HL-2A装置上的快扫Q波段微波反射系统研制

Development of a fast sweeping Q-band microwave reflectometer on HL-2A tokamak

介绍了在HL-2A装置上发展的一套快速扫频的Q波段外差微波反射系统，用于高时空分辨测量等离子体边缘到约束区的电子密度分布。该系统采用外差式连续波扫频调制技术（VCO），由外部任意波电压控制，工作频率为33～50GHz，全波段扫频周期达到6μs。在台面标定中发展了 VCO 源的动态标定技术，并解决了微波源及器件的非线性响应、波导的色散特性等因素造成差频频率动态范围过大的问题，使反射面固定时系统输出的差频为定频信号，有利于降低噪声干扰和数据处理。同时发展了直接相位处理技术，实现快速的电子密度分布反演。实验中用该微波反射系统测得了L模、H模等不同等离子体放电条件下的电子密度分布，观测ELM爆发前后台基区的形成与垮塌过程。

The fast sweeping frequency modulated continuous wave （FMCW） reflectometer was developed on the HL-2A tokamak to measure the electron density profile with high temporal and spatial resolutions. A wide band voltage controlled oscillator （VCO） is used as a local oscillator, which is driven by an arbitrary waveform generator （AWG）. The launching frequency covers Q band （33～50GHz） and the temporal resolution can reach about 6μs. Dynamic calibration technique of VCO is developed and the difference between static and dynamic responses of VCO is obtained. To acquire constant beat frequency when fixed reflector, nonlinear response of source and dispersion of waveguide are taken into account, making it easy to reduce noise and process data. A direct phase extracting technique is introduced to achieve rapid density profile inversion. With the reflectometer system, the density profiles at L-mode and H-mode have been observed, and the evolution of pedestal during ELM is demonstrated.