An investigation of the breakdown voltage characteristics of a 30 kWclass ammonia arcjet

Abstract

Public reporting burden for this collection of information is estimated to average 1 tiour per response, including ttie time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of infomnatlon, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of infomiation if it does not display a currently valid 0MB contr-oi number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. REPORT DATE (DD-MM-YYYY) Sep 1993 REPORT TYPE Technical Paper PL-TP-93-3029Approved for Public Release Distribution Unlimited The primary objective of this work was to identify methods to reduce the mean dynamic breakdown voltage (MDBV) of the ammonia propellant in the ESEX 26 kW arcjet. The approach to this objective was to establish a test matrix to investigate the influence of mass flow rate, electrode gap distance, cathode tip shape, and voltage ramp rate on the MDBV. Only the mass flow rale and voltage ramp rale were observed to significantly affect the MDBV; the MDBV was observed to rise as either of ihese parameters were increased. These test results provided the basis for the start circuit redesign for the ESEX flight experiment, and also represent an initial data base of breakdown characteristics in high power ammonia arcjeis. The second objective of this work was to identify the mechanisms associated with propellant breakdown in an arcjet. Progress towards this objective includes the following. A framework for investigating breakdown mechanisms in an arcjet was established which provides explanations for the strong dependence of the MDBV on the voltage ramp rale, the mass flow rate, and other parameters. A combination of a model, the calculation of electric field contours inside of the arcjet, and SEM photographs of a cathode tip indicate that the projection model for enhance field emission cannot alone account for the mean dynamic breakdown voltages observed in ihe high power arcjet. This result suggests that dielectric surface layers on the cathode tip are responsible for the enhanced field emission required to explain the observed MDBVs. SUPPLEMENTARY NOTES I. InU"oductionA reliable ignition method is essential for ihe space qualification of an arcjet propulsion system. Arcjeis are most commonly ignited by applying a high voltage pulse across the electrodes to breakdown the propellant. Following breakdown, the arcjet rapidly (on the order of lOO's of microseconds) transitions into a steady state high power arc. This study will focus on the initial phase of arcjet ignition: propellant breakdown. In ...