Plasma Sheaths Around Small Bodies: Classical, Space-Charge-Limited (SCL) and Inverse Sheaths

Abstract:

Plasma sheaths are formed above the surface of small bodies including the Moon, asteroids and spacecraft. Understanding of sheaths is important for understanding planetary surface processes, dust charging and transport, as well as in-situ measurements and exploration. Sheath formation can be complicated. Fundamentally, there are three kinds: classical (ion-dominated), space-charge-limited or SCL (non-monotonic), and inverse (electron-dominated). All the three kinds of sheaths can, in general, exist above the surface of small bodies that both collect (e.g., solar wind plasma) and emit (e.g., photoelectrons and secondary electrons) charged particles. For the first time, we do laboratory experiments to show the emergence of all these sheaths by changing a single parameter: the ratio of emitted to collected electron currents. In the experiments, plasma is created using a hot filament to emit energetic primary electrons to ionize neutral particles. Secondary electrons are also created from a surface where the primary electrons bombard. By changing the neutral pressure, the thermal electron density is changed from higher to lower than the secondary electron density, showing a change in sheath structure from classical through SCL to inverse profiles. We investigate the conditions for the formation of these sheathes and their properties.