  Laser ablation plasma thrusters are an emerging space propulsion concept that
provides promise for lightweight payload delivery. Predicting the lifetime and
performance of these thrusters hinges on a comprehensive characterization of
the expansion dynamics of the ablated plasma plume. While state-of-the-art
techniques for simulating plasmas are often particle-based, a grid-based direct
kinetic solver confers advantages in such a transient and inhomogeneous problem
by eliminating statistical noise. A direct kinetic solver including
interparticle collisions is employed on a plume expansion model problem
spanning one dimension each in configuration and velocity space. The high
degree of thermodynamic nonequilibrium inherent in plume expansion is
characterized, justifying the need for a kinetic rather than a hybrid or fluid
solver. Thruster-relevant metrics such as the momentum flux are also computed.
The plume dynamics are observed to be highly inhomogeneous in space with
insufficient time for thermalization in the region preceding the expansion
front, and the theoretical possibility of reducing the local grid resolution by
up to two orders of magnitude at the far end of the domain is established.
These grid-point requirements are verified via the employment of nonuniform
grids of various expansion ratios, several of which also employ coarsening in
velocity space. Longer domain lengths are explored to characterize
thruster-scale phenomena and larger ambient pressures are simulated as a
testbed to probe facility effects due to collisions with background particles.
