EAST托卡马克刮削层中低杂波平行波数谱测量及实验研究

The anomalous loss of lower hybrid current drive (LHCD) efficiency at high density has been a bottleneck and also a hot topic in LHCD research field. Recent studies show that the parallel wavenumber-spectrum broadening induced by parametric decay instability (PDI) or wave scattering may be one possible candidate. However, at present, almost all the investigations on LHCD experiments at high density are based on the measurements of frequency spectrum, and then the wavenumber-spectrum is obtained by theoretical calculations, namely, lacking the direct measurements. In this project, we plan to develop a measurement of parallel wavenumber of LH waves firstly, in order to measure the actual wavenumber directly when the LH waves leave away from the launcher and propagate in the scrape-off layer (SOL) during the first pass. Then, the characteristics of wavenumber at different plasma and wave parameters will be analyzed, and the relationship between CD efficiency and spectral broadening will be investigated by using this measurement. Further study will focus on the conditions to reduce spectral broadening by means of parameter optimization, hopefully to improve CD efficiency at high density. This project will be helpful to clarify the mechanisms for bridging the spectral gap and for the anomalous loss of CD efficiency at high density. Moreover, it is hopeful to obtain the conditions of efficiency improvement at high density.

高密度下低杂波电流驱动效率反常下降是低杂波电流驱动研究领域所面临的瓶颈，也是目前研究的热点。最近的研究表明因参量衰变不稳定性或波散射导致的平行波数谱展宽可能是原因之一。但是目前，几乎所有的高密度低杂波电流驱动实验研究都基于实验上频率谱测量诊断，然后通过理论计算得到波数谱，缺少直接的波数谱测量证据。本项目首先是在EAST上发展低杂波平行波数谱测量诊断，直接测量低杂波从天线端口发射出来后首次在刮削层中传播时的实际平行波数；然后基于此诊断，分析不同等离子体和波参数条件下的波数谱变化行为，研究低杂波驱动效率与波数谱展宽之间的关系；通过优化等离子体参数，进一步探索降低波数谱展宽的条件，从而有望改善高密度下驱动效率。此项目研究有助于理解波谱间隙填补的物理机制；同时有助于理解高密度低杂波电流驱动效率反常下降的物理机制，以及获得高密度下驱动效率改善的物理条件。

在EAST托卡马克装置上发展了8通道低杂波平行波数谱诊断测量系统。设计的射频探针具有测量平行于背景磁场方向的低杂波波数能力。为了避免“镜频干扰”问题的影响，设计了两级混频电路，将低杂波信号频率将至20 MHz，降频后的信号满足现有采集卡的采集能力（采样率 ~ 100 MS/s）。通过对8路信号进行空间傅立叶分析计算便可以获得低杂波平行波数谱。物理实验结果表明低杂波投入期间信号相干性系数从噪声水平急剧上升到 > 0.9，验证了系统的可靠性。通过开展相关实验研究获得了刮削层中N||值对等离子体密度、等离子体电流和天线相位的依赖关系。发现提高低杂波功率和等离子体密度会显著增强低杂波在刮削层中的非线性效应，导致频谱展宽，从而降低驱动效率。而提高等离子体电流能够有效抑制低杂波的非线性效应，从而提高低杂波驱动效率。此结果有助于理解高密度低杂波电流驱动效率反常下降物理机制以及获得高密度下驱动效率改善条件。