Analyses Of Teflontm Surface Charring And Near Field Plume Of A Micro Pulsed Plasma Thruster

Analyses of TeflonTM Surface Charring and Near Field Plume of a Micro-Pulsed Plasma Thruster

The Teflon(TM) ablation in a micro-Pulsed Plasma Thruster is studied with an aim to understand the charring phenomena. Microscopic analysis of the charred areas shows that it contains mainly carbon. It is concluded that the carbon char is formed as result of carbon flux returned from the plasma. A simplified model of the current layer near the Teflon(TM) surface is developed. The current density and the Teflon(TM) surface temperature have peaks near the electrodes that explain preferential ablation of these areas as was observed experimentally. The comparison of the temperature field and the ablation rate distribution with photographs of the Teflon(TM) surface shows that the area with minimum surface temperature and ablation rate corresponds to the charring area. This suggests that the charring may be related to a temperature effect. Electron densities predicted by the plume model are compared with near field measurements.

Electromagnetic Effects in the Near Field Plume Exhaust of a Micro-Pulsed Plasma Thruster

In this work we present a model of the near field plasma plume of a Pulsed Plasma Thruster (PPT). As a working example we consider a micro-PPT developed at the Air Force Research Laboratory. This is a miniaturized design of the axisymmetric PPT with a thrust in the 10 micro-N range that utilizes Teflon(Trademark(sup Trademark)) as a propellant. The plasma plume is simulated using a hybrid fluid-PIC-DSMC approach. The plasma plume model is combined with Teflon(Trademark) ablation and plasma generation models that provide boundary conditions for the plume. This approach provides a consistent description of the plasma flow from the surface into the near plume. The magnetic field diffusion into the plume region is also considered and plasma acceleration by the electromagnetic mechanism is studied. Teflon(Trademark) ablation and plasma generation analyses show that file Teflon(Trademark) surface temperature and plasma parameters are strongly non-uniform in the radial direction. The plasma density near the propellant surface peaks at about 10(exp 24)/cu m in the middle of the propellant face while the electron temperature peaks at about 4 eV near the electrodes. The plume simulation shows that a dense plasma focus is developed at a few millimeters from the thruster exit plane at the axis. This plasma focus exists during the entire pulse, but the plasma density in the focus decreases from about 2x10(exp 22)/cu m at the beginning of the pulse down to 0.3x10(exp 22)/cu m at 5 microsec. The velocity phase is centered at about 20 km/s in the axial direction. At later stages of the pulse there are two ion populations with positive and negative radial velocity. Electron densities predicted by file plume model are compared with near field measurements using a Herriot Cell technique and very good agreement is obtained.

Pulsed Plasma Thruster Contamination