Parametric decay instabilities (PDIs) lead to the generation of strong frequency-shifted radiation when powerful X-mode polarized microwaves, injected for electron cyclotron resonance heating (ECRH) of fusion plasmas, cross a region of non-monotonic electron density near the second-harmonic upper hybrid resonance (UHR). For the standard second-harmonic X-mode ECRH scenarios used at the ASDEX Upgrade tokamak, the second-harmonic UHR occurs near the plasma edge, meaning that PDIs occur in connection with phenomena leading to non-monotonic edge electron density profiles, e.g. blobs, edge-localized modes (ELMs), and inter-ELM modes. We present the first study of strong signals near half the ECRH frequency in fusion-relevant plasmas and demonstrate their PDI-like features, such as an ECRH power threshold required for their occurrence, through analog modulations of the ECRH power. The signals near half the ECRH frequency are found to be far more prevalent than expected based on previo...

中文翻译：

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聚变等离子体中二次谐波上混合共振附近的参数衰减不稳定性

当为聚变等离子体的电子回旋加速器共振加热（ECRH）注入的强大的X模式极化微波穿过第二个非单调电子密度区域时，参量衰减不稳定性（PDI）导致产生强频移辐射。谐波上混合共振（UHR）。对于ASDEX升级版托卡马克使用的标准二次谐波X模式ECRH场景，二次谐波UHR发生在等离子边缘附近，这意味着PDI与导致非单调边缘电子密度分布（例如斑点）的现象有关。边缘本地化模式（ELM）和内部ELM模式。我们目前对融合相关等离子体中接近ECRH频率一半的强信号进行了首次研究，并展示了其类似于PDI的特征，例如发生它们所需的ECRH功率阈值，通过ECRH功率的模拟调制。据发现，接近ECRH频率一半的信号比基于预期的预期要普遍得多。