Development and Characterization of an Ion Sourceto Simulate Solar Wind Plasma in a Vacuum Chamber

Blake A. Folta∗, Terence W. McGarvey IV†, Joseph C. Faudel‡, Kyle R. McMillen§, and Daoru Han¶

Missouri University of Science and Technology, Rolla, MO, 65409

As NASA and the United States look to return to the Moon with the upcoming Artemis Mission, many questionsmust first be answered. One of the more pressing issues with our return to the Moon is the environment that we will belooking to have extended missions on and one day mine. The Moon’s environment includes dusty plasma, i.e., plasmawith fine regolith grains in it, and thus presents a danger to equipment and astronauts alike. In an attempt to learn moreabout the lunar environment, the Gas and Plasma Dynamics Laboratory (GPDL) at Missouri University of Scienceand Technology has begun preparing its facilities for experimentation. In this vain, an RF generated ion source hasbeen fully integrated into the facility’s large scale vacuum chamber. Preliminary tests have been done with this RFsource to show its functionality. In addition, an experimental analysis of the vacuum chamber’s four diffusion pumpshas been done to determine the settling chamber pressure at various gas flow rates for each usable pump configuration.The mean free path was then calculated for each of these pump configurations in order to ensure the validity of thechamber’s experimental environment. At the conclusion of this work, the facility is ready for the installations of a fulldiagnostic probe array and will soon be ready for work with lunar regolith simulants.

The goal in installing a plasma source is to match the properties of solar wind as closely as possible. Characteristicssuch as plasma potential, ion flux, ion density, electron density, and electron temperature will be measured with an arrayof diagnostic probes. A commercial, off the shelf (COTS) RF plasma source (Veeco® ) was acquired and installedwithin the large-scale vacuum facility housed within the GPDL of Missouri University of Science and Technology(Missouri S&T). This plasma source was chosen because it can readily ionize Hydrogen gas. In future work, this canallow for experiments in which chemical interactions of solar wind with the lunar surface environment are of interest;as Hydrogen is the dominant ion species in solar wind plasma, this is a key element of importance. However, in thisand in near-future work, Argon will be used due to its chemical inertness. This will still allow several important factorson the lunar surface to be tested. Many factors play a role in the local charging environment on the Moon. Someof these factors include sheaths formed by nearby obstacles, such as a large boulder, and the Moons position relativeto the Earths magnetotail and plasma sheath [1]. A key parameter in scaling laboratory experiments to actual lunarphenomena is Debye length. Therefore, electron temperature and number density are also critical because they are thedetermining factors for this critical term. All of these properties are also important when determining the pathways ofions and electrons as they interact with the lunar regolith [2].

The background neutral gas is also of great importance for this type of experimentation. The molecular mean freepath must be much greater than the scale of the problem (in this case the distance between the plasma source andthe sample) in order to ensure that the plasma plume is not affected by charge exchange (CEX) collisions. This isimportant to consider because CEX collisions result in a population of low-energy ions that is non-existent in solarwind. Minimizing the number of ions in this low-energy population is imperative to maximizing the validity of theresults. The facility in which this experimentation took place was a 6-ft diameter, 10-ft long cylindrical vacuumchamber. High vacuum is achieved with four diffusion pumps, backed by a rotary vane pump and a roots blower. Thefour diffusion pumps are each capable of 200,000 L/s throughput for air. Figure 1, on the following page, illustratesthe setup and dimensions. Depending on which and how many pumps are turned on, the base pressure in the chamberranges from 10 to 30 𝜇Torr. Characterizing the impacts of which an individual or configuration of pumps play in theresulting collisional mean free path, when taking into consideration gas flow rate needed to generate plasma and thedesired background pressure throughout experimentation, is paramount to obtaining meaningful data in our plannedexperimentation.

In our planned experimentation, the COTS RF plasma source will be used where the chemical interactions betweensolar wind ions and lunar regolith are pertinent. The plasma source generates plasma with a triple coiled RF antenna.

∗Graduate Research Assistant, Department of Mechanical and Aerospace Engineering, 400 W. 13th St. Rolla, MO 65409†Graduate Research Assistant, Department of Mechanical and Aerospace Engineering, 400 W. 13th St. Rolla, MO 65409‡Undergraduate Research Assistant, Department of Mechanical and Aerospace Engineering, 400 W. 13th St. Rolla, MO 65409§Undergraduate Research Assistant, Department of Physics, 1315 N. Pine St. Rolla, MO 65409¶Assistant Professor, Department of Mechanical and Aerospace Engineering, 400 W. 13th St. Rolla, MO 65409, AIAA Member

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Fig. 1 Space and high-altitude vacuum facility as it currently resides in the GPDL of Missouri S&T.

The beam is extracted and focused using three molybdenum grids: a positively biased screen grid to maintain adischarge in the chamber, a negatively biased acceleration grid to extract ions from the discharge chamber, and agrounded deceleration grid to focus the beam.

Fig. 2 Photograph taken from inside the vacuum chamber of the Veeco® plasma source during operation.

References[1] Polansky, J. L., “Laboratory Investigations of the Near Surface Plasma Field and Charging at the Lunar Terminator,” PhD

dissertation, Viterbi School of Engineering, University of Southern California, Las Angelos, CA, 2013.

[2] Yu, W., “Numerical and Experimental Investigations of Dust-Plasma-Asteroid Interactions,” PhD dissertation, University ofSouthern California, Las Angelos, CA, 2018.

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