A quasi-optical Mach-Zehnder microwave interferometer operating at 140 GHz has been developed for the ENN's spherical tokamak XuanLong-50 (EXL-50), for the purposes of line-integral electron density measurement and plasma density real-time feedback control input. The EXL-50 is designed for long pulse operation (over 5 s) and the electron density of phase I is estimated below 10¹⁹ m^-3. Thus, the well-known microwave interferometer is suitable for the advantage of cost effectiveness and good stability. One of the major errors of the interferometer is vibration. To reduce it, the entire interferometer is supported by sand-filled stainless-steel columns of 0.3 m inner diameter and the vibration modes are calculated by finite elements analysis. Other sources of error, such as noise and thermal drift, are carefully handled. To reduce noise, the interferometer including cables and digitizers are carefully shielded and grounded. The phase error due to source frequency thermal drift, manifested due to uneven probe beam and reference beam path lengths, is observed in long term operation and explained by model calculation. A continuous 100 s test shows that it is reduced to about 0.04 /s when the Gunn oscillators are temperature controlled by Peltier coolers with the industrial Proportional-Integral-Derivative control method to maintain the frequency stability. The system has been in routine operation since August 2019, with 10¹⁶ m^-2 line-integral density resolution. The technical details of the interferometer and experimental results are presented.

为新奥的球形托卡马克旋龙50（EXL-50）开发了一种工作频率为140 GHz的准光学马赫-曾德尔微波干涉仪，用于线积分电子密度测量和等离子体密度实时反馈控制输入。EXL-50 设计用于长脉冲操作（超过 5 秒），I 相电子密度估计低于 10 ¹⁹ m ^-3. 因此，众所周知的微波干涉仪具有成本效益和稳定性好的优点。干涉仪的主要误差之一是振动。为了减少它，整个干涉仪由内径为 0.3 m 的填砂不锈钢柱支撑，并通过有限元分析计算振动模式。其他误差源，例如噪声和热漂移，都经过仔细处理。为了降低噪声，包括电缆和数字转换器在内的干涉仪都经过精心屏蔽和接地。源频率热漂移引起的相位误差，表现为探测光束和参考光束路径长度不均匀，在长期运行中观察到，并通过模型计算进行解释。连续 100 秒的测试表明它减少到大约 0。04 /s 当耿氏振荡器由帕尔贴冷却器进行温度控制时，采用工业比例积分微分控制方法以保持频率稳定性。该系统自 2019 年 8 月开始正常运行，有 10¹⁶ m ^-2线积分密度分辨率。介绍了干涉仪的技术细节和实验结果。