Feng X D, Zhuang G, Yang Z J, Gao L, Hu X W
State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan 430074, China.
Rev Sci Instrum. 2013 Apr;84(4):044705. doi: 10.1063/1.4802690.
A simple, single-channel millimeter-wave interferometer system has been designed, fabricated, and installed on the J-TEXT tokamak. For the plasma density anticipated on J-TEXT, a solid-state source operating at 150 GHz has been chosen to minimize errors due to both vibration along the beam path and refraction in the plasma. The new aspect of the interferometer design is to use a subharmonic mixer for detection with a frequency doubled 150 GHz source. It employs a single source which is bias-tuned and modulated with a sawtooth wave form up to 100 kHz in order to generate the intermediate frequency. The 12.5 GHz voltage-controlled oscillator is multiplied to 75 GHz before a final doubler raises it to 150 GHz. A portion of the 75 GHZ power is used for the local oscillator (LO) and is directly connected to the LO input of the subharmonic mixer. The phase is evaluated by a digital phase comparator using a software-based algorithm. Detection noise limits the minimum resolvable phase change with the interferometer to ±0.05 fringe, which corresponds to an averaged electron density change along the chord of ±1.1 × 10(17) m(-2). The maximum measurable electron density is expected to be ∼9 × 10(19) m(-3). A comparison of preliminary results from the millimeter wave interferometer with that from the far-infrared hydrogen cyanide laser (wavelength of 337 μm) interferometer shows good agreement during the pulse flat-top period. The millimeter wave interferometer system will be used as a part of the density feedback control system in the future.
一种简单的单通道毫米波干涉仪系统已被设计、制造并安装在J-TEXT托卡马克装置上。对于J-TEXT预期的等离子体密度，已选择一个工作在150GHz的固态源，以尽量减少由于沿光束路径的振动和等离子体中的折射所导致的误差。干涉仪设计的新特点是使用一个亚谐波混频器进行检测，其源频率为150GHz且经过倍频。它采用单个源，该源通过偏置调谐并用高达100kHz的锯齿波形进行调制，以产生中频。12.5GHz的压控振荡器在最终倍频器将其提升到150GHz之前先倍频到75GHz。75GHz功率的一部分用于本地振荡器（LO），并直接连接到亚谐波混频器的LO输入。相位由数字相位比较器使用基于软件的算法进行评估。检测噪声将干涉仪可分辨的最小相位变化限制在±0.05条纹，这对应于沿弦的平均电子密度变化为±1.1×10(17) m(-2)。预计最大可测量电子密度约为9×10(19) m(-3)。毫米波干涉仪的初步结果与远红外氰化氢激光（波长337μm）干涉仪的结果比较表明，在脉冲平顶期两者吻合良好。毫米波干涉仪系统未来将用作密度反馈控制系统的一部分。
