| The performance scaling of gas-fed pulsed plasma thrusters (GFPPTs) is investigated theoretically and experimentally. Analytical models of the discharge current suggest that close to critically damped current waveforms provide the best energy transfer efficiency. A characteristic velocity for GFPPTs that depends on the inductance-per-unit-length and the square root of the capacitance-to-initial-inductance ratio is also derived in these models. The total efficiency is predicted to be proportional to the ratio of the exhaust velocity to the GFPPT characteristic velocity. A numerical non-dimensional model is used to span a large parameter space of possible operating conditions and suggest optimal configurations. From the non-dimensional model, the exhaust velocity is predicted to scale with a non-dimensional parameter called the dynamic impedance parameter to a power that depends on the mass loading prior to the discharge.; To test the validity of the predicted scaling relations, the performance of two rapid-pulse-rate GFPPT designs, PT5 (coaxial electrodes) and PT9 (parallel-plate electrodes), has been measured over 70 different operating conditions with argon propellant. The performance measurements are made in a recently renovated facility that uses liquid nitrogen cooled baffles and a micro-thrust stand capable of measuring impulses <20 μNs within <10%. The measurements demonstrate that the impulse bit scales linearly with the integral of the discharge current squared, as expected for an electromagnetic accelerator. The measured performance scaling in both electrode geometries is shown to be in good agreement with theoretical predictions using the GFPPT characteristic velocity. Normalizing the exhaust velocity and the impulse-to-energy ratio by the GFPPT characteristic velocity collapses almost all the measured data onto single curves that represent the scaling relations for these GFPPTs. |
