  An extended thermal particle boundary condition is devised to more
efficiently and accurately model laser-plasma interactions in overdense
plasmas. Particle-in-cell simulations of such interactions require many
particles per cell, and a large region of background plasma is often necessary
to correctly mimic a semi-infinite plasma and avoid electron refluxing from a
truncated plasma. For long-pulse lasers of many picoseconds, such constraints
can become prohibitively expensive. Here, an extended particle boundary
condition (absorber) is designed that instantaneously stops and re-emits
energetic particles streaming toward the simulation boundary over a defined
region, allowing sufficient time and space for a suitably cool return current
to develop in the background plasma. Tunable parameters of the absorber are
explained, and simulations using the absorber with a 3-ps laser are shown to
accurately reproduce those of a causally separated boundary while requiring
only 20% the number of particles.
