Abstract
A kinetic-electron, fluid-ion model is used to study the 2D plasma expansion in an axisymmetric magnetic nozzle in the fully-magnetized, cold-ion, collisionless limit. Electrons are found to be subdivided into free, reflected, and doubly-trapped sub-populations. The net charge current and the electrostatic potential fall on each magnetic line are related by the kinetic electron response, and together with the initial profiles of electrostatic potential and electron temperature, determine the electron thermodynamics in the expansion. Results include the evolution of the density, temperature, and anisotropy ratio of each electron sub-population. The different contributions of ions and electrons to the generation of magnetic thrust are analyzed for upstream conditions representative of different thruster types. Equivalent polytropic models with the same total potential fall are seen to result in a slower expansion rate, and therefore to underpredict thrust generated up to a fixed section of the magnetic nozzle.
Highlights
Magnetic nozzles [1, 2] (MNs) act as the main plasma acceleration stage of electrodeless thrusters such as the helicon plasma thruster [3,4,5] (HPT) and the electron–cyclotron–resonance thruster [6,7,8] (ECRT), but is it an essential part of the applied-field magnetoplasmadynamic thruster [9] (AFMPDT), the variable specific impulse magnetoplasma rocket [10] (VASIMR), and other devices [11]
The focus of this work is on steady-state, near-collisionless MN plasma flows composed of hot, magnetized electrons and comparatively cold ions, which are relevant to helicon plasma thruster [3–5] (HPT) and electron–cyclotron–resonance thruster [6–8] (ECRT)
Given the low number of collisions in the plasma, electrons are typically away from local thermodynamic equilibrium, which hinders an accurate representation of the electron species by means of otherwise convenient fluid models, since a consistent closure relation for the fluid equation hierarchy requires to account for the full kinetic electron response
Summary
Magnetic nozzles [1, 2] (MNs) act as the main plasma acceleration stage of electrodeless thrusters such as the helicon plasma thruster [3,4,5] (HPT) and the electron–cyclotron–resonance thruster [6,7,8] (ECRT), but is it an essential part of the applied-field magnetoplasmadynamic thruster [9] (AFMPDT), the variable specific impulse magnetoplasma rocket [10] (VASIMR), and other devices [11]. Cool down as a result of the existence of effective potential barriers in phase space, arising from the interplay between electrostatic forces and magnetic mirror forces, which creates empty regions in the electron velocity distribution function (EVDF), as well as isolated regions populated with doublytrapped electrons that do not connect neither with the plasma source nor with infinity downstream. These results have been found analytically [13, 14], numerically [15], and experimentally [16, 17]. A first version of this research was presented in an international conference [27]
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