Mechanisms of dielectric breakdown of ljquid coolants under quench condition and estimation of withstand voltage

本研究旨在建立超导磁体的电绝缘技术和发展超导磁体的绝缘信任。研究了作为磁体冷却剂的液氮和氦气在高电场作用下的沸腾现象。并对液冷剂的击穿机理和耐压特性进行了讨论。所得结果总结如下：1。提出了池沸式he冷却超导线圈的电绝缘设计方法，并将其应用于某小型中小企业。研究发现，决定绝缘空间的主要参数有：熄灭保护开关延时时间、线圈初始电流、线圈电感、导体几何形状和尺寸。从理论和实验两方面研究了高电场作用下液冷剂的气泡行为。结果表明，气泡运动主要受电梯度力的影响。然后，为了在淬火条件下获得冷却剂的通道，注意间隙长度的设置是非常重要的。对LNi和LHe在超流体条件下的击穿特性进行了统计研究。结果表明，基于弱环节理论的击穿统计特性很好地符合Weibull分布，电极尺寸对击穿特性的影响主要是面积效应而不是体积效应。

In this study, establishment of electrical insulation technique and development of insulation trust of a superconducting magnet were aimed at. Boiling phenomena of liquid nitrogen and helium used for coolant of the magnet under high electric field was investigated. Moreover, mechanism of breakdown of those liquid coolant and characteristics of withstand voltage were discussed. Obtained results were summarized as follows.1.Electrical insulation design of pool-boiling He-cooled superconducting coilsInsulation design procedure was proposed and applied to a small SMES.It was found that the main parameters determining insulation space were delay time of quench protection switch, initial coil current, coil inductance, and conductor geometry and size.2.Bubble behavior in liquid coolant under quench conditionBubble behavior under high electric fields was studied theoretically and experimentally. The results showed that bubble motion was influenced mainly by an electrical gradient force acting on it. Then it is very important to pay attention to a setting of the gasp length in order to get a passage of the coolant under quench condition.3.Breakdown characteristics of LNi and LHe including a superfluid condition were investigated statistically.It was shown that the statistical characteristic of breakdown were well fitted to Weibull distribution based on weak-link theory and area effect was dominant rather than volume effect for size effect of electrodes to the breakdown characteristics.