Different Ablation Models for the Wall-plasma Interaction Process in Pulsed Plasma Thruster

Ablation excited by current pulses is a very critical physical process in pulse plasma thrusters(PPT).Its effects on wall-plasma interaction directly determine the PPT performances.In order to reveal the process of the ablated wall interaction with the discharge plasma in PPT,ablation models formulated by three different boundary conditions at the wall-plasma interface are studied.These are the two widely used high-speed evaporation models(Model-L and Model-M),and the recently developed Keida-Zaghloul model(Model-K)of the Knudsen layer that takes into account the internal degrees of freedom on the energy flux conservation.First,fundamental mechanisms of the three ablation models are clarified by comparative analysis in order to gain a comprehensive understanding of the wall-plasma interaction.Then,the applicability of different ablation models with the numerical solutions of LES-6 PPT is investigated in detail using magnetohydrodynamic(MHD)modeling.Results show that Model-L and Model-M are actually special cases of Model-K when a simplified jump conditions limited by high velocity at the vapor/plasma interface is used;A ratio of ablation rate in Model-L to that in Model-M is about 0.8at the same wall surface temperature,while it rises to 1 at different surface temperature determined by Model-L and Model-M in PPT.Even though Model-K solution requires significant computational time,it shows more accurate ablation feature for the wall-plasma interaction and possesses better computing precision of impulse bit during post-pulse which is useful for future studies of the late time ablation.

Ablation excited by current pulses is a very critical physical process in pulse plasma thrusters （PPT）. Its effects on wall-plasma interaction directly determine the PPT performances. In order to reveal the process of the ablated wall interaction with the discharge plasma in PPT, ablation models formulated by three different boundary conditions at the wall-plasma interface are studied. These are the two widely used high-speed evaporation models （Model-L and Model-M）, and the recently developed Keida-Zaghloul model （Model-K） of the Knud- sen layer that takes into account the internal degrees of freedom on the energy flux conservation. First, fundnmental mechanisms of the three ablation models are clarified by comparative analysis in order to gain a comprehensive understanding of the wall-plasma interaction. Then, the applicability of different ablation models with the numerical solutions of LES-6 PPT is investigated in detail using magnetohydrody- namic （MHD） modeling. Results show that Model-L and Model-M are actually special cases of Model-K when a simplified jump conditions limited by high velocity at the vapor/plasma interface is used; A ratio of ablation rate in Model-L to that in Model-M is about 0.8 at the same wall surface temperature, while it rises to 1 at different surface temperature determined by Model-L and Model-M in PPT. Even though Model-K solution requires significant computational time, it shows more accurate ablation feature for the wall-plasma interaction and possesses better computing precision of impulse bit during post-pulse which is useful for future studies of the late time ablation.