Microwave electrothermal thrusters using waveguide heated plasmas

This paper discusses the physical effects of power supply ripple on cathode erosion and cathode arc attachment in a water-cooled, 30 kW nitrogen arcjet. Experimental results are presented for 2 percent thoriated tungsten, which show that the long-term cathode erosion rate is a decreasing function of current ripple over the range 1-13 percent. Above this range, the cathode discharge becomes unstable, and the erosion rate rapidly increases. A qualitative model of this effect is given in terms of a magnetically induced radial motion of the arc column, and an overall increase in the cathode spot radius due to the higher peak current associated with higher ripple. The most important effect of power supply ripple is therefore shown to be its ability to collectively drive the cathode attachment away from the cathode center. This leads to an increase in the cathode attachment area, and a subsequent decrease in the cathode erosion rate.

Physical model and experimental results of cathode erosion related to power supply ripple

Development of a 1N, radiatively cooled arcjet

Near-term objectives and recent technological progress of JPL's electric propulsion program are discussed. Particular attention is given to accomplishments for ion, magnetoplasmadynamic (MPD), electron-cyclotron resonance (ECR), and arcjet thrusters. Xenon ion thruster erosion tests indicate a 15-fold reduction in tantalum baffle erosion when nitrogen is added to the xenon propellant and steady-state cylindrical MPD thruster tests at powers up to 72 kW show distinct self-constricted and diffuse discharge modes. An ECR thruster was operated at up to 7 kW with plasma acceleration at energies up to 7 kW; there was plasma acceleration at energies approaching 100 electron volts.

A brief review of JPL's electric propulsion technology activities

Next generation arcjets should have light-weight design and prolonged lifetime. For the former topic, it is shown that the radiator mass can be drastically reduced by the effective use of propellant as a coolant at the lower temperature region on the radiator. Resulting thruster weight of 2.0 kg including the radiator is possible for 15 kWe arcjet. For the latter topic, replaceable cathode system is proposed and some key issues are mentioned.

Technical Challenges for Advanced Arcjets

Endurance tests (100 hours) were conducted to examine the effects of geometry and spacing on 30 kW arcjet cathode erosion. The effects of input power source ripple were also examined. The preliminary results from a 413 hour endurance test are also discussed. This test was terminated voluntarily. The condition of the nozzle, boron nitride propellant injector, and cathode are discussed. A modeling effort is described which is aimed at quantifying the cathode tip heating phenomena. The results of the experiments and the model are compared.

W. D. Deininger

Papers

Cathode erosion tests for 30 kW arcjets