle of the distribution modify the EGAM frequency/growth rate by means of the resonance condition. Kinetic effects of the thermal electrons barely change the EGAM frequency, and have a weak damping effect on the EGAM. Benchmarks between the gyrokinetic particle simulations and a local EGAM dispersion relation exhibit good agreement in terms of EGAM frequency and growth rate.
Temperature Gradient, Toroidal and Ion FLR Effects on Drift-Tearing Modes
Hao Shi, Wenlu Zhang, Chao Dong, Jian Bao, Zhihong Lin, Jintao Cao, and Ding Li
Chin. Phys. Lett.    2020, 37 (8): 085201 .   DOI: 10.1088/0256-307X/37/8/085201
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The influences of the temperature gradient and toroidal effects on drift-tearing modes have been studied using the Gyrokinetic Toroidal code. After the thermal force term is introduced into the parallel electron force balance equation, the equilibrium temperature gradient can cause a significant increase in the growth rate of the drift-tearing mode and a broadening of the mode structure. The simulation results show that the toroidal effects increase the growth rate of the drift-tearing mode, and the contours of the perturbation field “squeeze” toward the stronger field side in the poloidal section. Finally, the hybrid model for fluid electrons and kinetic ions has been studied briefly, and the dispersion relation of the drift-tearing mode under the influence of ion finite Larmor radius effects is obtained. Compared with the dispersion relation under the fluid model, a stabilizing effect of the ion finite Larmor radius is observed.
Interface Width Effect on the Weakly Nonlinear Rayleigh–Taylor Instability in Spherical Geometry
Yun-Peng Yang, Jing Zhang, Zhi-Yuan Li, Li-Feng Wang, Jun-Feng Wu, Wun-Hua Ye, and Xian-Tu He
Chin. Phys. Lett.    2020, 37 (7): 075201 .   DOI: 10.1088/0256-307X/37/7/075201
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Interface width effect on the spherical Rayleigh–Taylor instability in the weakly nonlinear regime is studied by numerical simulations. For Legendre perturbation mode $P_n$ with wave number $k_n$ and interface half-width $L$, the commonly adopted empirical linear growth rate formula $\gamma_n^{\rm em}(L)=\gamma_n/\sqrt{1+k_nL}$ is found to be sufficient in spherical geometry. At the weakly nonlinear stage, the interface width affects the mode coupling processes. The development of the mode $P_{2n}$ is substantially influenced by the interface width. Moreover, the nonlinear saturation amplitude increases with the interface width.
Terahertz Radiation from a Longitudinal Electric Field Biased Femtosecond Filament in Air
Yi Liu, Shaojie Liu, Aurélien Houard, André Mysyrowicz, Vladimir T. Tikhonchuk
Chin. Phys. Lett.    2020, 37 (6): 065201 .   DOI: 10.1088/0256-307X/37/6/065201
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The terahertz (THz) temporal waveform and spectrum from a longitudinal electrically biased femtosecond filament is studied experimentally. The initial direction of the electron motion inside the unbiased filament plasma is deduced from the transformation of the THz temporal waveform with applied fields of opposite polarities. Furthermore, a spectrum shift to lower frequency of the THz spectrum is observed in the presence of a biased field. It agrees well with theoretical predictions.