Simulation of the Interaction Between Two Counterflowing Rarefied Jets

A preliminary analysis of the flow between a jet of argon plasma and one containing argon excited states is conducted using the direct simulation Monte Carlo method (DSMC). As a prelude to the use of more advanced models and to help design the accompanying experiment, a simplified model is formulated that ignores electron kinetics and field effects. This model allows the capture of most of the gas dynamics phenomena present with a standard DSMC code. Two interaction criteria are then used and compared to analyze the flow pattern of the interaction. An increase in the mass flow rate of the neutral jet is found to increase the level of interaction between the jets. PACS: 51.10.+y,52.25.Ya INTRODUCTION Motivations The accurate simulation of low temperature, weakly ionized rarefied plasmas is of paramount importance to optimize numerous industrial processes. Modern low-pressure plasma reactors used for etching or deposition [6] by the microelectronics industry frequently operate in the transition regime. Under these operating conditions, both electrons and heavy species (ions and neutrals) follow non-equilibrium distribution functions and require a kinetic modeling approach based on the Boltzmann equation. Any attempt at improving the behavior of most performance parameters of practical interest, such as the etching or deposition rates, can only be made by considering the distribution function (DF) of the various species of the flow. For certain species, desirable DF shapes that maximize the rate of given chemical reactions can further be identified. One should therefore ideally strive to selectively shape the DF of certain key species of the flow (usually that of the electrons) to optimize a particular process. At the University of Michigan, over the course of the next several years, an extensive study, both experimental and numerical, will be conducted to investigate the possibility of controlling the electron energy distribution function (EEDF) in an argon plasma by the introduction of argon metastables. An experimental investigation of the flow presented in this paper will be conducted at the University of Michigan’s Plasmadynamics and Electric Propulsion Laboratory (PEPL) while its numerical modeling will be conducted by the Nonequilibrium Gas and Plasma Dynamics Laboratory (NGPDL) of which both authors are members. Collaboration is also underway with the Computational Plasma Science and Engineering Group (CPSEG) to simulate the plasma source. Test Case Based on past work involving plasmas in the domain of electric propulsion and the layout of existing test facilities, the layout shown in Fig. 1 is chosen. A rarefied, weakly ionized, argon plasma (containing Ar, Ar and e−) produced by the helicon source described in [5] is subjected to a counterflowing jet of neutral argon containing metastables (primarily composed of Ar and Ar(4s), the first excited state, denoted Ar∗ in the following). The aim is to take advantage of the following de-excitation reaction: Ar∗+ e−→ Ar+ e− (1) to exchange energy between the excited states and the electrons so as to modify the shape of the EEDF. Although we have yet to fully characterize the composition of the two jets, the preliminary estimates shown in Table 1 was used in FIGURE 1. Sketch of the axisymmetric flow geometry