中子EDM实验的高压测试装置及液氦大电流下的介电强度下限
High Voltage Test Apparatus for a Neutron EDM Experiment and Lower Limit on the Dielectric Strength of Liquid Helium at Large Volumes
摘要：
一项新的研究正在使用超冷中子产生并保持在超流氦浴中来寻找中子的永久电偶极矩（EDM）。达到目标灵敏度需要在实验室中保持几十千伏每厘米的电场，该实验室标称宽7.5厘米，将包含约4升超流体。液氦的电性能预计将足以满足设计目标，但鲜为人知的是，这些性能的体积和电极间距适当的EDM实验。此外，从外部源直接向实验测试单元施加必要的电压是不切实际的。建立了一种在液氦环境中放大电压并测试大体积和电极间距下液氦电性能的装置。该装置由一个大面积的平行板电容器浸没在200升液氦杜瓦。初步结果表明，正常状态的液氦的击穿强度至少为90 kV/cm在这些体积，在氦蒸汽压对应于4.38 K。这些领域持有超过11小时的泄漏电流小于170 pA（约20%的最大容许在EDM实验）。该系统也被发现是强大的对预期的辐射背景。超流体的初步结果表明，至少30千伏/厘米的领域可以维持在EDM实验所需的体积，约60%的设计目标。这些结果可能受到必须保持在超流浴上方的低压的限制。
英文摘要：
A new search for a permanent electric dipole moment (EDM) of the neutron is underway using ultracold neutrons produced and held in a bath of superfluid helium. Attaining the target sensitivity requires maintaining an electric field of several tens of kilovolts per centimeter across the experimental cell, which is nominally 7.5 cm wide and will contain about 4 liters of superfluid. The electrical properties of liquid helium are expected to be sufficient to meet the design goals, but little is known about these properties for volumes and electrode spacings appropriate to the EDM experiment. Furthermore, direct application of the necessary voltages from an external source to the experimental test cell is impractical. An apparatus to amplify voltages in the liquid helium environment and to test the electrical properties of the liquid for large volumes and electrode spacings has been constructed. The device consists of a large-area parallel plate capacitor immersed in a 200 liter liquid helium dewar. Preliminary results show the breakdown strength of normal state liquid helium is at least 90 kV/cm at these volumes, at the helium vapor pressure corresponding to 4.38 K. These fields hold for more than 11 hours with leakage currents less than 170 pA (about 20% of the maximum tolerable in the EDM experiment). The system is also found to be robust against anticipated radiation backgrounds. Preliminary results for superfluid show that fields of at least 30 kV/cm can be sustained at the volumes required for the EDM experiment, about 60% of the design goal. These results are likely limited by the low pressure that must be maintained above the superfluid bath.
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