Investigation of the Breakdown Voltage Characteristics of a 30 kW class Ammonia Arcjet

摘要

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 rate and voltage ramp rate were observed to significantly affect the MDBV; the MDBV was observed to rise as either of these 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 arcjets. 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 rate, 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 the 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.