CATHODE PHENOMENA IN PLASMA THRUSTERS

For the optimization of cathodes of steady-state plasma accelerators, an extensive experimental program has been carried out at IRS to investigate the erosion mechanisms. On different thruster types and with an additional fundamental experiment, the influences of geometry, chamber pressure, amount of ThO2, cathode temperature, power, and gas species were explored. Propellants Ar, H-2, N2 were used which had been purified with a special filter to remove residual water and oxygen. The erosion rates during the ignition phase as well as during continuous operation were measured. The temporal evolution of the cathode erosion rate was measured by surface layer activation. At high current densities within the cathode, severe problems were encountered with all thruster geometries. The cathodes developed cracks and started to melt. To clarify the reason for this behavior, some cathodes were investigated metallurgically.