Propellant Charring in Pulsed Plasma Thrusters

The TeflonTM ablation in a micro-pulsed plasma thruster is studied with the aim of understanding the charring phenomenon. Microscopic analysis of the charred areas shows that it contains mainly carbon. It is concluded that the carbon char is formed as a result of carbon flux returned from the plasma. A simplified 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, such as was observed experimentally. 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 finding suggests that the charring may be related to a temperature effect. Nomenclature B = magnetic field C = capacitance e = elementary (electron) charge Ip = current peak j = current density L = circuit inductance n = electron density QF = heat flux due to the particle convection Q j = Joule heat Qr = radiation heat flux R = equivalent circuit resistance r = radial distance Te = electron temperature Uo = initial voltage on the capacitor V = plasma velocity µ = permittivity σ = plasma (Spitzer) conductivity ωτ = Hall parameter I.