The nonequilibrium region of an electrode in contact with a flowing thermal plasma

A model for a high-pressure thermal plasma flowing onto an electrode in stagnation flow geometry based on a continuum description of the flow is used to establish anode current density limitations. These limitations can be overcome at high anode current densities when ionization in the nonequilibrium boundary layer supplies the required ion current to the distant plasma. Two modes of anode operation (low and high current density modes) are identified and interpreted to be a difference between a diffuse and constricted mode of arc attachment. Although the flow field is highly idealized, the results provide insight as to the coupling of the ionizational nonequilibrium boundary layer to the flow field and the concomitant effect that this coupling has on anode current limitation. The quasi-neutral approximation used is expected to accurately describe the dynamics within the ionization nonequilibrium boundary layer. The anode saturation current (current density above which the anode must emit ions) strongly increases with increasing free stream velocity and anode temperature. These results have important implications for the understanding of high-power arc jet operation, and the use of high-power arc jets in low thrust space propulsion