The supersonic expansion of an underexpanding argon plasma from a high dens
ity arc source with small dimensions into a low-pressure vessel with large
dimensions is studied by an extended one-dimensional nonlocal thermal equil
ibrium fluid model, called SPIRIT. In an expanding plasma the velocity incr
eases and the pressure, the density, and the temperatures decrease severely
. In this article the virtual source model is discussed first, which is a m
odel describing the expanding plasma as originating from a virtual source.
The virtual source model includes some viscosity and heat transport in simp
lified form, but most of the viscosity and heat transport contributions are
neglected. The SPIRIT code includes the full energy and momentum balances.
The inclusion of viscosity and heat sources may lead to deviations from an
adiabatic and/or isentropic expansion. The SPIRIT code can analyze the dev
iations. When deviations are small, the isentropic expressions from gas dyn
amics can be used to model expanding plasma too. Model outcomes are compare
d with experimental data. (C) 2001 American Institute of Physics.