Breakdown characteristics of helium in uniform gaps at MPa-level pressures: effects of N 2 impurities

Abstract MPa-level high pressure helium is used as the heat transfer media for the High Temperature Gas-cooled Reactor (HTGR), and also the insulation gas for the motors of the main helium circulator. As an untypical insulation gas, the high-pressure helium was seldom studies for its insulation properties, especially with trace of gas impurities these were allowed with 10-6 mole fraction in the design of HTGR. In present study, we built a one-dimensional plasma-fluid model that was capable of fast calculation of the critical breakdown voltage (CBV) of helium at MPa-level pressure with trace of nitrogen impurity. The model was justified by taking the influence of gas pressure on the transporting properties of each species, and the reaction rates for 3-body reactions which might subject to the high-pressure limit. Good agreement was achieved between the predicted CBV and experiment measurement. Results indicated the CBV exhibits a significant non-monotonic dependence on N2 content—it decreased sharply initially, reached a minimum at a critical content, and then recovered slowly, forming a U-shaped curve. This phenomenon arose from the nonlinear variation of Penning ionization rate with N2 content, where Penning ionization acted as the dominant reaction for breakdown in helium with impurities. When the N2 content was below the critical content, the Penning ionization rate increased with impurity content, leading to a decrease in CBV. Beyond the critical content, the abundant low-energy excited states of N2 enhanced electron energy loss, reducing the density of high-energy electrons and the yield of He*, which ultimately inhibited the Penning ionization rate and caused CBV to rise.