9th Spacecraft Charging Tec EPOCHAL TSUKUBA, T Proposed Modifications to Engineering Design Guidelines Related to Resistivity Measurements and Spacecraft Charaina v v J.R. Dennison Prasnnna Swaminathan, Randy Jost and Jerilyn Brunson Utah Stute University Nelson Green and A. Robb Frederickson Jet PropLrlsion Laboratory UtahState UNIVERSITY Proposed Modifications to Engineering Design Guidelines Related to Resistivity Measurements and Spacecraft Charging Supported by the NASA SEE Program, USU Research Office, and A. Robb Frederickson USU College of Science Jet Propulsion Laboratoty Abstract A key parameter in modeling differential spacecraft charging is the resistivity of insulating materials. This determines how charge will accumulate and redistribute across the spacecraft, as well as the time scale for charge transport and dissipation. Existing spacecraft charging guidelines recommend use of tests and imported resistivity data from handbooks that are based principally upon ASTM methods that are more applicable to classical ground conditions and designed for problems associated with power loss through the dielectric, than for how long charge can be stored on an insulator. These data have been found to underestimate charging effects by one to four orders of magnitude for spacecraft charging applications. .A_ rp vipw is presentprt nf methnds to measure the resistive of highly insulating materials- including the electrometer-resistance method, the electrometer-constant voltage method, the voltage rate-of-change method and the charge storage method. This is based on joint experimental studies conducted at NASA Jet Propulsion Laboratory and Utah State University to investigate the charge storage method and its relation to spacecraft charging. The different methods are found to be appropriate for different resistivity ranges and for different charging circumstances. A simple physics-based model of these methods allows separation of the polarization current and dark current components from long duration measurements of resistivity over day- to month-long time scales. Model parameters are directly related to the magnitude of charge transfer and storage and the rate of charge transport. The model largely explains the observed differences in resistivity found using the different methods and provides a framework for recommendations for the appropriate test method for spacecraft materials with different resistivities and applications. The proposed changes to the existing engineering guidelines are intended to provide design engineers more appropriate methods for consideration and measurements of resistivity for many typical spacecraft charging scenarios. 8-Apnl-05 9"'SCTC 2005 Outline 3. Two methods for measuring resistivity 1. Constant-voltage (ASTM) 1. Method 2. Instrumentation 3. Equations Capacitor Model of 4. Results for PET 2. Charge Storage Method 1. Method 2. Instrumentation 3. Equations 4. Results for: 1. 2. 3. 4. Conclusions sms and time 1. Time scale easur and 5. A charge picture 3. Conclusions to be drawn 6. Two ways charge can change 7. The voltage or current picture 8-.4pril-05 9Ih SCTC 2005 4 Resistivty and Spacecraft Charging As before, spacecraft accumulate charge and adopt potentials in response to the plasma environment. The distribution and migration of this charge determines the extent of difierentiai charging. Resistivity of insulating materials determines: Where charge will Spacecruji Clturging resultsf rom accumulate charge accumulation. transport and dissipation. 8-April-05 9Ib SCTC 2005 5 Our Spacecraft Charging Issues New testin have identified a problem > Charge Storage resistivity tests done on Polyimides, ds find p 101-104 8-April-05 9hS CTC 2005 6 Definition of Resistivity Familiar with concept of resis as the proportionality constant in Ohm’s Law: sic device property that sistance to flow of o a driving force V. is the proportionality in another form of Ohm’s 91 hthat p=R.A/L=I/a . aterial prope 8-April-05 9Ih SCTC 2005 7 Thin-film Capacitor Model for Spacecraft Dielectrics Most critical charging systems can be approximated as thin- film capacitors. Charge accumulation and dissipation on a oarallel-rAate Capacitor Discharge 8-April-05 9Ih SCTC 2005 R Orbit Time and Charge Decay Time Treating thin film insulator as simple capacitor, charge It o 24 hours. decay time proportional to I resistivity. 8-April-05 91h SCTC 2005 9 Critical Time Scales and Resistivites x MARGINA 1 .lo4 E .- c) 1.10'~ 1 1 1 1 1 1 1 resistivity (ohm-crn) X-.4pril-O5 !Yb SCTC 2005 10 Time Independent Capacitor Voltage: A SimDle Model E=V/d Er= I I D=EE=E,E, E = E E~+ P +- Qo . + i+* + s I+ +T + ... .'. + .++ + + f 2. D = coE I T" I 1 n=E OE+ P T" 8-April-05 9jh SCTC 2005 11 Capacitor Voltage with Leakage Consider decay of free charge: 8-April-OS 9'h sac zoos 12 -. Capacitor Voltage with Polarization 1 I I ! ! Consider *.. Q build-up of I) =,..t: . P i. Ir.: ......................................... . ,I i -I -----........ ........ -1' bound 1 . I / I 1 charge: 0 II J I 8-April-05 9Ib SCTC 2005 13 Polarization Time Scales and Mechanisms \1 orientational atomic electronic 41 I , , , , , , , , , , , Yi I , r 1 Id' 0 io* IO' IO6 IO8 IO'" lo" IO" IO" Frequency (Hr) 9Ib SCTC 2005 14 Polarization in Polymers E I P 8-April-OS qbS CTC ZOOS IS Comparison of Resistivity Test Methods ASTM Capacitor Method Adjustable Voltage * I 1 p = 1 ~ 1 0 (~ohm~ -c m) p > 5x1020 For measurements on same sample of polyir n id e 8-April-OS 9Ih SCTC 2005 16 Comparison of Typica Resistivity Results + Polyimide Surface Voltage 500 I I I I a5n l.....iy;~i LaRC-SI401 400 350 300 I 250 4 I I I 0 10 20 30 40 Days Since First Charge Established p = -2x1Ol6 (ohm-cm) p > 5x1Ol9 For measurements on samples of Kapton H 8-Apnl-05 9Ib SCTC 2005 17 Constant-voltage (ASTM Resistivity: Methods Constant voltage replenishes free charge and supplies current to balance polarization. Limited by small current measurements. 8-.4pril-05 9Ib SCTC 2005 Constant-voltage (ASTM) Resistivity: Instruments Vwlable Voltmge Aluminum 1 Frame J I ' I J I dnmr cardboard Experimental setup of the classical ASTM method I S-April45 9Ib SCTC 2005 19 Constant-voltage (ASTM) Resistivity: Equations For the Constant Voltage Method, the measured current as a function of elapsed time is where t = time, seconds t, = polarization decay constant, seconds t,, = dark current decay constant, seconds ep = initial relative dielectric constant, Flm erm= asmyptotic relative dielectric constant, F/m I,,=measured current, amp V,,=constant applied voltage, volt C, = capacitance of the sample with e=l, farads 8-April-OS 9" SCTC 2005 20