Early stage of the discharge in ablative pulsed plasma thrusters

Abstract

Currently the renewed interest in ablative pulsed plasma thrusters (APPTs) mostly results from the needs of satellites of the nano and micro sectors. However, the efficiency of APPTs significantly decreases when they are scaled down. A better understanding of the physical processes involved in plasma generation and maintenance in this type of thruster may help to minimize APPTs’ performance degradation, when they are miniaturized. One of the issues, which up to now seems to be only marginally clarified, is an initial phase of discharge between the APPT’s electrodes. In this paper a novel analytical model of a flashover development is proposed. In the model, movement of electrons between the PPT’s electrodes is described relying on the concept of a saturated secondary electron avalanche. Two analytical approaches are considered. In the simplest approach, secondary electrons are assumed to be monoenergetic, whereas in the second, the Maxwellian distribution function is assumed. Simple, analytical formulas for surface current density, electron density and the thickness of the electron layer are derived. The predicted values are in a good agreement with measurements and computer simulations of other authors. The work was inspired by experimental examination of a micro APPT fed with polymeric nonvolatile propellant. For completeness, the thruster, known as an LμPPT (liquid micro pulsed plasma thruster), is described and its performance is presented.