Radiofrequency Plasmas as Vacuum Ultraviolet Sources for Space Environment Simulations

Abstract

This thesis presents results from experiments using radiofrequency (RF) plasmas as vacuum ultraviolet (VUV) sources, primarily to simulate Solar radiation in the 100-200 nm wavelength range for experiments on Lunar regolith charging.

The Lunar surface is covered in a layer of loose, unconsolidated rock and dust known as regolith. As well as being particularly fine and abrasive, this regolith dust is known to electrostatically charge and mobilise upwards in a 'lofting' motion. Apollo astronauts reported several dust-related equipment malfunctions or failures during surface operations, as well as several negative health impacts due to dust contact and inhalation. This thesis presents an experiment examining dust charging under 172 nm VUV radiation and a low-density argon plasma (10^6 cm^-3), simulating Lunar dayside conditions. Dust exposed to VUV alone exhibited lofting, while no motion occurred under plasma alone. With both present, the number and charge of mobilised particles increased by factors of 3x and 2.1x respectively, suggesting the relevance of the Solar wind plasma to the charging and mobilisation of dust particles on the Lunar dayside, where the Solar VUV is typically assumed to dominate.

Given that both the wavelength and flux of the VUV radiation used in this experiment differs significantly from what is expected at the Lunar surface, further study of these processes would benefit from the use of VUV sources that more closely replicate the solar emission spectrum. The remainder of this thesis characterises a hydrogen RF plasma source designed to simulate the Solar VUV, particularly in the 121.6 nm Ly-a line. Hydrogen RF plasmas (13.56 MHz, up to 400 W) were operated over 10-100 mTorr. The Ly-a intensity peaked at 50 mTorr, then declined despite rising electron density. Comparisons with an optically-thin, collisional model driven by plasma parameters obtained by probe and optical diagnostic indicate this is likely the result of self0absorption by ground-state atomic hydrogen. These results imply a pressure-dependent maximum in the Ly-a emissivity for a given RF input power.

The VUV spectra of hydrogen plasmas mixed with argon, neon and helium were then observed as a function of hydrogen concentration at fixed pressure (40 mTorr, 400 W). The addition of the inert gases enhanced the Ly-a intensity, primarily by the increase in electron density due to the heavier ion masses of the admixed gases. The Ly-a intensity reached a maximum with H2 concentration, and by varying the total pressure at which the mixtures were maintained (20-80 mTorr), it was observed that higher total pressures resulted in greater maximum intensities, also occurring at lower H2 concentrations. The maximum Ly-a 'enhancement' observed was 3.6 x, compared to a pure H2 at the equivalent pressure, with a mixture of 27.5% H2 : 72.5 % Ar at 80 mTorr and 400 W RF power. Again, divergence of measured intensities from an optically thin model at higher partial hydrogen pressures indicated that self-absorption is still a highly relevant for mixed gas plasmas at these pressures and densities.

A Solar VUV simulator was then designed and characterised, based on a transformer coupled plasma of hydrogen mixed with an inert gas and an RF power of 400 W. Spectra from pure H2 and mixed H2:Ar/Ne/He plasmas were compared against the Solar VUV spectrum, with the closest match found to occur for a 35 % H2 : 65 % Ar plasma at 20 mTorr. Spatially resolved measurements of the H-a (656.3 nm) line intensity were used to develop a model of the spatial distribution of the plasma emission, and compared with photodiode measurements to obtain an RF- to-VUV power conversion efficiency of 32% +- 1.8%. Measured VUV fluxes as high as 40 mW/cm2 were observed at the source output aperture, approximately 4000 x the Solar VUV flux. In the current configuration, a 30 cm diameter spot can be illuminated with 100 - 200 x relative Solar fluxes simultaneously. Show more