Coaxial thermal pulsed plasma thrusters

This thesis investigates the operation and performance of a thruster design that utilizes the discharge within a conical cavity existing of a copper cathode, a plastic propellant and a tungsten anode to produce a pulse of plasma. Analysis suggests that thermal processes are more important than electromagnetic forces in the acceleration of the plasma propellant. A pulsed plasma thruster experiment was designed, fabricated and operated to test the hypothesis and to investigate the effects of discharge energy, nozzle design and propellant material. A capacitive discharge circuit was designed and fabricated. This circuit was used to power a thruster design which utilized different solid plastic propellants. Two different propellants were used for the experiments to test this hypothesis, Teflon (CF_{2)n and polyethylene (CH2)n. The polyethylene was expected to give a higher specific impulse because of its lower average molecular weight. The exhaust velocity was measured using a two beam laser interferometer. In order to verify the correctness of the results with the interferometer also a high speed camera was used to take pictures of the exhaust plume from which the velocity could also be measured. The mass ablation was also measured to give an insight in the ablation behavior of the thruster of a broader initial stored energy range. Besides the values mentioned above we also measured the voltage and current input into the thruster. Spectrometer data of the exhaust plume were also recorded. The results of the experiments supported the hypothesis. The specific impulse and the velocities of the polyethylene propellant with up to 3000 s were approximately twice as high as the values for the Teflon propellant with up to 1450s. We also found that the larger nozzle worked much better than the small nozzle, which supported the hypothesis that mainly electrothermal processes accelerated the propellants as the plasma was better expanded. Thrust efficiencies up to 10 % for polyethylene and 5 % for Teflon were measured using the large nozzle.}