Experimental and theoretical investigation of pulsed plasma thrusters

An experimental and theoretical effort was carried out to investigate and explore various design modifications to improve pulsed plasma thruster (PPT) performance in the energy range between 5 and 60 J. Experimentally, a benchmark PPT was fabricated, and its performance evaluated and found to be comparable to previous flight type PPTs. A thermal management study to determine and elucidate energy loss mechanisms occurring in PPTs was conducted. It was determined that Teflon surface temperatures in excess of 370°K were attained during steady state thruster operation after the current pulse has ended contributing to the “late time ablation” of Teflon molecules. Also, radiation losses from the PPT structure accounted for approximately 50% of the thruster's stored energy.; An inductively-driven plasma-actuated PPT was designed and fabricated to evaluate and investigate the effect of changing the discharge current waveform on PPT performance. A 1 μH inductor was used but resulted in reduced discharge current peaks and reduced Teflon ablation; this resulted in low thrust efficiency. In addition, the crowbar electrode placement contributed to the thrust degradation of the thruster since it resulted in a lateral JXB force that was not directed along the thrust axis.; Coaxial inverse-pinch PPT configurations were designed, fabricated, and tested in an attempt to achieve efficient mass utilization. Axisymmetric discharge current operation was successfully achieved. Experimental and MACH2 predicted enclosed current contours agreed for 20 and 30 J thruster operation. Comparisons between the experimental and theoretically determined ablation rates indicated good agreement for 20 J but not for 30 J thruster operation. For 20 J, the temperature of the Teflon propellant surface never exceeded 6730K, thus indicating that no “late time ablation” due to Teflon molecule decomposition was occurring. For the 30 J thruster operation the MACH2 predicted ablation rate magnitude was only 40% of the experimentally measured value. Analysis of the temperature profile found that the temperature in the Teflon solid exceeded 673°K; this resulted in additional mass ablation after the current pulse has ended. To account for the difference between the experimental and theoretical ablation rates, it was found that Teflon decomposition occurs for Teflon temperatures above 705°K.; Coaxial inverse-pinch PPT thrust efficiency of 5.3% was achieved at a specific impulse of 2420 s for 20 J thruster operation. This 150% improvement in specific impulse over the 20 J benchmark PPT operation is an indication of improved mass utilization.