JOURNAL OF SPACECRAFT AND ROCKETS Vol. 42, No. 3, May–June 2005 Solar Arrays for Direct-Drive Electric Propulsion: Arcing at High Voltages Todd A. Schneider ∗ NASA Marshall Space Flight Center, Huntsville, Alabama 35812 I. G. Mikellides † and G. A. Jongeward ‡ Science Applications International Corporation, San Diego, California 92121 and T. Peterson, § T. W. Kerslake, ¶ D. Snyder, ∗∗ and D. Ferguson †† NASA John H. Glenn Research Center at Lewis Field, Cleveland, Ohio 44135 The results from an experimental investigation to assess arcing during operation of high-voltage solar arrays in a plasma environment are presented. The experiments were part of an effort to develop systems that would allow safe operation of Hall-effect thruster(s) in direct-drive mode. Arc discharges are generated when the array is biased negative with respect to the plasma. If sustained for long periods of time between adjacent solar cells, arcs can severely damage a solar array, thus significantly shortening its lifetime. Most often sustained arcs are triggered by plasma produced during short-duration discharge arcs (∼20 ms). These “trigger” arcs are sparked between the semiconducting cell and the covering dielectric. Both trigger and sustained (>1-ms) arcs have been captured during the tests. Current and voltage waveforms associated with the different arc events are presented. The test results have defined operational limits (thresholds) for the various array concepts studied that minimize the likelihood of damage from sustained arcs. Experimental trends regarding the effect of the solar-array substrate on arc duration are also presented. Introduction A GUIDING precept of spacecraft design is to seek out savings in system mass as well as power system efficiency. Conse- quently, work is being conducted in the aerospace industry to create high-voltage low-current solar-array systems, which allow the use of smaller current-carrying conductors in the power generation and distribution systems. The usefulness of high-voltage solar arrays has been further refined for the case of spacecraft that employ Hall- effect thrusters (HET). 1,2 In this case, considerable savings could be realized by driving the thruster directly from a high-voltage array. 3 Such a system would significantly reduce the weight and complex- ity of the power processing unit, which is commonly used to step up the array voltage. Previous work suggested the operation of a Hall-effect thruster in direct-drive mode is feasible. 4 However, no detailed tests were per- formed to assess life-limiting issues, such as arcing, associated with prolonged operation of such a high-voltage system in a Hall-effect- thruster plasma environment. The present work is part of a larger Presented as Paper 2003-5017 at the AIAA/ASME/SAE/ASEE 39th Joint Propulsion Conference, Huntsville, AL, 20–23 July 2003; received 6 Octo- ber 2003; revision received 7 April 2004; accepted for publication 15 April 2004. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rose- wood Drive, Danvers, MA 01923; include the code 0022-4650/05 $10.00 in correspondence with the CCC. ∗ Physicist, ED31, Space Environmental Effects Group. Member AIAA. † Scientist; currently Scientist, Mail Stop 125-09, Advanced Propulsion Technology Group, Jet Propulsion Laboratory, California Institute of Tech- nology, Pasadena, CA 91109. Member AIAA. ‡ Division Manager, Mail Stop X1, Defense Technology Group, 10260 Campus Point Drive. § Program Manager, 500-103, Power and Propulsion Office, 21000 Brookpark Road. ¶ Aerospace Engineer, 500-103, Power and Propulsion Office, 21000 Brookpark Road. ∗∗ Electrical Engineer, 302-1, Photovoltaics and Space Environments Branch, 21000 Brookpark Road. Member AIAA. †† Group Leader for Space Environments, 302-1, Photovoltaics and Space Environments Branch, 21000 Brookpark Road. Senior Member AIAA. effort that aims (in part) to identify requirements for the safe opera- tion of a direct-drive Hall-effect thruster (D2HET) system. A more specific objective of the D2HET program is to identify solar-array technologies that are capable of operating at high voltages in the charge-exchange plasma environment induced by the HET. Natu- rally, technologies developed for this particular application will also be relevant to other spacecraft that operate with high-voltage solar arrays in a dense plasma environment (e.g., low-earth-orbit iono- sphere). As NASA looks to high-power electric propulsion, lessons learned from the D2HET program promise to elucidate many of the serious interaction issues associated with high-voltage spacecraft systems in a dense plasma environment. Among others, the solar array must operate under two major re- quirements: 1) minimal electron current collection, also known as parasitic or leakage currents, by the solar array and 2) no prolonged arcing across adjacent cells. Parasitic currents can degrade the per- formance of the solar array and can become a significant design obstacle if the collected electron current is a substantial fraction (more than a few percent) of the array operating current. Sustained arcing can lead to permanent electrical shorts and has been found to occur only when parts of the array are at negative electric potentials and can therefore attract ions. (No arc events were observed when the array was biased positive.) Our current understanding of the arc process is based on earlier ex- periments conducted by Vaughn et al. in support of the International Space Station. 5 Arcs were initiated on biased anodized aluminum in a background plasma. The experiments confirmed that a trigger arc releases a plasma that expands from the arc site and covers the nearby surfaces with a low impedance plasma. If there is positive charge on the surfaces, the plasma can conduct electrons from the arc site to the surface, thereby discharging the effective capacitance of the surface. This configuration occurs when a chassis is negative with respect to a background plasma. For the D2HET system we expect this to occur in low Earth orbit when the HET is off and the arrays are generating 300 V, causing the system to float negative. In geosynchronous orbit or higher orbits where magnetospheric envi- ronments can induce inverted chassis voltages, then ion collection will also occur. Ground tests have been performed under conditions of both elec- tron and ion collection. Results from the electron collection tests, 543