Diagnostic development for parallel wave-number measurement of lower hybrid waves in EAST

An eight-channel magnetic probe diagnostic system has been designed and installed adjacent to the 4.6 GHz lower hybrid (LH) grill antenna in the low-field side of the Experimental Advanced Superconducting Tokamak (EAST) in order to study the n∥ evolution of LH waves in the first pass from the launcher to the core plasma. The magnetic probes are separated by 6.6 mm, which allows measurement of the dominant parallel refractive index n∥ up to n∥ = 5 for 4.6 GHz LH waves. The magnetic probes are designed to be sensitive to the magnetic field component perpendicular to the background magnetic field with a slit on the casing that encloses the probe. The intermediate frequency stage, which consists of two mixing stages, down-coverts the frequency of the measured wave signals at 4.6 GHz to 20 MHz. A bench test demonstrates the phase stability of the magnetic probe diagnostic system. By evaluating the phase variation of the measured signals along the background magnetic field, the dominant n∥ of the LH wave in the scrape-off layer has been deduced during the 2019 experimental campaign. In the low density plasma, the measured dominant n∥ of the LH waves is about 2.1, corresponding to the main peak 2.04 of the launched n∥ spectrum. n∥ deduced by the least-squares linear fit method remains near this value in the low density plasma with a high spatial correlation magnitude of 0.9. With an eight-channel probe system, a wave-number spectrum has also been deduced, which has a peak near to the measured dominant n∥.

为研究低杂波（LH）从发射器首次传播至芯部等离子体过程中平行折射率n∥的演化规律，实验先进超导托卡马克（EAST）低场侧、4.6 GHz低杂波（LH）栅格天线附近已设计并安装一套八通道磁探针诊断系统。磁探针间距为6.6 mm，可测量4.6 GHz低杂波的主导平行折射率n∥，最高可达n∥=5。磁探针外壳开有狭缝，使其对垂直于背景磁场的磁场分量敏感。中频级包含两个混频级，可将测得的4.6 GHz波信号频率下变频至20 MHz。台架试验验证了该磁探针诊断系统的相位稳定性。通过分析沿背景磁场测得的信号相位变化，2019年实验运行期间推导出了刮削层内低杂波的主导n∥。在低密度等离子体中，测得的低杂波主导n∥约为2.1，与发射n∥谱的主峰2.04对应；采用最小二乘线性拟合方法推导的n∥在低密度等离子体中仍接近该值，且空间相关系数高达0.9；借助八通道探针系统还推导得到波数谱，其峰值位置与测得的主导n∥相近。