Research on the mechanism of the electromagnetic emission bursts suppressed by lower hybrid wave in tokamak plasmas

doi:10.16568/j.0254-6086.202504014

Abstract

Abstract: Intense intermittent plasma frequency emission bursts near the electron plasma frequency (ω_pe) with high-energy narrowband features have been observed in many tokamak plasmas, related to the beam instability caused by a suprathermal tail. However, the ωpe emission bursts are not seen during lower-hybrid current drive (LHCD) experiment, despite the presence of a suprathermal electron tail, and tend to appear almost immediately after the LHCD is turned off. So far, there is no physical model to give reasonable explanation for this phenomenon. In this paper, a theoretical model based on four-wave interaction under the fluid approximation is proposed. In this model, the lower-hybrid wave as a pump wave, interacts with a beam-driven wave and itself sidebands, leading to parametric damping. The nonlinear dispersion relationship of the four-wave interaction is analytically derived. The spatial distribution of the beam-driven instability near the magnetic axis of tokamak plasma is given by solving the nonlinear dispersion relation numerically. The simulation results reveal that when the power of the pump wave is relatively low, the growth rates of the beam-driven instability decrease significantly with the increase of pump wave amplitude until it becomes negative, which indicates that lower-hybrid wave can effectively reduce the beam instability, and then suppress the occurrence of ω_pe emission bursts. The simulation results are consistent with the experimental ones.

Key words: Nonlinear dispersion relations, Beam-driven wave, Plasma frequency emission burst

摘要： 在托卡马克等离子体中，普遍能观测到具有高能窄带特征的电子等离子体频率(ω_pe)附近的电磁迸发辐射，通常认为其与超热尾部电子引起的束流不稳定性有关。但在低杂波电流驱动(LHCD)实验中，即使存在超热电子尾部，也不能观测到电磁迸发辐射的发生，一旦关闭 LHCD，该辐射又立刻发生并被观测到。到目前为止尚无合适物理模型对这一物理现象给出合理的解释。提出了一个流体近似下的四波相互作用理论模型，即低杂波作为泵浦波与束流漂移波和低杂边频波相互作用发生参量衰变，并推导出四波相互作用的非线性色散关系。通过数值求解非线性色散关系，给出了束流不稳定性在托卡马克等离子体芯部的空间分布。数值模拟结果发现，在泵浦波功率较小的情况下，就可导致束流不稳定性增长速率随泵浦波振幅的增大而明显降低直至变为负。这表明低杂波可以有效抑制束流不稳定性，进而有效抑制 ω_pe 迸发辐射的发生，该模拟结果与实验结果相吻合。

关键词: 非线性色散关系, 束流漂移波, 等离子体迸发辐射

CLC Number:

O534

WANG Liu-qing, HU Ye-min, BAI Shu-hang, LIU Yong. Research on the mechanism of the electromagnetic emission bursts suppressed by lower hybrid wave in tokamak plasmas[J]. Nuclear Fusion and Plasma Physics, 2025, 45(4): 467-474.

王柳青, 胡业民, 白书航, 刘永 . 托卡马克等离子体中低杂波抑制电磁迸发辐射的物理机制的研究[J]. 核聚变与等离子体物理, 2025, 45(4): 467-474.

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