PMG

Plasma Motor Generator

The Plasma Motor-Generator experiment was launched from Cape Canaveral Air Force Station as a Delta/GPS secondary payload on June 26, 1993, 3 months after SEDS-1. TAI developed the PMG wire and deployer, and assisted NASA on mission design. The hardware and mission objectives were quite different from SEDS-1, but the hardware interfaces and Delta operations were very similar. This eased integration and reduced the cost of flying PMG.

The PMG experiment started after the GPS satellite had left and the Delta second stage had done its propellant depletion burn. A 28 kg "far end package" was ejected straight up from the Delta at 2.4 m/s. This caused deployment of an insulated 18AWG copper wire. The last 60 meters of the 500m wire was weakly bonded to the deployer, to serve as a passive brake. This length never deployed, because the deployment energy went instead into speeding up a "high-order skiprope" in the deployed wire. This energy storage was reversible and resulted in a ~50% recoil, based on EMF measurements. The skip-rope largely died out during this bounce, as indicated by a far smaller voltage change during a second bounce. These dynamics were expected from tests and analyses done after hardware delivery, but before launch.

The goal of PMG was to measure the characteristics of electron collection and emission by hollow cathodes at well-separated locations in a tenuous ionospheric plasma. The current varied strongly with ambient plasma conditions, over a range from 8 to 300mA. PMG results suggest that hollow cathodes should be good electron emitters for electrodynamic tethers, but that other concepts should be used for collection. Since then hollow cathodes have been used operationally on the ISS, to neutralize charging caused by electron collection on positively-biased parts of the large high-voltage solar arrays.

PMG winding.

PMG on Delta.