Mode transitions in a magnetically shielded Hall thruster. I. Experimentally informed model

Abstract

An experimental evaluation is presented of a two-equation model for the low frequency (<25 kHz), large amplitude (>100% of mean) discharge oscillations exhibited by a 9-kW class magnetically shielded Hall thruster. The model is based on a theoretical treatment of the “breathing mode” oscillations in Hall thrusters (Barral and Peradzyński, “A new breath for the breathing mode,” IEPC-2009-070) and includes governing equations for fluctuations in the discharge current and the spatially averaged neutral density in the thruster channel. The derivation of the governing equations is reviewed, and the key simplifying assumptions are formulated in terms of comparisons between the magnitudes of relative fluctuations in spatially averaged plasma properties. Experimental measurements are performed of these plasma properties at an operating condition of 300 V discharge voltage and 10 A discharge current. It is found that all quantities of interest such as drift speed, electric field, and temperature fluctuate on the timescale of the low frequency oscillations. However, the relative phasing of these properties combine in such that the key assumptions of the model are satisfied—all but the neutral density and discharge fluctuations can be neglected in the equations for neutral density and current oscillations. A physical interpretation of the validity of the assumptions is presented, and the model is discussed in the context of its extensibility to other operating conditions. The validated model forms the basis for a parametric study presented in Part II of mode transitions and the criterion for these transitions in a magnetically shielded Hall thruster.