Analyses of nonequilibrium transport in atmospheric-pressure direct-current argon discharge under different modes

The key plasma parameters under different discharge modes, such as heavy-particle and electron temperatures, electron number density, and nonequilibrium volume of plasmas, play important roles in various applications of gas discharge plasmas. In this study, a self-consistent two-dimensional nonequilibrium fluid model coupled with an external circuit model is established to reveal the mechanisms related to the discharge modes, including the normal glow, abnormal glow,arc, and glow-to-arc transition modes, with an atmospheric-pressure direct-current(DC) argon discharge as a model plasma system. The modeling results show that, under different discharge modes, the most significant difference between the preceding four discharge modes lies in the current and energy transfer processes on the cathode side. On one hand, the current to the cathode surface is mainly delivered by the ions coming from the plasma column under the glow discharge mode due to the low temperature of the solid cathode, whereas the thermionic and secondary electrons emitted from the hot cathode surface play a very important role under the arc mode with a higher cathode surface temperature and higher ion flux toward the cathode. On the other hand, the energy transfer channel on the cathode side changes from mainly heating the solid cathode under the glow mode to simultaneously heating both the solid cathode and plasma column under the arc mode with an increase in the discharge current. Consequently, the power density in the cathode sheath(P_c) was used as a key parameter for judging different discharge modes, and the range of(0.28–1.2) × 10^(12) W m^(-3) was determined as a critical window of P_c corresponding to the glow-to-arc-mode transition for the atmospheric-pressure DC argon discharge, which was also verified by comparison with the experimental results in this study and the data in the previous literature.

Experimental study and numerical analysis of vacuum thermal characteristics of brush DC motor

Brush direct current(DC)motors have several qualities that make them very attractive for space flight applica-tions.Considering the high reliability requirements of aerospace missions,the thermal characteristics and ther-mal failure of the brush DC motor in the space environment were studied.Using a motor thermal resistance network model,a special thermal test method was determined and combined with a thermal conductivity anal-ysis model,the thermal parameters were obtained via item-by-item stripping,and the motor temperature field was constructed.By introducing the arc discharge factor to evaluate the electric-corrosion heat consumption,the numerical analysis results were in good agreement with the test results under the conditions of stalled rotor,nor-mal rotation,single brush,and multiple brushes.The analysis and test results show that continuous operation for 110 s will lead to melting of the brush solder joints,and electrical corrosion heat consumption is one of the main factors that cannot be ignored.The reliability model of vacuum applications should be established in the normal working mode of at least two brushes in both the positive and negative electrodes.To improve the reliability,a sealed air-filled structure of the motor was proposed,a heat-flow co-simulation model of a continuous medium flow with a large curvature and constant without a gravity field was established,and the temperature and ve-locity fields under different sealed pressures were obtained.The results show that the temperature of the single brush reduced to below 140°C from 204.5°C in vacuum,which can meet the long-term continuous working requirement of high reliability of brush motors in space missions.In addition,it was found that with the decrease in pressure,the effect of convective heat transfer gradually weakens,the temperature gradually increases and converges to the unique heat conduction process of the gas,while the effect of convection is negligible.As the pressure continues to decrease,the sealed gas evolves from continuous medium flow to transitional and free molecular flow,and the heat conduction effect of the gas weakens again until it approaches the singleness solid conduction process.