Collisionless electron cooling in unmagnetized plasma thruster plumes

Abstract

A kinetic model of electrons in a steady-state, collisionless, paraxial plasma plume is presented. The model is based on the conservation of an adiabatic invariant related to the oscillating radial motion of the electrons in the electric potential of the plume. The electron phase space can be divided into distinct regions according to their connectivity with the upstream or downstream boundary conditions, and may include isolated regions of trapped electrons. A particular plasma potential is used to illustrate the capabilities of the model. This electron model is the first and fundamental piece of a full plasma plume model that will allow the self-consistent computation of the ion, electron, and electric potential responses in order to investigate electron collisionless cooling mechanisms in an unmagnetized plasma plume.

Hybrid 3D model for the interaction of plasma thruster plumes with nearby objects

“…If collisions are introduced through standard resistive terms, the main concern is in the expressions of the pressure tensor  e (in the momentum equation) and of the heat flux (in the energy equation), in the collisionless limit. Both should be derived from a kinetic approach, and several attempts are under progress in this regard, both for magnetized and unmagnetized plumes [23,[61][62][63]71] and even for anisotropic plasmas [64], showing a combination of near-isothermal and polytropic behaviors.…”

“…For the purposes of the present paper, we will limit the electron fluid model to an unmagnetized plume ( = B 0) and to the simple polytropic electron closure: where γ is the constant polytropic coefficient, and n e0 , T e0 are the electron density and temperature at the plume location where we have set f = 0. Such a polytropic cooling approximation allows us to reproduce the major features of a plume expansion in terms of plasma density, as demonstrated by experiments [19,[65][66][67][68][69][70], fully kinetic simulations [71], and comparisons between polytropic fluid and full-PIC models [72]. Thus, equation (13) becomes is the electron Bernoulli function, and…”

Kinetic electron model for plasma thruster plumes

“…Finally, section 6 presents some additional comments on the kinetic model in the light of the obtained results, including a discussion of its limits of validity, and section 7 gathers the main conclusions. A preliminary version of this work was presented in [47].…”