| Insulation structures are of great significance to the security and stability of large low temperature superconducting devices.Insulation failure is considered to be the most likely problem leading to fault of superconducting devices.Due to the complex operating condition including cryogenic temperatures near helium boiling point,high fault voltage and various levels of vacuum,high electrical performance is required for the insulation structures in low temperature superconducting devices.Currently,most of insulation structures are designed for operating at high vacuum.However,considering the local low pressure condition which could occur at long time operation is able to affect the electrical strength of insulation system,it is necessary to study the electrical properties of dielectrics under conditions of different cryogenic temperatures and vacuum levels.In this study,the surface flashover characteristics of polyimide,which is widely used as electric insulation at low temperatures,were analysed at cryogenic temperatures and vacuum conditions,and mechanism of the cryogenic temperature effects on the surface flashover at different vacuum levels were discovered.The key issues and results are listed below:Cryogenic temperatures and vacuum regulation platform was developed,which could provide environment with temperatures of 300?20 K and pressure of 10-5?105 Pa.The test platform could reduce the influence from coolant,and it is suitable for various electrical tests for different types of dielectrics.Based on this platform,DC surface flashover characteristics of polyimide film were tested at conditions of 300?20 K temperatures and 10-3?103 Pa helium pressure.The results show that vacuum level is the most critical factor to the flashover voltage,and the temperature dependences on flashover voltage are different at high vacuum,medium vacuum and low vacuum.There are turning areas between surface flashover at different vacuum levels,and the corresponding gas molecular densities are constant at different temperatures.At high vacuum,the flashover voltage is relatively high,and for a certain temperature,the flashover voltage is independent of gas molecular density,and for a certain gas molecular density condition,the flashover voltage shown a general downtrend with temperature decreasing;At medium vacuum,for a certain temperature,the flashover voltage declines rapidly with higher gas molecular density,and for a certain gas molecular density,flashover voltage increases and then decreases with temperature decreasing;at low vacuum,the flashover voltage is lowest,and for a certain temperature,the relationship between flashover voltage and gas molecular density is characterised as a Paschen's curve,and for a certain gas molecular density,the flashover voltage decreases,increases and then decrease with decreasing temperature,which shown a highest flashover voltage at 100 K,and relative lower voltage at 200 K and 20 K.Helium breakdown characteristics are considered to affect surface flashover of polyimide at low vacuum.Helium breakdown were tested under small gap and uniform DC field,and at temperatures of 300?20 K and pressure of 1?105 Pa.Combining the studies of discharge spectrum,gas effective ionisation parameter and cathode emission at cryogenic temperatures,the mechanism of temperature effect on helium breakdown was revealed.According to the test results,helium molecular density is the major factor affecting the breakdown voltage,and discharge at each temperature accords with Paschen's law,and the gas molecular density corresponding to the Paschen minimum remains at different temperatures;for a certain molecular density,helium breakdown voltage show a non-monotonic trend with temperature decreasing:within 300?100 K,impurities desorption contributes to the gas ionisation by Penning effect,but the cathode emission is suppressed by low temperatures,and the breakdown voltage increase after a decrease with temperature decreasing;within 100?20 K,according to the enhance of helium adsorption on cathode,the work function is declined and second electron emission is enhanced at lower temperatures,which leading to a decrease in helium electric strength.By comparing the surface flashover of polyimide and breakdown of helium,it was found that the mechanism of cryogenic temperature effect on polyimide surface flashover at low vacuum is similar to that on helium breakdown.The effect of space charge accumulation on polyimide DC surface flashover at cryogenic temperatures and high vacuum were studied,and the mechanism of temperature effect on DC surface flashover characteristic of polyimide at high vacuum was revealed.By using association test of pulsed electroacustic-conduction current,high field conduction current test at cryogenic temperatures and thermally stimulated current method,the space charge characteristics of polyimide at low temperatures,and the trap characteristics in the bulk as well as the space charge transport properties were studied.The test results shown that,at temperatures below 273 K,electron injection decreases and converges to a low level as temperature decreasing,As for the charge transportation at low temperatures,obvious charge migration in the bulk could only be found when the external field exceeds 125 kV/mm,and for the lower electric field,the injected charges are rapidly captured by the deep traps distributed on the subsurface layer of polyimide,and the trap charge mobility keeps decreasing with temperature.The temperature dependence on difference between charge injection and trap charge migration leads to the change of space charge accumulation,which presents a decrease trend followed by an increase with temperature decreasing.The space charge accumulation effect on flashover voltage was studied by surface flashover tests with pre-stress,and compared with the temperature dependence on DC surface flashover characteristic,the cryogenic temperature effect on space charge accumulation was considered to be the main factor affecting DC flashover voltage of polyimide at vacuum. |
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