Floating potential of emitting surfaces in plasmas with respect to the space potential

Strongly emitting surfaces affect the plasma-wall interaction in many applications, for example in divertors and limiters in fusion devices, emissive probes, and thermionic cathodes. The potential difference between a floating emitting surface and the surrounding plasma strongly impacts particle and energy transport to the surface. A variety of sheath models describe this potential difference, including the space-charge-limited sheath, the electron sheath with high emission current, and the inverse sheath with ion trapping. Our measurements reveal that each of these methods has its own regime of validity. We determine the charge state of an emissive filament immersed in a variety of plasmas, emphasizing variations in emitted current density and neutral particle density. Depending on the regime chosen, emitting surfaces can float positively or negatively with respect to the plasma potential.