Fully Kinetic Modeling of Emissive Sheaths in Hypersonic Boundary Layer Plasmas

Electrostatic Particle-in-Cell (PIC) and Direct Simulation Monte Carlo (DSMC) methods are used to model thermionic emissions from an emitting wall in partially ionized boundary layer plasmas in the hypersonic regime. Emissive sheaths submersed in a high-density
$(\sim 10^{16}\mathrm{m}^{3})$
plasma are examined using a PIC-DSMC solver, CHAOS
^l
. We will show that collisionless emissive sheaths with plasma domains of sufficient length (30 and 60 Debye lengths,
$\lambda_{\mathrm{D}}$
) are subject to strong oscillations in the quasi-neutral region due to a two-stream electron instability. The dispersion relation for the two-stream instability is solved to determine a minimum domain size
$(\sim 18\lambda_{\mathrm{D}})$
for instabilities to occur. The most dominant frequencies
$(\omega))$
and wavenumbers (k) in our PIC simulations are shown to be in the range of
$0-2 \text{GHz}\ (\omega:0-1.1\omega \text{pe})$
and
$625 - 10,000 \mathrm{m}^{-1}(\mathrm{k}\lambda_{\mathrm{D}}:0.017-0.27^{1})$
respectively, for the case with long domain size,
$60 \lambda_{\mathrm{D}}$
.The effect of ion-neutral collisions (MEX and CEX) are analyzed for low-
$(\sim 10^{19}\mathrm{m}^{-3})$
and high-
$(\sim 10^{20}\mathrm{m}^{-3})$
, cold
$(\sim 300\mathrm{K})$
and hot
$(\sim 2,000\mathrm{K})$
neutral density backgrounds. We plan to show the trapping mechanism of slow CEX ions inside the virtual cathode (VC). With CEX ion-trapping, we find that thermionic emission from the wall can be increased by %8 compared to the collisionless case.