  VPIC is a general purpose Particle-in-Cell simulation code for modeling
plasma phenomena such as magnetic reconnection, fusion, solar weather, and
laser-plasma interaction in three dimensions using large numbers of particles.
VPIC's capacity in both fidelity and scale makes it particularly well-suited
for plasma research on pre-exascale and exascale platforms. In this paper we
demonstrate the unique challenges involved in preparing the VPIC code for
operation at exascale, outlining important optimizations to make VPIC efficient
on accelerators. Specifically, we show the work undertaken in adapting VPIC to
exploit the portability-enabling framework Kokkos and highlight the
enhancements to VPIC's modeling capabilities to achieve performance at
exascale. We assess the achieved performance-portability trade-off through a
suite of studies on nine different varieties of modern pre-exascale hardware.
Our performance-portability study includes weak-scaling runs on three of the
top ten TOP500 supercomputers, as well as a comparison of low-level system
performance of hardware from four different vendors.
