Relationship between anode spots and onset voltage hash in magnetoplasmadynamic thrusters

Experimental results are presented which indicate a direct relationship between voltage transients in magnetoplasmadynamic thrusters operating above onset and the time-resolved appearance of destructively released anode material in the thruster plume. Such a relationship gives support to previously discussed anode spotting theories, Langmuir probe measurements of plasma density fluctuations and measurements of argon and copper (anode material) ion luminosity are compared with the voltage transients. The onset of spikes in the thruster voltage is directly correlated with the onset of similar spikes in the plasma density at the probe location, and with a rise in the copper luminosity, but a fall in that of argon, in the plume. The voltage hash is categorized into two types: large-amplitude spikes at currents well above the onset current, and lower-amplitude random fluctuations at currents just above the onset current. It is shown that the two categories of voltage hash can be related to two classes of damage on the anode surface: pitlike damage of 10 to 100 μm extent, caused by explosive emission due to voltage spikes, and shallow surface melting due to the lower-amplitude random fluctuations, which may be responsible for observed density and luminosity oscillations at 600 kHz, and a dip in the voltage power spectrum at the same frequency.