Modeling of Electron Emitting Surfaces Submersed in Boundary Layer Plasmas

The present study investigates the thermionically emitting surfaces immersed in flowing plasmas by using an electrostatic Particle-in-cell (PIC) code, CHAOS [1] , which is an efficient GPU-based solver developed at the University of Illinois Urbana-Champaign. The nature of emissive sheaths submersed in low density (~10 14 m -3 ) and high density (~10 16 m -3 ) plasmas is examined by performing 1-D collisionless simulations. In this work, we will show that, in both cases, there is a virtual cathode formation near the emitting surface where an electron cloud accumulates and a space-charge-limiting (SCL) sheath forms. The SCL sheath is about 10 Debye lengths in thickness with a 16 Debye lengths thick quasi-neutral, and 8 Debye lengths thick inlet-sheath region. Since the electrons are emitted with a surface temperature of 0.17 eV, a voltage drop of the same order is found for the virtual cathode. We validate our PIC simulations by comparing the SCL currents with the estimates from Child-Langmuir Law [2] and an improved sheath theory developed by Hanquist et al [3] . We plan to show how variables of emissive surface temperature and plasma density affect the SCL current with application to electron-transpiration-cooling over hypersonic surfaces.