Study On Electron Swarm Parameters Of Helium At Cryogenic Temperature And Low Gas Pressure

The insulation structure in the large-scale low-temperature superconducting device plays an important role in the safe and stable operation of the entire device.Insulation failure is the most likely problem that causes the failure of the superconducting magnet.In the low-temperature superconducting device,the insulating structure faces complicated operating conditions,including the extremely low temperature environment and the low-pressure environment in the fault state,and the electrical properties of the insulating material are relatively high.At present,the insulation system is mostly designed with high vacuum as the background.However,considering the local low-pressure environment during the operation of the superconducting device and reducing the voltage withstand level of the insulation system,it is necessary to measure electric property of the dielectric material in a low-temperature and low-pressure environment.In this paper,helium is used as the test object,and the electron swarm parameters test is carried out under low temperature and low pressure environment.The main results obtained are as follows:Based on the steady-state Townsend method,a set of low temperature and low pressure environmental parameter control platform was developed.The test platform can keep the temperature of the copper test chamber stable at any set temperature within the temperature range of 50～290 K.The control error does not exceed ± 1 K,and the pressure regulation in the range of 10-5 to 105 Pa.Based on the platform,the reduced ionization coefficient of helium in the temperature range of 50～290 K and the pressure range of 0.5～5.0 kPa was studied.The experimental results show that the helium reduced ionization coefficient at each temperature increases with the increase of the electric field strength.In the case of the same reduced electric field strength,the effect of temperature on the helium reduced ionization coefficient is not monotonous.The increase in the reduced ionization coefficient of helium in the range of 50～290 K is generally first rising and then decreasing,and the maximum temperature is around 200 K,The overall upward trend is affected by the enhancement of cathode electron emission.In the vicinity of 200 K,due to the desorption of impurity gases such as nitrogen and participation in the Penning effect,the degree of ionization is increased,resulting in a maximum value of the helium reduced ionization coefficient.The electron drift velocity and diffusion coefficient of helium in the temperature range of 50～290 K were obtained by Boltzmann equation calculation method.By comparing the reduced ionization coefficient obtained by the experiment with the reduced ionization coefficient calculated by the Boltzmann equation,the ionization collision cross section of the helium gas at low temperature is corrected,and the transport parameters of helium at low temperature are calculated.The electron drift velocity and diffusion coefficient of helium show a downward trend with temperature rise from 50 to 200 K.The temperature rises slightly from 200 to 290 K with a slight increase in temperature,and the minimum value is near 200 K.The main influencing factor is the impurity gas content represented by nitrogen.