Experimental study of a photoelectron sheath

Abstract

We describe a set of laboratory experiments to reproduce and investigate the photoelectron layer that occurs above UV-illuminated surfaces in space. The experiments are done in vacuum with UV illumination at 172 nm that is sufficiently intense for the creation of a photoelectron layer above a large, planar metal surface with a Debye shielding distance of ∼7 centimeters, small in comparison with the scale of the experiment. The emitting surface electrically floats to a potential approximately 1.5 V more positive than a nearby equipotential surface. Retarding potential analysis of the energy distribution of the electrons emitted from the electrically floating surface, as well as Langmuir probe data, show an effective electron temperature of 1.4 (±0.3) eV and a density of 4×1010 m−3. Langmuir probe measurements are taken throughout the photoelectron sheath to determine the electron density, which show good agreement with results from a 1-D particle-in-cell simulation. These experiments enable the better understanding of the plasma environment of spacecraft, the moon, and other airless bodies in the solar system, and the processes that might be responsible for the charging, mobilization, and transport of dust particles on their surfaces.