A Discrete Boltzmann Method (DBM) with a Maxwell-type boundary condition is constructed to investigate the influence of rarefaction on laminar Shock Wave/Boundary Layer Interaction (SWBLI). Due to the complexity of compressible flow, a Knudsen number vector Kn, whose components include the local Knudsen numbers such as Kn_ρ and Kn_U, is introduced to characterize the local structures, where Kn_ρ and Kn_U are Knudsen numbers defined in terms of the density and velocity interfaces, respectively. Since first focusing on the steady state of SWBLI, the DBM considers up to the second-order Kn_ρ (rarefaction/non-equilibrium) effects. The model is validated using Mach number 2 SWBLI and the necessity of using DBM with sufficient physical accuracy is confirmed by the shock collision problem. Key findings include the following: the leading-edge shock wave increases the local density Knudsen number Kn_ρ and eventually leads to the failure of linear constitutive relations in the Navier-Stokes (N-S) model and surely also in the lower-order DBM; the non-equilibrium effect differences in regions behind the leading-edge shock wave are primarily correlated with Kn_ρ, while in the separation region are primarily correlated with Kn_U; the non-equilibrium quantities D₂ and D_4,2, as well as the viscous entropy production rate Ṡ_NOMF can be used to identify the separation zone. The findings clarify various effects and main mechanisms in different regions associated with SWBLI, which are concealed in N-S model.

In the past few decades, ion engines have been widely used in deep-space propulsion and satellite station-keeping. The aim of extending the thruster lifetime is still one of the most important parts during the design stage of ion engine. As one of the core components of ion engine, the grid assembly of ion optic systems may experience long-term ion sputtering in extreme electro-thermal environments, which will eventually lead to its structural and electron-backstreaming failures. In this paper, the current studies of the grid assembly erosion process are systematically analyzed from the aspects of sputtering damage process of grid materials, numerical simulations, and measurements of erosion characteristics of grid assembly. The advantages and disadvantages of various erosion prediction models are highlighted, and the key factors and processes affecting the prediction accuracy of grid assembly erosion patterns are analyzed. Three different types of experimental methods of grid assembly erosion patterns are compared. The analysis in this paper is of great importance for selecting the sputter-resistant grid materials, as well as establishing the erosion models and measurement methods to accurately determine the erosion rate and failure modes of grid assembly. Consequently, the working conditions and structure parameters of ion optic systems could be optimized based on erosion models to promote the ion engine lifetime.

Hollow cathode, with the highest plasma density, current density, and temperature, becomes one of the most important components in the electro-thruster system. As the electric-propulsion thruster performance is directly related to the ionization rate, reliability, and lifetime of the hollow cathode, this paper develops a global model to study the effects of discharge current, gas flow rate, and gas species on the discharge characteristics in the insert and orifice regions of the hollow cathode. The emitter wall temperatures of hollow cathodes predicted by the global model are compared with experimental results from NSTAR thruster neutralization cathodes, confirming the model's validity. The influence of hollow cathode emitter material and structure sizes on the plasma parameters in the internal regions was also evaluated. The simulation results show that there is an optimal matching relationship between the discharge current and gas flow rate to guarantee the maximum ionization rate. The optimal working region for the hollow cathode has been determined under different energetic, regime and structural parameters. The global model established in this paper can quickly determine the key structure and operating parameters of hollow cathode at the design stage, and provide the theoretical basis for hollow cathode design and development.