Development of a vacuum arc thruster for nanosatellite propulsion

[摘要] This thesis describes the development of a vacuum arc thruster (VAT) to be used as a potential lowmass (< 500 g), low power (< 5–10W) propulsion system for nanosatellites. The thruster uses a highvoltage capacitive circuit to initiate and power the arc process with a 400 ns high current (150–800A)pulse. A one-dimensional steady state analyticalmodel describing the cathode region of the vacuumarc was developed. The model made use of mass and energy balances at the sheath region andcathode surface respectively to predict key quantities such as thrust, ion velocity, ion-to-arc currentratio and erosion rate. Predicted results were shown to be within the limits of reported literature(∼63 μN/A, 26.12 km/s, 0.077 and 110 μg/C respectively). A sensitivity analysis of the analyticalmodel found that a high electric field in the cathode region impedes and decelerates ion flow, whichis used for thrust. This was confirmed experimentally for thrust values at arc voltages greater than2000 V.Both direct and indirect means of measuring thrust were achieved by using a deflecting cantileverbeam and an ion collector system, respectively. The transient response of the cantilever beam to impulsivethrust was analytically modeled, whilst the ion current was found by measuring the currentinduced on a plate subject to ion bombardment. Knowledge of the ion current density distributionwas successfully used to approximate the effective normal thrust vector. Direct and indirect thrustlevels were roughly 140 and 82 μN/A of average arc current, respectively. Measured thrust wasfound to be higher than predicted thrust due to thrust contributions fromthe ablation of Teflon insulation.The discrepancy is also due to the uncertainty in quantifying free parameters in the analyticalmodel such as the fraction of generated ions flowing away from the cathode region. The thrust-topowerratio, specific impulse and efficiency of the vacuum arc thruster at an average arc current of200 A was measured to be 0.6 μN/W, 160 s and 0.05 %, respectively.A thruster performance analysis and specification showed that the VAT is capable of achieving specificorbital and slew manoeuvres within a constant 5–10 W average power. It was concluded thatthruster performance could be improved by using a two-stage arc circuit consisting of a high voltage,low current, short pulse trigger and a low voltage, high current, long pulse driver.

[发布机构] Stellenbosch University