Electron transpiration cooling relies on thermionically emitted electrons to escape from a hot surface and transport energy through the surrounding plasma. We use kinetic particle-in-cell simulations to progressively investigate this process from local emissive-sheath physics to full-gap electron transport and electrostatic instability. Local sheath simulations show that increasing thermionic emission can drive the sheath toward space-charge-limited and virtual-cathode states, while introducing a downstream absorbing collector can qualitatively modify the near-cathode charge distribution, electric field, and potential structure, indicating that the emitting sheath cannot always be treated independently of the downstream plasma system. Motivated by recent full-diode studies showing that emitted-electron current can also be limited by plasma backflow[1], we further model the complete cathode-to-anode gap and find that increasing emission does not necessarily improve useful electron transport because strong backflow and global potential restructuring can develop [2]. When the neutral gas density is further reduced, electron transmission improves, but the collected current becomes substantially less steady, accompanied by an interior traveling electrostatic instability (Fig. 1). A state-matched multistream theoretical model quantitatively reproduces the main wave characteristics. These results show that electron transpiration cooling should be evaluated through the coupled effects of emission, sheath formation, full-gap transport, electron backflow, downstream collection, and plasma stability, rather than by emission strength alone.

Electron transpiration cooling; thermionic emission; particle-in-cell simulation; emissive sheath; electron backflow; electrostatic instability