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首页> 外文期刊>Plasma physics and controlled fusion >Trapped upper hybrid waves as eigenmodes of non-monotonic background density profiles
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Trapped upper hybrid waves as eigenmodes of non-monotonic background density profiles

机译：被困的上部混合波作为非单调背景密度剖面的特征模

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摘要

Non-monotonic plasma density structures such as blobs and magnetic islands give rise to trapped upper hybrid (UH) waves. Trapped UH waves which satisfy Bohr-Sommerfeld quantization can be thought of as eigenmodes of a cavity. Using fully kinetic particle-in-cell simulations, we verify the existence of these UH eigenmodes and demonstrate their significance as only eigenfrequencies become unstable to three-wave interactions. The eigenmodes can be excited through parametric decay instabilities (PDIs) of an X-mode pump wave at approximately twice the UH frequency, as could be the case for a gyrotron beam traversing a blob in a magnetically confined fusion plasma. We derive a closed expression for the wavenumber of UH waves, which is accurate both close to the UH layer and to the electron cyclotron resonance. This allows for fast analysis of eigenmodes in a non-monotonic structure. An expression for the amplification of PDI daughter waves in an inhomogeneous plasma is extended to a decay region where the first several derivatives vanish. From the amplification in a convective PDI, we estimate the growth rate of the absolute PDI involving the trapped waves. We show that the excitation of eigenmodes through PDIs in our simulations are indeed absolute rather than convective due to the trapping of the daughter waves. Additionally, we show that only eigenmodes get excited through the PDIs, and that we are able to predict the growth rates of the daughter waves and how they scale with the pump wave intensity. This is evidence supporting a fundamental assumption of analytical theory describing low threshold strong scattering observed in magnetically confined fusion experiments during second harmonic electron cyclotron resonance heating (ECRH). Such low threshold instabilities can degrade ECRH performance but also offer novel uses for ion heating or as diagnostics.
机译：非单调的等离子体密度结构，如斑点和磁岛，会产生被俘获的上杂波。满足玻尔-索末菲量子化的陷波可以被认为是腔的本征模。利用全动力学粒子模拟，我们验证了这些UH本征模的存在，并证明了它们的重要性，因为只有本征频率对三波相互作用变得不稳定。本征模可以通过X模泵浦波的参数衰减不稳定性（PDI）激发，其频率约为UH频率的两倍，就像回旋管束穿过磁约束聚变等离子体中的水滴一样。我们推导出了UH波的波数的闭合表达式，它在UH层和电子回旋共振附近都是精确的。这允许快速分析非单调结构中的本征模。将非均匀等离子体中PDI子波放大的表达式推广到前几阶导数消失的衰变区域。根据对流PDI中的放大，我们估计了包含陷波的绝对PDI的增长率。我们的模拟表明，由于子波的捕获，通过PDI激发的本征模确实是绝对的，而不是对流的。此外，我们还表明，只有本征模通过PDI被激发，并且我们能够预测子波的增长率，以及它们如何随泵浦波强度缩放。这是支持分析理论基本假设的证据，描述了在二次谐波电子回旋共振加热（ECRH）期间磁约束聚变实验中观察到的低阈值强散射。这种低阈值不稳定性会降低ECRH性能，但也为离子加热或诊断提供了新的用途。

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