Low Current Surface Flashover for Initiation of Electric Propulsion Devices

Omar G Dary, Purdue University

Abstract

There has been a recent increase in interest in miniaturization of propulsion systems for satellites. These systems are needed to propel micro- and nano-satellites, where platforms are much smaller than conventional satellites and require smaller levels of thrust. Micro-propulsion systems for these satellites are in their infancy and they must manage with smaller power systems and smaller propellant volumes. Electric propulsion systems operating on various types of electric discharges are typically used for these needs. One of the central components of such electrical micropropulsion systems are ignitor subsystems, which are required for creation the breakdown and initiation of the main discharge. Ignitors have to provide reliable ignition for entire lifetime of the micropropulsion system. Electric breakdown in vacuum usually require high voltage potentials of hundreds of kilovolts per mm to induce breakdown. The breakdown voltage can be significantly decreased (down to several kVs per mm) if dielectric surface flashover is utilized. However, classical dielectric surface flashover operates at large electric current (100s of Amperes) and associated with overheating and damage of the electrodes/dielectric assembly after several flashover events. The central idea of this work was to eliminate the damage to the flashover electrode assembly by limiting the flashover currents to low values in milliampere range (Low Current Surface Flashover -LCSF) and utilize LCSF system as an ignition source for the main discharge on the micropropulsion system. The main objective of this research was to create a robust LCSF ignition system, capable producing a large number of surface flashover triggering events without significant damage to the LCSF electrode assembly. The thesis aims to characterize the plasma plume created at LCSF, study electrodes ablation and identify conditions required for robust triggering of main discharge utilized on micro-propulsion system. Conditioning of a new LCSF assembly (flashover current was limited to <100 mA in all>experiments) was measured and breakdown voltages in the range of 8kV to 12kV were observed for the fully conditioned assembly. No damage to the LCSF electrode assembly was observed after about 104 LCSF events. The LCSF assembly created sufficient amount of seed plasma in order to bridge a vacuum gap between the high-current electrodes and to reliably ignite high-current arcs (10A–12A arc were used in this work). Ignition of the high-current arc was observed at three different cases of LCSF with limiting currents 100 mA, 33 mA and 20 mA respectively. Plasma parameter measurements were conducted with variety of Langmuir probes inside the LCSF plume. Ion currents created by the LCSF were primarily expelled directly perpendicular from the insulator surface. The plasma expansion for the LCSF assembly was measured to be 2 × 106–6 × 106 cm/s. Plasma density was measured to range 10 10–1011 cm–3. The plasma density was maximal near the LCSF assembly and quickly reduced radially. Temporal decay of the plasma was observed on a time scale of about 5 µs after the LCSF event. The results of this work are significant for creation of ignitor for micropropulsion systems. LCSF system offers reliable triggering for numerous ignition pulses for entire lifetime of the micropropulsion system and reduces complexity and volume of the system by excluding moving parts and the need for an external gas tanks.

Degree

M.S.A.A.

Advisors

Shashurin, Purdue University.

Subject Area

Aerospace engineering|Plasma physics

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