The Electromagnetic Physics Laboratory (EMPL) at the Kennedy Space Center Is a Fully Functional

NASA Kennedy Space Center

Testing and Research Capabilities at the

Electromagnetic Physics Laboratory (EMPL)

The Electromagnetic Physics Laboratory (EMPL) at the Kennedy Space Center is a fully functional testing and research facility, specializing in Electrostatics [1-3]. The laboratory is under the direction of Dr. Carlos Calle, the lead scientist. Dr. Ray Gompf, with over 30 years of testing experience, is considered a leader in Electrostatics testing [4]. Dr. Charles Buhler, a Condensed Matter physicist, is a postdoctoral scientist under Dr. Calle. Other members of the group include Dr. James Mantovani, a professor of Condensed Matter physics at the Florida Institute of Technology and a NASA faculty fellow in our lab; Michael Hogue, M.S.; Ellen Groop, B.S.; and Andrew Nowicki, B.S.

I. Testing Capabilities at the EMPL

The EMPL is fully equipped to handle any Electrostatic test under various environmental conditions. A list of measurement capabilities is given below.

·  Surface Resistance

·  Charge Decay and Charge Measurements

·  Triboelectric Charging Measurements

·  Volume Resistivity

·  Adhesion Coefficient

·  Electric Field Measurements

·  Surface Charge Distribution

·  Surface Analysis Techniques

Surface Resistance Measurements

In general, materials with a low surface resistance tend to not build up electrostatic charge. Therefore, measurements of the surface resistance have been used as an industry standard for many years. There are several types of surface resistance probes. A few of the instruments in the EMPL are listed below:

·  Prostat PRS-801 Resistance System
·  Prostat PRS-812 Resistance Meter
·  PRS-801 Resistance System
·  Surface Resistance Probes (2)
·  PRF-914 Resistance Fixture
·  PAR-809B Variable Resistance
·  ETS Series 2000 Static Control Audit Kit
·  Surface Resistance Probe Model 850
·  Wide Range Resistance Indicator 870A
·  Resistance to Ground Tester Model 255
·  ETS Surface Resistance Probe Model 850
·  ETS Resistance/Resistivity Model 823

Charge Decay and Charge Measurements

Measurements of the surface resistance alone cannot truly identify a material as being static dissipative. In order to see charge bleed off with time, charge decay measurements must be taken. The EMPL has the following instruments:

·  ETS Static Decay Meter

·  JCI 155v4 Charge Decay Test Unit
·  PDT-740B Static Decay Timer
·  JCI 176 Electrostatic Pan
·  Model 210HS Q/m Test System
·  JCI 147 Faraday Pail
·  PFC-721 Faraday Cup Assembly
·  ETS Faraday Cup
·  Large ME Faraday Cup

In the JCI 155v4 Charge Decay Test Unit (Figure 1) a high voltage corona discharge (+ or – polarity) is used to deposit a patch of charge on the surface of the material to be tested [5-9]. A fast response electrostatic fieldmeter observes the voltage generated by this charge and measures how quickly the voltage falls as the charge mitigates away. It is particularly useful in that the data analysis is done directly and Voltage vs. time graphs can be plotted simultaneously. Charge decay curves are classified as the charge remaining per unit time by the decay constant t given by the equation Q(t) = Q0 exp(-t/t) if ohm’s law is obeyed. The JCI 155v4 can give the value for the initial charge deposited Q0 as well as the decay constant t for each test and the data can be displayed and printed from a computer. This unit is capable of testing almost any material including fabrics, plastics, polymers, liquids and powders.

Figure 1

Particle charge measurements can be done with a charge to mass (Q/m) meter. These instruments are standard tools used in the photocopier industry to detect the amount of charge on toner and carrier particles. The Model 210HS Q/M Test System instrument is capable of measuring single as well as dual component powders or soils. Faraday cups are also widely used to measure the total charge of a soil or powder sample. These two instruments are capable of measuring any electrostatic charging of powders, dust particles or soils.

Triboelectric Charging Measurements

Another way to deposit charge to a surface is by rubbing one material with another and then separating. This process of charge deposition is called triboelectrification. Successful triboelectric testing of materials for charge generation and decay has been performed at this NASA/KSC laboratory since 1963. During this period KSC has specialized in this form of static charge generation through the use of a triboelectric rubbing machine, evaluating thousands of materials [4]. Our lab developed the current standards used at the Kennedy Space Center for the electrostatic certification of materials used in the Space Shuttle and the International Space Station preparation facilities.

The current version of the triboelectric rubbing machine, the ESD Robot (Figure 2) is a fully automated robot capable of testing up to six materials in a variety of environments [10,11]. This instrument is designed to measure both the electrostatic generating potential and the discharge time of the material under test. It is capable for testing films, clothing materials, space suits, solid foams, gloves, paints and coatings. The robot operates in a laboratory test chamber (after proper material acclimation time) under the following environmental conditions:

·  Atmospheric pressure: 0.4 torr to 760 torr ( 0.533 mb to 1010 mb)

·  Temperature: -190°F to 392°F (-123°C to 200°C)

·  Humidity: 0.5% to 100%

·  Various atmospheric gas combinations.

Figure 2

Volume Resistivity Measurements

The Chilworth Volume Resistivity Kit can measure the volume resistivity (W m) of a soil or powder. With an applied voltage, the current can be measured and the volume resistivity r = V/I * 0.4 can be calculated. With r less than 1 X 106 W m the soil is said to be conductive in electrostatic terms and materials having a r greater than 1 X 1010 W m are considered insulating. In between they are said to be antistatic.

Adhesion Coefficient

The adhesion coefficient gives a measure of how well a dust or powder adheres to a surface. There are a number of ways to define this coefficient. It can be defined as the ratio of the number of particles detached to a surface to the total number of particles originally attached to the surface. It can also be the mass ratio of the two or a ratio of the area covered before and after detachment. There are several forces involved in attachment including the electrostatic force (set up by image charges), the Van der Waals force, the capillary action force (moisture), forces due to the double electric layer and forces due to contact potential. Measuring the force of detachment needed to dislodge the particles can make an estimate of these forces. Detachment can be caused and measured by subjecting the sample to sliding, vibrating, rotating, bombarding it with air pressure, applying DC and AC electric fields and magnetic fields. For example, a Scanning Electron Microscope (SEM) can show the area (or count the number) of particles covered before and after a material is placed in a dust-laden flow. This ratio gives the adhesion coefficient.

Electric Field Measurements

An electric field may be present once charge is deposited or removed from a surface. We have several instruments capable of determining the resulting electric field:

·  ETS Electrostatic Voltmeter Model 105
·  JCI 148 Electrostatic Voltmeter
·  HP 7015B Electrostatic X-Y Recorder
·  JCI 140F Static Monitor (2)
·  JCI 149 Charge Dissipation Test Unit
·  Credence Technologies EM-Eye
·  JCI Giant Electrometer (Brass)
·  Champman EOS 100 Electrostatic Meter
·  SSD Statiron-M2 Electrometer
·  Statiron-M Electrometer
·  2501 Static Detecting Head
·  Prostat Kit
·  PFM-711A Electrostatic Fieldmeter
·  PCS-730 Electrostatic Charging Source
·  Series 2000 ESD Test System Gun
·  Series 2000 ESD Test System Model PSC-1
·  Series 2000 ESD Test System DN-2
·  Series 2000 ESD Test System DN-10
·  ETS Series 2000 Static Control Audit Kit
·  Model 205C Charge Plate Detector
·  Model 212 Static Meter
·  Static Watch I IMCS

The EMPL is also equipped with many ionizers that can be used to discharge any material. By creating ions of both polarities, the ionizers blow ions onto a surface to neutralize any unbalanced charge.

Surface Charge Measurements

The build up of surface charge on insulator surfaces and its decay can be measured in real time with a multisensor electrometer developed by our lab in collaboration with the Jet Propulsion Laboratory (Figure 3) [12]. This instrument, developed as a flight instrument for a Mars landing mission, is currently being used in our laboratory for research and testing purposes [13-15]. An advanced multisensor electrometer (Figure 3) has recently been developed to measure the surface charge deposited onto a surface by means of dust transport [16-19]. This electrometer is suited to test materials exposed to powders/dusts at high speeds under various environmental conditions to measure the charge buildup as particles come in contact with it. The instrument can be used not only to test a materials response to charge accumulation but also a material’s ability to keep dust adhering to it.

Figure 3

Surface Analysis Techniques

Materials at the EMPL can be analyzed using X-ray photoelectron spectroscopy (XPS) or ion scattering spectroscopy (ISS). With XPS both elemental information as well as chemical state and bonding information are available. The instrument includes a provision for surface etching to allow study of sub-surfaces. The etching is done with argon ions and the energy and diameter of the beam can be controlled. ISS is a true surface technique that is useful for looking at the surface monolayer of a sample. It allows elemental analysis of the very surface of samples. The XPS looks deeper into the surfaces of samples, down to around 80 angstroms, depending on the sample. Both of these analyses can be done in a single instrument so that a great deal of information on surface composition can be derived from a single sample.

The lab also has access to a Scanning Electron Microscope (SEM) of which images of a surface can be made. The SEM cannot only provide images with a resolution down below a few microns, it can also provide an elemental analysis of the material. This is a standard test used to perform surface analysis quickly and reliably.

II.  Testing Conditions at the EMPL

The EMPL is capable of performing many of the above tests in a variety of environmental conditions including:

·  High and Low Temperatures and Pressures

·  High and Low Humidities

·  Various Atmospheric Conditions

Vacuum Chambers

The MEC Chamber

The large Mars Electrostatics Chamber (MEC) (Figure 4) has a volume of 1.5 m3 and was designed to simulate the Martian environment for electrostatic studies [20,21]. This is a fully automated chamber with a graphical user interface that is capable of bringing the chamber down to 10 mbar and –90o C. A liquid N2 cold plate may be used to cool the 1.43 m ´ 0.80 m deck if necessary. The deck and shroud can also be heated above 250o C if a bake out is required. The atmospheric control subsystem also provides an atmosphere of most gases at pressures from 10-2 torr up to 760 torr.

Figure 4

The small vacuum chamber

The EMPL is also fitted with a small automated vacuum chamber that is capable of pressures from atmospheric down to 10-7 torr. It too can be cooled with a liquid N2 cold plate and backfilled with most gases.

Environmental Chambers

The large walk-in Environmental Chamber

This large chamber (Figure 5) is capable of performing tests from –34oC to 85oC at relative humidities ranging from 5% to 95%.

Figure 5

Large Thermal Vacuum Chamber

With a volume of 1.22m X 1.22m X 1.52m, this large chamber is not only capable of cooling down to –72oC and heating up to 177oC, it is also capable of pumping down to 1 torr simultaneously.

Environmental Chambers

The lab is also equipped with three low humidity chambers that measure and control the relative humidity (from 0.5% to 100% RH) at atmospheric pressures. Each is fitted with glove ports to ensure the safety of the employees when working with dusts and powders.

Simulated Conditions

Dust Impeller

Besides controlling the atmospheric conditions (temperature, pressure, and gases), the EMPL has a further ability to reproduce windy conditions as a way to disperse powders/dusts onto a material [15-18]. This need derived from the inability of convection to occur at low pressures. Convection alone cannot propel dust at low pressures (~ 1 torr) so a piezoelectric device was created to disperse a systematic amount of dust onto a surface. Shown below (Figure 6) is the Dust Impeller that is capable of reproducing winds up to 80 m/s at various pressures.

Extended Capabilities

Prototype and Development Laboratory

The Electromagnet Physics Laboratory is provided full fabrication support through the Prototype and Development Laboratory. This shop is part of the Labs and Testbeds Division at the Kennedy Space Center.

Figure 6

References

1. Calle, C.I, "Measuring Electrostatic Phenomena on Mars and the Moon," Proceedings of the Institute of Electrostatics Japan, pp. 169-279, Tokyo, 2001.
2. Calle, C.I., E.E. Groop, J.G. Mantovani, and M.G. Buehler, "Maximum Frictional Charge Generation on Polymer Surfaces," Bulletin of the American Physical Society, 46,1165 (2001).
3. Kim, H.S., D. Jackson, C. Calle, R. Gompf, R. Lee, D. Lewis, P. Richuso, M. Parenti, J. Bayliss, J. Rauwerdink, and, M. Buehler, "Electrostatic Discharge of Materials in a Simulated Martian Environment," in Research and Technology 1999 Annual Report, J.F. Kennedy Space Center, NASA Technical Memorandum 208567, p. 44, 2000.
4. Gompf, R.H., “Physical and Chemical Test Results of Plastic Films”, MTB-402-85, Kennedy Space Center, March 3, 1986.
5. Chubb, J.N., “Instrumentation and standards for testing static control materials” IEEE Trans. Ind. Appl. 26 (6) 1990, p 1182.
6. Chubb, J.N, “Dependence of charge decay characteristics on charging parameters” Proceedings of ‘Electrostatics 1995’ Conference, University of York, April 3-5, 1995 Inst Phys Confr Series 143 p103.
7. Chubb, J.N., P. Malinverni “Experimental comparison of methods of charge decay measurements for a variety of materials” EOS/ESD Symposium, Dallas, 1992.
8. Chubb, J.N.,. “Corona charging of practical materials for charge decay measurements” J. Electrostatics 37 (1&2) 1996 p53.
9. Chubb, J.N., “New approaches for testing materials” Proceedings ESA Annual Meeting, Brock University, Niagara Falls, June 18-21, 2000.
10. Gompf, R.H., “Triboelectric Testing For Electrostatic Charges On Materials At Kennedy Space Center”, 1984 EOS/ESD Symposium Proceedings, EOS-6, 58-63.
11. Gompf, R.H., “Robotic Testing For Triboelectric Properties In A Computer Controlled Environment At Kennedy Space Center”, 1986 EOS/ESD Symposium Proceedings, EOS-8, 151-155.
12. Buehler, M.G. L-J. Cheng, O. Orient, D. Martin, R.H. Gompf, C.I.Calle, J. Bayliss, and J. Rauwerdink “From Order to Flight in Eighteen Months: The MECA Electrometer Case Study,” Proceedings of the 2000 IEEE Aerospace Conference, (2000).
13. Calle, C.I., C.R. Buhler, J.G. Mantovani, E.E. Groop,M.D. Hogue, and A.W. Nowicki, “Experimental Results of a Mission-Ready Triboelectric Device for Mars Robotic Missions,” Proceedings of the Electrostatics Society of America-Electrostatics Society of Japan, 106, (2002)
14. Mantovani, J.G., C.I. Calle, E.E. Groop, A.W. Linville, R.H. Gompf, and M.G. Buehler, "Performance Status of the Mars Environmental Compatibility Electrometer," Proceedings of the 38th Space Congress, (2001).
15. Calle, C.I., J.G. Mantovani, C.R. Buhler, and E.E. Groop, “Particle Charging with Regolith Simulant Particles at Martian Environmental Conditions,” Proceedings of the Institute of Electrostatics of Japan Annual Meeting, (2001).
16. Calle, C.I., J.G. Mantovani, C.R. Buhler, M.D. Hogue, A.W. Nowicki, and E.E. Groop, "Electrostatic Charging of Polymers by Particle Impact at Low Pressures," Proceedings of the Fourth International Conference on Applied Electrostatics, Dalian, China, October 2001.
17. Mantovani, J.G., Calle, C.I., E.E. Groop, and M.G. Buehler, "Studies of Surface Charging of Polymers by Indirect Triboelectrification," Bulletin of the American Physical Society, 46,1165 (2001).
18. Mantovani, J.G., C.I., Calle, E.E. Groop, and M.G. Buehler, "Neutralizing Triboelectrically Generated Charge on Insulators under Martian-like Atmospheric Conditions," Bulletin of the American Astronomical Society, 32, 1640 (2000).
19. Mantovani, J.G., C.I., Calle, E.E. Groop, and M.G. Buehler, "Triboelectric Charging of an Insulator's Surface Using Martian Soil Simulant," Bulletin of the American Physical Society, 45, 26 (2000).
20. Buchanan, R.K., A.C. Barnett, and C.I. Calle, "Controlling Cryogenics for Creating Mars Environment," Proceedings of the ISA Aerospace and Test Measurement Division, 2001.
21. Calle, C.I., D.C. Lewis, R.K. Buchanan, "Capabilities of the Mars Electrostatics Chamber at Kennedy Space Center," Proceedings of the 38th Space Congress, 2001.

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