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，发展超导磁体的绝缘可靠性。研究了在高电场作用下磁体冷却剂液氮和氦气的沸腾现象。此外，还讨论了这些冷却液的击穿机理和耐压特性。主要研究结果如下：1.池沸腾氦冷超导线圈的电气绝缘设计提出了一种小型SMES的绝缘设计方法，并将其应用于小型SMES的绝缘设计中，发现决定绝缘间距的主要参数是失超保护开关的延迟时间、线圈初始电流、线圈电感，2.淬火条件下冷却液中的气泡行为对高电场下的气泡行为进行了理论和实验研究。研究结果表明，气泡的运动主要受电场梯度力的影响，因此，在淬火条件下，要获得冷却剂的通道，必须注意间隙长度的设置; 3.对LNi和LHe在超流条件下的击穿特性进行了统计研究，发现击穿的统计特性符合Weibull分布电极尺寸效应对击穿特性的影响主要表现为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.