Study On DC Breakdown Characteristics Of Helium At Cryogenic Temperature

Liquid helium leakage accident may occur in low temperature system of large superconducting equipment,which may evaporate into the insulation system of superconducting magnet,resulting in a local cryogenic temperature and low pressure environment,and leading to helium breakdown between the high voltage conductor and the ground,which may cause a catastrophic effect on the whole device.In this paper,taking helium as the test object,the experiment was designed and carried out from the aspects of pressure,temperature,inter-electrode distance and electrode materials by using the self-developed cryogenic temperature dielectric test system.The DC breakdown characteristics of helium at different temperature(10K-300K),different electrode spacing(1mm,2mm and 4mm),different pressure(less than 101.3kPa)and different electrode materials(copper,nickel and aluminum)were studied.The results show:There exits intersection of breakdown curves under different electrode gaps in the temperature range of 10K-300 K.The breakdown voltage changes differently with pd value on both sides of the intersection.The influence of electrode spacing decreases gradually with the decrease of temperature,In the temperature range of 10K-300 K,the breakdown voltage of helium first decreases and then increases with the increase of pd value,and there is a minimum value in the middle.When the electrode spacing is fixed,the breakdown voltage of helium decreases with the decrease of temperature in the range of 200K-300 K under the same gas density.In the temperature range of 78K-200 K,the breakdown voltage of helium increases with the decrease of temperature.The changing trend of helium breakdown voltage in the temperature range from 10 K to 78 K depend on gas density.When in low gas density(less than 0.1kg/m~3),the breakdown voltage of helium decreases with the decrease of temperature.But in larger gas density(greater than 0.1kg/m~3),however,it increases with the decrease of temperature.When the electrode spacing is fixed,the color of the gas discharge is related to gas type and gas pressure.In air,the color is pink with blue,and there will be bright white in the middle with the increase of pressure.When in helium,the color is light red,and it will deepen with a light purple with the increase of pressure.At low pressure,there is a clear discharge channel at the edge of electrodes,the discharge area gradually gathered into the electrode gap and the light is getting stronger with the rise of pressure.In vacuum,the initial breakdown voltage increases with the increase of times,and then tends to be stable.In the temperature range of 78K-300 K,the stable breakdown field strength increases with the decrease of temperature,and is basically unchanged in the range of 10K-78 K.Based on the mathematical model of the Townsend theory,the helium breakdown characteristics were fitted.The fitting degrees of each temperature point were above 0.8,and obtained the empirical formula of the variation of breakdown voltage with the change of pd value in the temperature range of 10K-300 K.The difference of the electrode material is different in the empirical formula.In the end of this paper,the DC breakdown characteristics of helium at cryogenic temperature are fully analyzed from the aspects of temperature,electrode and impurity gas.