Experimental Investigations of the Lunar Photoelectron Sheath

Details Experimental Investigations of the Lunar Photoelectron Sheath Experimental Investigations of the Lunar Photoelectron Sheath

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p51c1460d&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P51C-1460

Other

[5421] Planetary Sciences: Solid Surface Planets / Interactions With Particles And Fields, [5465] Planetary Sciences: Solid Surface Planets / Rings And Dust, [5494] Planetary Sciences: Solid Surface Planets / Instruments And Techniques, [6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

Solar ultraviolet radiation incident upon the dayside lunar surface produces a photoelectron gas that dominates the near-surface plasma environment, with a typical density of 60 cm-3 and a characteristic scale-length of ~1 m. It has traditionally been difficult to produce a photoelectron gas with sufficient density in a laboratory settings to study its properties. In our initial experiments, the characterization of the photoelectron density above a Zr surface (work function W=4.4 eV) illuminated by Xe excimer lamps (peak emission at a wavelength of 172 nm) indicated that a sheath with a Debye length on the order of 10 cm formed. We characterize the photoelectron population above the surface by utilizing an emissive probe to map the electric potential distribution above the surface, and a Langmuir probe to determine the number density and temperature of the photoelectrons. A grid is placed 7.5 cm above the Zr surface to repel photoelectrons emitted from the chamber walls. Emissive probe measurements show a potential dip of about 2 V extending ~1 cm above the zirconium surface. The size of this potential well is dependent on the number of lamps illuminating the surface, as the density of photoelectrons above the surface increases with greater illumination. The electrons in the sheath have a Maxwellian distribution with an electron temperature around 1 eV (maximum energies are expected to be approximately 2.8 eV). We will use this experimental apparatus to characterize the photoelectron sheath above other surfaces; powders, such as CeO2 have similar work functions, but different photoelectric yields. Lunar soil simulants are expected to have approximately an order of magnitude smaller yield than metallic surfaces, which will act to increase the characteristic length of the photoelectron sheath above the surface. The experiments and accompanying computer simulations are used to guide the development of new instrument concepts for future in situ plasma measurements on the lunar surface.

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