Adrienne Dove

Experimental study of a photoelectron sheath

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 unde...

NanoRocks: Design and performance of an experiment studying planet formation on the International Space Station

In an effort to better understand the early stages of planet formation, we have developed a 1.5U payload that flew on the International Space Station (ISS) in the NanoRacks NanoLab facility between September 2014 and March 2016. This payload, named NanoRocks, ran a particle collision experiment under long-term microgravity conditions. The objectives of the experiment were (a) to observe collisions between mm-sized particles at relative velocities of < 1 cm/s and (b) to study the formation and disruption of particle clusters for different particle types and collision velocities. Four types of particles were used: mm-sized acrylic, glass, and copper beads and 0.75 mm-sized JSC-1 lunar regolith simulant grains. The particles were placed in sample cells carved out of an aluminum tray. This tray was attached to one side of the payload casing with three springs. Every 60 s, the tray was agitated, and the resulting collisions between the particles in the sample cells were recorded by the experiment camera. During the 18 months the payload stayed on ISS, we obtained 158 videos, thus recording a great number of collisions. The average particle velocities in the sample cells after each shaking event were around 1 cm/s. After shaking stopped, the inter-particle collisions damped the particle kinetic energy in less than 20 s, reducing the average particle velocity to below 1 mm/s, and eventually slowing them to below our detection threshold. As the particle velocity decreased, we observed the transition from bouncing to sticking collisions. We recorded the formation of particle clusters at the end of each experiment run. This paper describes the design and performance of the NanoRocks ISS payload.

Reducing particle adhesion by material surface engineering

We have developed surface chemical modification processes which when applied to a variety of surfaces renders the surfaces resistant to particulate contamination. Chemically modified surfaces are shown to shed particles at a dramatically higher level as compared to native surfaces. This is demonstrated on a variety of surfaces that include optics, polymers, metals and silicon. The adhesive force between lunar stimulant particles (JSC-1AF) and black Kapton is measured to decrease by 95% when the black Kapton surface is chemically modified. The chemical modification process is demonstrated to not change the surface roughness of a smooth silicon wafer while decreasing particle affinity. The optical properties of chemically modified surfaces are reported. The surface modification process is robust and stable to aggressive cleaning. The particle shedding properties of chemically modified surfaces are retained after simulated extraterrestrial vacuum ultra-violet light exposure and temperature excursions to 140°C. This technology has the potential to provide a robust passive particle mitigation solution for optics, mechanical systems and particle sensitive applications.

Operation of a Langmuir Probe in a Photoelectron Plasma

Dust transport on the lunar surface is likely facilitated by the variable electric fields that are generated by changing plasma conditions. We have developed an experimental apparatus to study lunar photoelectric phenomena and gain a better understanding of the conditions controlling dust transport. As an initial step, Langmuir probe measurements are used to characterize the photoelectron plasma produced above a Zr surface, and these techniques will be extended to CeO2 and lunar simulant surfaces.