Pulsed Plasma Thrusters

Pulsed plasma thrusters (PPT's) operate by striking a discharge using a spark plug. A potential difference applied between a cathode and anode in different geometric configurations leads to current flowing over a Teflon surface. The Teflon decomposes and creates a carbon-fluorine plasma which accelerates by electromagnetic and thermal mechanisms to very high speeds (10's of km/s).

As with modeling of other electric propulsion devices, the exhaust plume is computed by a combination of the direct simulation Monte Carlo (DSMC) and Particle-In-Cell (PIC) techniques.

Micro - Pulsed Plasma Thruster

The Teflon 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. Carbon char is formed as result of carbon flux returned from the plasma. A model of the current layer near the Teflon surface is developed. The current density and the Teflon 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 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.

Carbon char on the propellant face (Scanning Electron Microscope)

A model of the near field plasma plume of a Pulsed Plasma Thruster (PPT) is developed. 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 N range that utilizes TeflonTM as a propellant. The plasma plume is simulated using a hybrid fluid-PIC-DSMC approach. The plasma plume model is combined with Teflon 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 ablation and plasma generation analyses show that the Teflon surface temperature and plasma parameters are strongly non-uniform in the radial direction. Electron and neutral densities predicted by the plume model are compared with near field measurements using a two-color interferometer and good agreement is obtained.

Comparison with Herriot cell electron density data measurements


 

Ablation pattern


 

Kinetics of Teflon ablation

The vaporization of Teflon in contact with high-current discharge plasmas is considered. A kinetic numerical method named the direct simulation Monte Carlo (DSMC) and analytical kinetic approaches based on the bimodal distribution function approximation are employed. The solution of the kinetic layer problem depends upon the velocity at the outer boundary of the kinetic layer which varies from very small, corresponding to the high-density plasma near the evaporated surface, up to the sound speed, corresponding to evaporation into vacuum. The present is coupled with a plasma discharge model to describe self-consistently the electrical discharge. An example of calculated ablation rate contours are shown in the Figure for the range of parameters typical for a PPT.

Electrothermal Pulsed Plasma Thruster

P>The physical processes of a pulsed discharge in a Teflon cavity of a co-axial pulsed plasma thruster (PPT) are analyzed. We consider electrothermal PPT that has this central Teflon cavity to produce a high- pressure cloud of ablation products during the discharge pulse. The mathematical model includes the Teflon thermal conduction, the plasma energy balance, mass and momentum conservation in a quasi-neutral plasma region and the relation between the plasma parameters in a non-equilibrium layer near the ablated Teflon surface. It was found that the plasma parameter variation along the cavity length is important feature of the ablation controlled discharge in the cavity that affects plasma energy balance, mass and momentum conservation, and thermal conductivity. Performance characteristics of the PPT such as mass ablation and impulse thrust bit are calculated. Predicted plasma temperature, ablation rate and gasdynamic thrust are found to be in agreement with available experimental data.

 

Electrothermal PPT-4

Calculated ablated rates for two electrothermal PPT designs

Acknowledgments

Funding for this work provided by the Air Force Office of Scientific Research.

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