Effect of Air Impurities on the Characteristics of Helium Discharge at High Temperature and High Pressure

Abstract The high temperature gas-cooled reactor is one of the fourth-generation reactors, which uses helium as the coolant for its low neutron-absorption cross-section and stable chemical properties. In the reactor, the main helium blower, the active magnetic bearings and some other electrical equipment work in high-temperature and high-pressure helium gas. Therefore, it is necessary to study the discharge characteristics of helium under high temperature and high pressure. In this paper, helium models with air impurities were established by COMSOL Multiphysics. To validate the model, the results of these models were compared with those of the pure helium model and the experimental data. A concept named “reaction chain” was introduced via analyzing the reaction rates to elucidate the effects of the air impurities and describe the underlying mechanisms. We observed that the process of helium discharge can be perfectly described by the reaction chains, just like the nuclear reaction. Besides, the results revealed that the Penning ionization of the He* and air impurities can consume He* constantly and directly, which can shorten the reaction chain of the production of dominant ions. Furthermore, the direct ionization of ground state air impurities molecules can produce more electrons to strengthen the effects of electron avalanche, which will further intensify the reactions. As a result, the presence of air impurities can decrease the breakdown voltage remarkably. Finally, due to the differences of energy thresholds of air impurities, the nitrogen, oxygen, and water vapor impurities will have different effects on the breakdown voltage of helium gas.