Experimental Investigations of Dusty Spacecraft Charging at the Lunar Terminator

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1 Missouri University of Science and Technology Scholars' Mine Mechanical and Aerospace Engineering Faculty Research & Creative Works Mechanical and Aerospace Engineering Experimental Investigations of Dusty Spacecraft Charging at the Lunar Terminator Kevin Chou William Yu Daoru Frank Han Missouri University of Science and Technology, Joseph J. Wang Follow this and additional works at: Part of the Aerospace Engineering Commons Recommended Citation K. Chou et al., "Experimental Investigations of Dusty Spacecraft Charging at the Lunar Terminator," Proceedings of the AIAA SPACE 2015 Conference and Exposition (2015, Pasadena, CA), (AIAA), Sep The definitive version is available at This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in Mechanical and Aerospace Engineering Faculty Research & Creative Works by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu.

2 AIAA SPACE Forum 31 Aug-2 Sep 2015, Pasadena, California AIAA SPACE 2015 Conference and Exposition Experimental Investigations of Dusty Spacecraft Charging at the Lunar Terminator Kevin Chou, William Yu, Daoru Han, and Joseph J. Wang University of Southern California, Los Angeles, CA , USA An experimental investigation is conducted to understand the I-V characteristics and floating potential of a surface covered by varying amounts of dust in a plasma environment similar to that at the lunar terminator. I-V curves are measured to determine dust coverage effects on surface charging, and a non-contacting electrostatic voltmeter is used to measure dust surface charging. Results show that as dust coverage increases, the electric field created between the dust surface layer and conducting surface layer increases the possibility of arcing and breakdown. I. Introduction The lunar surface is directly exposed to various space plasma environments and charged by ambient plasma collection and photoelectron emission because the Moon s weak magnetic field cannot deflect ionized particles. Typical day-time lunar surface potentials have been reported to be on the order of a few tens of volts positive due to photoelectron emission, and night time potentials have been reported to be on the order of hundreds to even thousands of volts negative Both solar illumination and plasma flow have a substantial influence on lunar surface charging. The effect of plasma flow is especially complex near the lunar terminator, where the transition from sunlight-driven positive surface potential to plasma-charged negative surface potential occurs The solar wind plasma is typically mesothermal, where the directed plasma flow speed is larger than ion thermal speed but less than electron thermal speed. At the terminator region, the solar wind plasma flows over the rugged terrain at a low elevation angle and generates localized plasma wake regions. This, combined with localized shadows, generates zig-zag divisions of positively charged and negatively charged regions. Many difficulties with lunar dust have been documented by Apollo astronauts. The M agellan spacecraft mission to Venus also experienced major technical issues to its star tracker due to dust contaminants present in the Venus environment. 17 Particulates and dust have been observed on the Mars Observer, and similarly to Magellan, the star tracker experienced problems. 40 Because of its adhesive properties, abrasive surfaces, and electrostatic nature, lunar dust adheres to spacesuits and equipment, decreasing the lifetime of equipment and creating major problems during mission operation. 15 Spacecraft charging is well understood for a clean surface with no contaminants, but charging on a dusty surface, where dust accumulates on the surface, has not been studied extensively. Dusty environments are not limited to only the lunar surface. It is important to obtain a fundamental understanding of dust effects on spacecraft charging for future missions to an asteroid or a planet with dust. 41, 42, 40 Spacecraft charging has been a subject of extensive investigations. The floating potential of a clean spacecraft is dictated by current balance, where the total current collected by the spacecraft is equal to zero. This includes the collection of ion and electron current, as well as any secondary electron or photoelectron emission. On a dusty surface, where the dust grains are electrically coupled with neighboring dusts because the inter-dust distance is much less than the Debye length, and a single sheath forms over the dust surface, 16 however, the charging changes. The spacecraft floating potential is still dependent on current balance, but the dust layer capacitance and dust surface potential must also be taken into consideration. This Graduate Research Assistant,Department of Astronautical Engineering, 854B Downey Way, Los Angeles, California , AIAA Student Member Associate Professor, Department of Astronautical Engineering, 854B Downey Way, Los Angeles, California , AIAA Associate Fellow 1 of 13

3 is because the dust grains are electrically coupled with the neighboring dust grains as well as the spacecraft surface. There is limited knowledge on the I-V characteristics and floating potential of a dust covered surface in plasma. Spacecraft charging studies have mostly looked at clean surfaces. Langmuir probe theories are for a probe in a clean plasma. This paper considers the electric properties of a dust covered surface in plasma and looks at the change in current-voltage characteristics and floating potentials due to dust. It will also be important to look at differential charging and the possibility of arcing or breakdown between the dust layer and spacecraft surface. 19 II. Experimental Setup The experiment was conducted in a cylindrical, stainless steel vacuum chamber, measuring 91.5 cm in diameter and 122 cm in length. An Alcatel mechanical pump is used for roughing, and a CVI TM500 cryogenic pump with a pumping speed of 8,500 L/s brings the chamber to high vacuum. The chamber pressure is maintained between 10 7 to 10 6 Torr with 2.5 sccm of argon gas flow. A 4 cm diameter electron bombardment gridded ion thruster with a hot filament neutralizer was used for this experimental investigation. Figure 1 shows the ion source configuration. To generate a mesothermal plasma, argon gas flows through the back of the ionization chamber and thermal electrons emitted from the hot tungsten filament surface ionize the neutral argon gas. A magnetic field, created by ring magnets and a back magnet, confines the electrons along magnetic field lines to enhance collisions and ionization with the neutral gas molecules. The ionization chamber is biased to 1100 V above ground, and the anode cup is biased 50 V higher than the ionization chamber to absorb any low energy electrons that exist after collisions. This maintains continuous ionization. Ion optics accelerate the ions, generating a mesothermal plasma to simulate average solar wind conditions, and a hot-filament neutralizer is placed directly downstream of the source exit plane to reduce space charge effects and arcing. 31 The source is contained in a grounded enclosure to screen the accelerated beam from the high voltage internal components and prevent potential perturbations. The plasma source was run at 10 ma with 2.5 sccm of argon gas flow for all the experimental cases. Figure 1. Ion source configuration Figure 2. Experimental setup An electrostatic Langmuir probe, axial and radial Faraday probe, and emissive probe were used to obtain plasma field parameters. A commercial Trek non-contacting electrostatic voltmeter (ESVM) and vibrating capacitative probe were also used to measure the sample surface potential, φ s. The diagnostic tools were placed on a 3D traversing system that moved the probes with an accuracy of 1 mil. A 7.62 cm 12.7 cm floating aluminum plate was used to simulate the spacecraft surface. To examine the effects of dust coverage, borders were used to create 15 equal 2.54 cm 2.54 cm grids. Ten different combination of grids were then filled with JSC-1A, a lunar simulant, as shown in Figure 3, and used as test samples. Dust quantity effects were studied by varying dust coverage from no dust to full dust, shown in Table 1, and dust positioning was examined by filling the same number of grids but changing the dust grid positions. The dust layer thickness was 1.27x10 2 cm. Before each case was tested, the dust samples were held under vacuum and baked with a Watlow polyimide sheet heater to outgas residual moisture. Each dust sample configuration will be referred to by the number and position of dust grids, as shown in the labels in Figure 3. 2 of 13

4 (a) N o dust coverage (b) 1 sq (c) 7 sq checker (d) 7 sq (e) 7 sq (f) 7 sq (g) 7 sq (h) 8 sq checker (i) 14 sq (j) F ull dust coverage Figure 3. Sample setups Table 1. Dust sample coverage N o Dust 1 sq 7 sq checker 7 sq sq sq sq sq checker 14 sq F ull dust coverage Dust Coverage Percentage 0% 6.7% 46.7% 46.7% 46.7% 46.7% 46.7% 53.3% 93.3% 100% The plasma source was placed opposite the cryogenic pump, along the chamber centerline, and set at 0 angle of attack with respect to the target sample, shown in Figure 2. The target sample was placed 17.8 cm downstream of the source. Plasma plume measurements were taken over a 12.7 cm axial by 7.62 cm radial scan region to obtain a 2D plume profile. The scanning area was divided into 77 measurement points with a spatial resolution of 1.27 cm by 1.27 cm. III. III.A. Results and Discussions Plasma Plume Parameters The plasma diagnostics discussed in Section II were utilized to obtain a full set of plasma parameters for a 10 ma beam with 2.5 sccm argon gas flow. Figure 4 contains the plasma potential, φp, current density, Ji, ion density, ni, electron density, ne, and space charge, ne ni. The closest measurements to the target sample were made 1.91 cm above the surface. III.B. Current-Voltage Characteristics Figure 5 shows the measured I-V curves for each sample. The I-V curves represent the current collected as the aluminum plate is biased from -40 to 40 V. The floating potential of the plate is the potential where the ion and electron current are equal, which corresponds with where total current collected equals zero. It can be seen that as the dust coverage increases, the total current collected by the aluminum plate begins to decrease. When dust covers the entire surface of the aluminum plate, the plate is not charged, and only noise is measured. From the plots, it is clear that the no dust case has the highest floating potential, and 3 of 13

5 (a) φ p, plasma potential (b) J i, ion current density (c) n i, ion density (d) n e, electron density (e) n i n e, space charge (f) T e, electron temperature Figure 4. Plasma environment 4 of 13

6 (a) (b) (c) (d) (e) (f) (g) (h) (i) Figure 5. I-V curves 5 of 13

7 (a) (b) Figure 6. I-V curve comparison Table 2. Floating potential with respect to ambient plasma potential Measured [V] No Dust sq sq checker sq sq sq sq sq checker sq F ull Dust Coverage all 7-square samples have very similar I-V curves. As more dust covers the surface, however, it is surprising to see that dust coverage quantity does not have much of an affect on floating potential. Figure 6 presents all the I-V curves together. The decrease in total current collected as dust coverage increases can be explained by current balance, and a circuit can model the system. The total current collected is dependent on the plate potential with respect to the ambient potential as well as the difference between the ion flux and electron flux multiplied by the collecting surface area. Secondary electrons are not consideration because the plasma species have such low energies. Because the exposed surface area decreases as dust coverage increases, it can be seen that the total current collected should decrease, shown in Eq. (1), where c s is ion acoustic velocity, T e is electron temperature, Φ amb is ambient potential, V bias is the plate biased potential, and A exposed is the exposed plate collection area. The sheath that forms over the aluminum plate can also be modeled as a resistor. This resistance, Eq. (2), follows Ohm s law and is dependent on the plate potential with respect to the ambient potential. I tot (V bias ) = (en i c s en e kb T e 2πm e exp ( e(φamb V bias ) k B T e ) A exposed (1) R tot (V bias ) = Φ ambient V bias I tot (2) C dust = ɛ 0 ɛ rd A/d (3) 6 of 13

8 Table 3. Sheath resistance as function of dust coverage [Ω] Bias : -40 V 0 V 40 V No Dust sq sq sq sq Table 3 lists the total sheath resistance as a function of the plate bias for each sample. Because the 7 square dust samples all have the same exposed collection area, their sheath resistance are all the same. It can be seen that as dust coverage increases, the resistance increases as well. In fact, the total resistance of the 14 square case is the sheath resistance of one exposed square and can be used to model the total sheath resistance of all other samples. For example, the no dust case has 15 exposed squares and can be modeled sq resistors in parallel. This can be mathematically proven and is illlustrated in Figure 7, where R tot (V bias ) is the total sheath resistance. When dust is introduced, the dust squares can be modeled as a resistor and capacitor in series. The resistor models the sheath formed due to the dust surface potential, and the dust layer acts as a parallel plate capacitor following Eq. (3), where ɛ 0 is the permittivity of free space, ɛ rd is the relative permittivity, A is the surface area, and d is the dust layer thickness. The relative permittivity of JSC-1A is Figure 8 illustrates the circuit model for the 7 square case, but this model can be adapted to all partially covered cases. Again, the 14 sq-resistors are placed in parallel, depending on the number of exposed aluminum squares, and the resistor-capacitor groups are also placed in parallel, depending on the number of dust squares. The simplified partial dust circuit model illustrates the fact that once the dust capacitance is filled, current no longer flows through the dust, and current can only be collected by the exposed aluminum surfaces. Figure 9 shows an equivalent circuit model for the full dust case. III.C. Dust Surface Potentials A non-contacting electrostatic Trek probe was used to measure the surface potential of each sample immediately after the plasma source was shut down. During the run, the probe would traverse to and measure the surface potential of the center of each grid square. The recorded values were then adjusted with respected to ground, which was measured at the beginning and end of each run, to account for any charge build-up on the probe. Black squares represent dust covered grids, and white squares represent exposed aluminum grids. The results can be seen in Figure 10. Figure 11 shows a 1-D potential profile of each sample along its centerline as well as the ambient plasma potential at the same position. It can be seen that the dust surface charges more negatively than the aluminum plate in each dust coverage condition, and as dust coverage increases, the dust surface potential becomes more negative. When there is full dust coverage, however, the dust surface potential becomes more positive than the floating potential of the no dust sample. Similarly to the circuit models discussed earlier, a circuit model can be created to explain how the dust and aluminum plate charge. When there is no dust, the plate acts as a capacitor with the ambient plasma, and the sheath formed over the plate acts as a resistor. With partial dust coverage, the dust layer and its sheath act as a capacitor and resistor in series, which is both in parallel with the plate s sheath resistance. When the dust capacitance is filled, current only flows through the aluminum plate s sheath to the plate, and the circuit behaves like the no dust circuit model. With full dust coverage, the dust and plate act as two capacitors in series, and no current can flow to the floating plate when the dust capacitance is filled. This explains how no current could be collected when we measured the full dust I-V curves. III.D. Discussions Due to its dielectric properties, the dust acts as a parallel plate capacitor and follows Eq. (3). It can be seen that as the dust surface area increases, the capacitance should increase and more charge can be stored in the dust surface. From the plots in Figure 10, this trend is clear. The dust surface charges more negatively 7 of 13

9 (a) Full circuit model (b) Simplified circuit model Figure 7. No dust circuit model (a) Full circuit model (b) Simplified circuit model Figure 8. 7 sq./partial dust circuit model (a) Full circuit model (b) Simplified circuit model Figure 9. Full dust circuit model 8 of 13

10 (a) 1 sq. (b) 7 sq. checker (c) 7 sq (d) 7 sq (e) 7 sq (f) 7 sq (g) 8 sq. (h) 14 sq. (i) F ull dust coverage Figure 10. Surface potentials 9 of 13

11 (a) 1 sq. (b) 7 sq. checker (c) 7 sq (d) 7 sq (e) 7 sq (f) 7 sq (g) 8 sq. (h) 14 sq. (i) F ull dust coverage Figure D potentials 10 of 13

12 (a) No dust (b) Partial dust (c) Full dust Figure 12. Dust/plate circuit model Table 4. Dust layer capacitance, charge, and electric field strength Capacitance [F] Charge [C] Field [V/m] No Dust sq sq checker sq sq sq sq sq checker sq F ull Dust Coverage than the aluminum plate, and as the quantity of dust increases, the dust surface potential becomes more negative. Table 4 lists the capacitance of each sample. It is interesting to note that the full dust coverage surface potential is more positive than all the other dust sample surface potentials. The numerical simulation mentioned previously will be used to assist in explaining the reasoning behind this phenomenon. Given that the dust surface potential becomes negative more rapidly than the aluminum plate floating potential as dust quantity increases, differential charging between the dust surface and spacecraft surface will occur. Because the dust layer is only cm thick, having even a small potential difference between the dust surface and plate floating potential can generate a large electric field. As dust coverage increases and the dust surface potential becomes more negative while the floating potential remains the same, the electric field created increases dramatically, shown in Table 4. This can increase the risk of arcing and breakdown. Though the electric fields shown are below the threshold for breakdown, the ion and electron energies in this experimental investigation are far lower than average solar wind conditions at the lunar terminator region. Therefore, differential charging must be mitigated. IV. Conclusion An experimental investigation was conducted to understand the I-V characteristics and floating potential of a conducting surface covered by varying amounts of dust in a plasma environment similar to that at the lunar terminator. It has been found that the I-V characteristics of a dust covered surface is influenced significantly by the dust coverage area. However, the floating potential of a surface partially covered by 11 of 13

13 dust does not seem to be very sensitive to dust coverage unless it is entirely covered by dust. The dust surface potential becomes more negative as dust coverage increases. This, in turn, increases the electric field strength between the dust layre and conducting surface, increasing the risk of arcing and breakdown. Future work will expand the measurements for different dust layer thickness, grain size, and dust coverage, and the effects of secondary emissions from dust on surface charging potential will be investigated. Acknowledgments This work is supported in part by NASA grant NNX11AH21G. References 1 D.L. Reasoner and W.J. Burke, Characteristics of the lunar photoelectron layer in the geomagnetic tail, Journal of Geophysical Research, vol. 77, no. 34, pp , R.H. Manka, Plasma and potential at the lunar surface, Photon and Particle Interactions with Surfaces in Space, pp , J.W. Freeman and M. Ibrahim, Lunar electric fields, surface potential and associated plasma sheaths, Earth, Moon, and Planets, vol. 14, no. 1, pp , J.S. Halekas, R.P. Lin, and D.L. Mitchell, Large negative lunar surface potentials in sunlight and shadow, Geophysical Research Letters, vol. 32, no. L09102, J.S. Halekas, G.T. Delory, D.A. Brain, R.P. Lin, M.O. Fillingim, C.O. Lee, R.A. Mewaldt, T.J. Stubbs, and W.M. Farrell, and M.K. Hudson, Extreme lunar surface charging during solar energetic particle events, Geophysical Research Letters, vol. 34, no. L02111, J.S. Halekas, G.T. Delory, R.P. Lin, T.J. Stubbs, and W.M. Farrell, Lunar prospector observations of the electrostatic potential of the lunar surface and its response to incident currents, Journal of Geophysical Research, vol. 113, no. A09102, J.S. Halekas, G.T. Delory, R.P. Lin, T.J. Stubbs, and W.M. Farrell, Lunar surface charging during solar energetic particle events: Measurement and prediction, Journal of Geophysical Research, vol. 114, no. A05110, J.S. Halekas, Y. Saito, G.T. Delory, and W.M. Farrell, New views of the lunar plasma environment, Planetary and Space Science, vol. 59, no. 14, pp , O.E. Berg, A lunar terminator configuration, Earth and Planetary Science Letters, vol. 39, no. 3, pp , N. Borisov and U. Mall, Charging and motion of dust grains near the terminator of the moon, Planetary and Space Science, vol. 54, no. 6, pp , W.M. Farrell, T.J. Stubbs, R.R. Vondrak, G.T. Delory, and J.S. Halekas, Complex electric fields near the lunar terminator: The near-surface wake and accelerated dust, Geophysical Research Letters, vol. 34, no. L14201, J. Wang, X. He, and Y. Cao, Modeling electrostatic levitation of dust particles on lunar surface, IEEE Trans. on Plasma Science, vol. 36, no. 5, pp , W.M. Farrell, T.J. Stubbs, J.S. Halekas, R.M. Killen, G.T. Delory, M.R. Collier, and R.R. Vondrak, Anticipated electrical environment within permanently shadowed lunar craters, Journal of Geophysical Research: Planets, vol. 115, no. E3, A. Poppe, M. Piquette, A. Likhanskii, and M. Horányi, The effect of surface topography on the lunar photoelectron sheath and electrostatic dust transport, Icarus, vol. 221, no. 1, pp , T.J. Stubbs, R.R. Vondrak, and W.M. Farrell, Impact of dust on lunar exploration, Dust in Planetary Systems, X. Wang, J. Colwell, M. Horányi, and S. Robertson, Charge of dust on surfaces in plasma, IEEE Trans. on Plasma Science, vol. 35, no. 2, pp , J. Goree and Y. Chiu, Dust contamination of the spacecraft environment by exposure to plasma, Journal of Spacecraft and Rockets, vol. 30, no. 6, pp , J. Conger and D. Hastings, Control of particle-spacecraft interactions in leo near-spacecraft environment, in 31st Aerospace Sciences Meeting and Exhibit, M. Hornáyi, Charged dust dynamics in the solar system, Annual Review of Astronomy and Astrophysics, vol. 34, pp , M. Hornáyi, B. Walch, S. Robertson, and D. Alexander, Electrostatic charging properties of apollo 17 lunar dust, Journal of Geophysical Research, vol. 103, no. E4, pp , J. Colwell, M. Horányi, S. Robertson, X. Wang, A. Poppe, and P. Wheeler, Lunar dust levitation, Journal of Aerospace Engineering, vol. 22, no. 1, pp. 2 9, A. Barkan, N. D Angelo, and R. Merlino, Charging of dust grains in a plasma, Physical Review Letters, vol. 73, pp , A. Sickafoose, J. Colwell, M. Horányi, and S. Robertson, Experimental levitation of dust grains in a plasma sheath, Journal of Geophysical Research: Space Physics, vol. 107, no. A11, Z. Sternovsky, S. Robertson, A. Sickafoose, J. Colwell, and M. Horányi, Contact charging of lunar and martian dust simulants, Journal of Geophysical Research: Planets, vol. 107, pp , T.J. Stubbs, R.R. Vondrak, and W.M. Farrell, A dynamic fountain model for lunar dust, Advances in Space Research, vol. 37, no. 1, pp.59 66, J. Colwell, S. Batiste, M Horányi, S. Robertson, and S. Sture, Lunar surface: Dust dynamics and regolith mechanics, Reviews of Geophysics, vol. 45, no. 2, of 13

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