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Experimental Study of a Low-Voltage Pulsed Plasma Thruster for Nanosatellites
The commercial CubeSat industry has experienced explosive growth recently, and with falling costs and growing numbers of launch providers, the trend is likely to continue. The scientific missions CubeSats could complete are expanding, and this has resulted in a demand for reliable high specific impulse nanosatellite propulsion systems. Interest in liquid-fed pulsed plasma thrusters (LF-PPTs) to fulfill this role has grown lately. Prior work on a nanosatellite LF-PPT was done in the Purdue Electric Propulsion and Plasma Laboratory, but its high operational voltage and electrode size would be disadvantageous for integration on a CubeSat, which have strict volume limitations and provide only tens of Watts in power at low voltages. This work aims to address those disadvantages and further advance the development of a nanosatellite LF-PPT by reducing the operating voltage and removing long plate electrodes to prevent energy losses on components other than the expelled plasma sheet. Two major objectives are pursued: to construct a coaxial pulsed plasma thruster operating with 10s to 100s of volts and to characterize the temporal evolution of the discharge parameters in this low-voltage operation scenario.
It took three experimental design iterations, all of which used a 260 uF , 400 V film capacitor, to arrive at a functional coaxial pulsed plasma thruster. First, a button gun was tested. It produced a peak current of ~16 kA, which serves as the expected maximum for the later experiments. Due to the presence of parasitic arcing, it revealed that electrical lines needed to be removed from vacuum chamber to enable testing at a wide range of pressures. Second, a coaxial PPT was designed, built, and tested. This design confirmed operation at discharge voltages <100 V across the plasma, achieving one of the project’s aims, and produced a peak current of 7.4 kA. However, necessity to better align the cathode and provide an unobstructed camera view for observation of the discharge column attachment to the cathode surface forced additional system redesign. Third, a revised coaxial PPT was built and tested. Using air as a propellant, the discharge generated a peak current of 10.4 kA at a mass flow rate of 2 mgs. The PPT cathode was imaged with an ICCD camera over a wide range of pressures, and the photos indicated “spotless” diffuse arc attachment to the cathode, which serves as evidence to expect low erosion rates. The direct measurements of the cathode erosion rate are planned for future.