ABSTRACT. A low-mass solution to electrical insulation in the lunar environment may be pos- sible by embedding bare HV conductors within the lunar soil itself.
Vol. 3 No. 1, February 1996
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Experimental Study on Simulated Lunar Soil High Voltage Breakdown and Electrical Insulation Characteristics Hulya Kirkici Electrical Engineering Department, Auburn University, Auburn, AL
M. Frank Rose Space Power Institute, Auburn University, Auburn, AL
Todd Chaloupka Space Power Institute, Auburn University, Auburn, AL
ABSTRACT A low-mass solution to electrical insulation in the lunar environment may be possible by embedding bare HV conductors within the lunar soil itself. In this paper, a 'standard' NASA soil (lunar simulant) representing chemical and physical conditions found in some lunar samples was used in laboratory experiments, and its HV electrical breakdown data were presented for the first time. Insulation characteristics and leakage currents representative of variations in nonlinear resistivity of the soil were investigated. The data were compared to earlier published data for a typical lunar soil's electrical characteristics. A pair of bare planar transmission lines, having a parallel plate geometry, was buried in the lunar soil simulant and used to measure the simulant's electrical parameters. The soil simulant and the electrode assembly were in a high vacuum environment throughout the experiment series. For this parallel plate electrode configuration, electrical breakdown limits and temporal evolution of voltage breakdown of the simulated lunar soil were measured. The data were analyzed in terms of physical mechanisms. Results presented here may find applications in terrestrial uses of porous ceramics as insulators as well as in future lunar colony transmission line insulation techniques and design purposes.
1. INTRODUCTION
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LECTRICAL insulation of HV power systems to be used in a space environment is currently a major research area because of the interest in HV, high-power systems intended to support such components as inter-satellitemodules collecting galactic data or manned or unmanned interplanetary mission vehicles to the Moon and Mars. This paper provides data on a non-traditional insulation method, i.e. using lunar soil as an insulator. Major environmental factors, such as the Moon's high vacuum, cosmic radiation, solar particle flux, meteorites, gravity, thermal variation, and dust migration due to thermal cycling [ll should be considered when designing HV power systems for use in this hostile environment. It is not advantageous to use solid dielectric materials for insulation in the lunar environment due to the mass penalty which must be incurred for transportation of these solid
dielectrics to space. In addition, the lunar surface's extreme temperature variation, high energy photon (Wradiation) and particle flux from the sun, dust migration, and meteoroid impact phenomena tend to shorten the lifetime of solid dielectric materials, thereby reducing the reliability of traditional solid insulation materials. Vacuum insulationmay be a viable solutionto the solid dielectric material mass penalty problem [2]. However, when bare conductor materials are exposed to the lunar vacuum, they are likely to interact with the environment, similar to solid dielectric materials [31. Specifically, the electrode surfaces will be altered due to meteoroid impact, charged particles, and thermal stress causing enhanced field emission, photoemission or thermionic emission, respectively. Micro or bulk discharges would then be initiated, resulting in system failure. Additionally, the electrode surface may be contaminated by lunar dust, or other charged and
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neutral particles, causing electrical breakdown of the system. As an alternative solution to resolving electricalpower system insulation challenges in the lunar environment, one may possibly utilize the lunar soil as the insulating material by imbedding the bare conductor within the soil. The lunar soil is known to be water free and mainly composed of insulating materials. It consists of small, sharp-edged particles with the main compounds being oxides, such as SiOz,Al202, Ti02 and FeO. One other distinct property of the lunar soil is the ubiquitous presence of native iron (Feo) as a stable phase, in combination with the absence of ferric iron ) in iron bearing minerals [4].This combination of properties of the soil makes it a good candidate for electricalinsulation of the power systems to be employed in the lunar environment. Bare conductor material used in the power system can be imbedded in this high dielectric (low conductivity) soil to meet insulation requirements. Although charging phenomena due to a temperature change at the lunar surface and consequent soil migration or levitation have been reported [11, similar charging effects due to an electrically charged conductor material within the lunar soil have not been studied or reported in the literature to date. Additionally, the insulation properties of the lunar soil, and breakdown limits with respect to the operation of power systems, are not well known. Finally, all of the effects described above may play important roles in the breakdown events. A major objective of this work is to provide data for future lunar power system designs as well as to study the mentioned physical phenomena. In a recent paper by Kirkici and Rose [5], the characteristicsof a small HV transmission line segment imbedded in simulated lunar soil and operated in a high vacuum environment have been described. In the cited paper, a parallel plate electrode configuration was chosen as the transmission line geometry, and electrical parameters of the simulated soil, such as resistivity and leakage current as a function of applied field, were gtven. In this paper, the experimental results of the electricalbreakdown characteristics of simulated lunar soil are measured, again using a parallel plate transmission line geometry similar, but not identical, to the work in [5]. As a result, the two sets of data are complimentary and comparable. In both cases, the electrodes were imbedded within the soil and dc field applied in a high vacuum typical of lunar conditions.
2. EXPERIMENTAL SETUP The details of the experimental setup were given in a previous paper [5], and are briefly repeated herein. The setup consists of an Ultra High Vacuum ( U W ) chamber, a variable dc HV,power supply, and diagnostic equipment (Figure 1).The simulated soil was placed in a 4 cm deep, 15 cm long rectangular quartz container. The parallel plate electrodes were embedded in the soil at a depth of 2 cm. The soil was tightly packed into the container (in between and around the electrodes),and the assembly was baked M 8 h at 200°C to reduce water contamination in the soil before putting it into the vacuum chamber. The vacuum chamber was heated to promote outgassing while being pumped down. Initially, the chamber was pumped down at a relatively slow rate
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Figure 1. Experimental setup showing the electrical diagnostic equipment and UH vacuum chamber. For the transient current measurements a Rogowski coil, and for the leakage current measurements an electrometer was used. The electrometer was removed from the circuit during the breakdown experiments, and external circuitry was attached to the electrometerfor protection from unexpected breakdown events.
to prevent any disturbance or eruption of the soil at its surface. There was no visible movement of the soil during pump down. Overall pumping time to reach the desired vacuum range was approximately two to three weeks. Although, there was no pressure control between the electrodes,it was assumed that the electrodes were tightly embedded in the soil and they experienced constant static force throughout the series of experiments with this packing and pumping procedure. The experiments were conducted at a pressure of lop7 Pa and at a temperature of 20°C. The electrode surfaces were polished using standard polislung techniques (usinga polishing paste containing 0.25 pm size diamond grains) before the electrodes were placed into the soil. This polishing reduced any surfaceirregularities which might produce field enhancement and promote field emission at these asperities points. The electrode material was aluminum, and each plate was 10 cm in length, 1cm in width, and 0.2 cm in thickness. The electrode separation was 1 mm. The simulated lunar soil sample MLSl (Minnesota lunar simulant #1) is crushed, ground, and sieved basalt. This material is a simulant for the rock fragments in lunar sample 10084 from the Apollo-11 site [61, and was obtained from the University of Minnesota. The SEM analysis of the soil shows that it consists of sharp edged particles with a grain size of 75 to 105 pm (Figure 2).
A variable dc HV power supply was used as the power source. The diagnostic equipment consisted of an oscilloscope (Tektronix 2430 A) to record the breakdown events, a high-frequency HV probe (Tektronix P6015) for voltage measurements, and a Rogowski coil (high-current high-frequency current probe) for transient current measurements. The Rogowski coil was designed and built in the laboratory, based on the original theory given by W. Rogowslu [7]. The probe is essentially a toroidal winding of N turns of small area, linked by the magnetic flux created by the current to be measured, which is enclosed by this toroidal coil. The induced secondary current is the output current of the coil and is proportional to the actual current to be measured [8]. The applied voltage was increased slowly from zero until breakdown
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supply. However, most of the breakdown events did not have transient currents of magnitudes which shortcircuited the power supply. The average maximum transient current was 5 A. A 35 mm camera was used to determine whether optical emission was present during the breakdown event. No optical emission was observed from the surface or bulk of the soil. Based on this result, it was concluded that the breakdown had occurred within the soil between the electrodes. N
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soil. Soil grain size is in the range of 75 to705 pm. occurred, and the event was captured through the diagnostic system. The voltage ramp-up time was M 1 min per 5 kV voltage increase. After the breakdown event, the applied voltage was removed. Tests were conducted at approximately 15 min intervals. The main reason for this waiting period was to allow the static charge, which might have been stored in the soil, to dissipate. Approximately 25 individual shots were conducted per day. The same procedure was repeated for a total of 5 data sets (one data set per day). Each data set was conducted at 72 h intervals. Leakage current measurements, before the breakdown events, were recorded with an electrometer. The electrometer was included in the circuit only when the leakage current measurements were conducted, and the Rogowski coil was removed from the circuit. In this case, the applied voltage was increased in 300 V steps initially, then with 200 V steps, and the corresponding current values were recorded until breakdown voltage was reached. External circuitry was added to the electrometer to protect it from unpredicted breakdown events which might have occurred during the measurements. There was delay of 30 s between the step increase of the voltage and the data recording to allow transient effects to dissipate. The corresponding current values were recorded when a stable current reading was obtained.
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3. EXPERIMENTAL RESULTS A series of breakdown experimentswere conducted and the results were tabulated for analysis. Electrical breakdown was observed within the soil between the electrodes at 6 kV. Some breakdown events were associated with large transient current values. The impedance of the system at breakdown was sufficiently low so as to activate the protective circuits of the power
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Figure 3. (a) A typical voltage waveform of the electrical breakdown of simulated soil under a dc field. (b)A typical current waveform of the breakdown. The 25 MHz ringing characteristic of the pulse is due to the circuit inductance.
3.1. VOLTAGE AND CURRENT WAVEFORMS In the experiments, the applied voltage was increased slowly until breakdown occurred. The oscilloscope was set to trigger when there was a voltage collapse between the electrodes; the voltage waveform and the associated current pulse were recorded as the characteristics of a breakdown event. Figures 3(a) and (b) show voltage and current waveforms for a typical breakdown event. The voltage waveforms, as well as the current waveforms, were highly repeatable. The duration of the breakdown event, based on the voltage waveform and the voltage collapse time, was measured to be N 50 ns (Figure3(a)).A Fourier transform analysis of the voltage and current waveforms was carried out to determine the noise level and the system inductance of the diagnostics instruments. The current diagnostic showed a 25 MHz characteristic frequency (Figure 3(b)),and was caused by the diagnosticcircuit inductance and the current probe (Rogowskicoil) inductance.
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This conclusion was reached based on data obtained in a separate the electrically-stressedmedium, composed of crushed solid parexperiment using the same circuitry and current probe, and aIso ticulates, does not lend itself to the generation of free or mobile based on the design parameters of the current probe [71. Howev- ions. As a result, it is not expected to have mobile energy carrier, detailed calculations to determine the effective circuit induc- ers, such as positive ions, that would promote secondary electron tance and capacitance were not conducted. The voltage diagnos- emission from the cathode surface to contribute in the breakdown tic (Figure3(a))did not show exactly this characteristic('ringing'), event. as it did exhibit a small random noise signal. The transient curResults obtained in this experimental series were also observed rent (Figure 3(b))reached ,-.,4.2 A for this particular event. to be similar to breakdown characteristicsin solid dielectric materials given in the literature [ll].In the cited work, breakdown was explained to develop as a result of inelastic displacement of bound charges in the dielectric material due to the external field. Consequently, breakdown occurs molecules within the solid dielectricwith i strength. In the present work, a similar ern the leakage current characteristi down. Additionally, since the diele consisted of small dielectric particle migrate as a result of an electric forc I oo-i ' I ' ' ' and thereby change the potential di 0 i 2 3 4 5 Applied field FY/ml electrodes, further enhancing or affecting the breakdown. Figure 4. LeakaEe current as a function of applied field strength. Field emission of the electrons from the cathode surface genExperimentaldata and curve-fit data are dispfayed in one gap
