Development of miniaturized, spectroscopically assisted Penning gauges for fractional helium and hydrogen neutral pressure measurements

Direct measurements of the helium (He) fractional neutral pressure in the neutral gas around fusion devices is challenging because of the small mass difference between the abundant D2 molecules and the He atoms which will be produced by D-T fusion. To study He exhaust, Penning gauges were used to measure total neutral pressure assisted by spectroscopy to resolve the D and He partial pressures. In this contribution, initial results are shown from developing this technique into a miniaturized configuration for direct in-situ measurements in the divertor of fusion devices. The configuration is based off a gauge originally designed for the National Spherical Tokamak Experiment-Upgrade (NSTX-U). The goal of this new miniaturized design it to reduce the space required by the gauge on the device and use of the inherent magnetic field of the machine rather than permanent magnets inside the gauge, enabling it to be adapted into a system that can be extended directly into the divertor region. The feasibility test of the method for NSTX-U and the Wendelstein 7-X (W7-X) stellarator are surveyed. For W7-X, a commercial Penning Gauge has been installed on an outboard vacuum flange as a generic feasibility test in the neutral gas environment of a stellarator. At an integration time of 25s, helium lines can be seen down to 10^-5 mbar and H-alpha lines down to 10^-6 mbar. Successful measurement of the total as well as the fractional neutral pressures of He and H has been shown. A first prototype of the miniature Penning gauge has been tested in Madison and shows a near linear power law scaling between current and pressure: I = C*P^n with n = 1.0 - 1.2. Pressure measurements were achieved starting at 10^-3 mbar and down to 10^-6 mbar. A modular gauge is being assembled, which allows easy interchangeability of the anode to test new anode geometries, in order to improve optical access and increase spectroscopic sensitivity. This shall enable an increase of the time resolution of the spectroscopically assisted fractional neutral pressure measurements to up to 1kHz.

由于丰度较高的D₂分子与D-T聚变产生的氦原子之间质量差极小，直接测量聚变装置周边中性气体中的氦（He）分压强极具挑战。为研究氦的抽排行为，研究团队采用潘宁规（Penning gauge）测量总中性压强，并辅以光谱法解析氘与氦的分压强。本文介绍了将该技术开发为微型化构型的初步结果，用于聚变装置偏滤器内的直接原位测量（in-situ measurements）。该微型构型基于一款专为国家球形托卡马克实验升级装置（National Spherical Tokamak Experiment-Upgrade, NSTX-U）设计的原规开发。新型微型化设计的目标在于缩减规体在装置上占用的空间，不再使用规体内部的永磁体，转而依托装置自身的固有磁场，使其可适配为可直接延伸至偏滤器区域的系统。本研究调研了该方法在NSTX-U与文德尔斯坦7-X（Wendelstein 7-X, W7-X）仿星器上的可行性测试。针对W7-X，研究团队已在外侧真空法兰上安装了商用潘宁规，作为仿星器中性气体环境下的通用可行性测试。在25s积分时长下，可检测到低至10^-5 mbar的氦谱线，以及低至10^-6 mbar的Hα谱线。实验已成功实现氦与氢的总压强及分压强测量。首个微型潘宁规原型已在麦迪逊完成测试，其电流与压强间呈现近似线性的幂律缩放关系：I = C*P^n，其中n取值为1.0~1.2。压强测量范围覆盖10^-3 mbar至10^-6 mbar。目前团队正在组装模块化规体，该规体可便捷更换阳极以测试新型阳极几何构型，从而优化光学通光性并提升光谱灵敏度。这将有望将光谱辅助式中性分压强测量的时间分辨率提升至1kHz量级。