Bursting core-localized ellipticity-induced Alfvén eigenmodes driven by energetic electrons during EAST ohmic discharges

Pengjuan Su, Heng Lan, Chu Zhou, Jian Bao, A. Liu, Xiang Zhu, Mingfu Wu, Chenxi Luo, Jiangyue Han, Jialei Wang, Haiqing Liu, Tao Zhang, Ruijie Zhou, Shiyao Lin, Hailing Zhao, Youwen Sun, Z. Qiu, N. Chu, T. Tang, Yi Tan, Long Zeng, Zhe Gao
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Abstract

A series of high-frequency (400~1000 kHz) bursting core-localized Alfvén instabilities have been observed during ohmic discharges in EAST tokamak. The instability trigger favours the discharge conditions of low toroidal magnetic field and low electron density. The toroidal mode numbers are mainly n=2~3 and they propagate in the ion diamagnetic drift (co-current) direction. These modes are radially localized in the range of ρ_tor=0.2~0.35 based on Doppler BackScatter (DBS) measurement. They are identified as ellipticity-induced Alfvén eigenmodes (EAEs) occurring at q=1 rational surfaces by magnetohydrodynamics (MHD) simulations using the realistic geometry and plasma profiles. The EAEs show regular bursts with ~10 milliseconds duration along with the mode frequency chirping downwards and upwards rapidly. It is also found that sawtooth events can interrupt the growth and evolution of the EAEs, causing the modes to disappear immediately. Passing energetic electrons (EEs) that move much faster than Alfvén velocity are responsible for the destabilization of these EAEs, which attribute to the fact that the large poloidal and toroidal frequencies mostly cancel each other and satisfy the EAE resonance condition with primary energy exchange. These novel experimental results of the wave-particle interaction between EAEs and EEs are helpful for extrapolating alpha particle physics that are characterized by small orbit width with respect to machine size in future fusion reactors.
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在 EAST 欧姆放电过程中,由高能电子驱动的迸发核心定位椭圆性诱导阿尔费文特征模式
在EAST托卡马克的欧姆放电过程中,观测到了一系列高频(400~1000 kHz)迸发的核心局部阿尔弗韦恩不稳定性。触发不稳定的条件是低环形磁场和低电子密度。环形模式数主要为 n=2~3,沿离子二磁漂移(同流)方向传播。根据多普勒反向散射(DBS)测量结果,这些模式在ρ_tor=0.2~0.35范围内径向定位。利用现实几何和等离子体剖面进行的磁流体力学(MHD)模拟将它们确定为发生在 q=1 理性表面的椭圆性诱导阿尔弗恩特征模(EAEs)。EAEs 显示出持续时间约为 10 毫秒的规律爆发,同时模式频率快速向下和向上鸣叫。研究还发现,锯齿事件会打断 EAE 的增长和演化,导致模式立即消失。通过的高能电子(EEs)的运动速度远高于阿尔弗韦恩速度,是这些 EAEs 失稳的原因,这归因于大极坐标频率和环坐标频率大多相互抵消,并满足主要能量交换的 EAE 共振条件。这些关于 EAE 和 EE 之间波粒相互作用的新实验结果有助于推断未来聚变反应堆中轨道宽度相对于机器尺寸较小的阿尔法粒子物理学。
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