Progress on an energy-conserving, asymptotic-preserving orbit integrator for implicit PIC simulations of arbitrarily magnetized plasmas

We build on previous work, which developed an implicit, asymptotic preserving (AP) time-integrator for charged particle motion that (a) correctly recovers guiding center motion when stepping over the gyro-period, (b) converges to full-orbit motion in the small time-step limit, and (c) conserves energy exactly. We extend the scheme to additionally capture the finite Larmor radius effects that appear in many applications. This is done by alternating large and small time-steps in a coordinated fashion. New restrictions on time-step size are derived, which leads to an adaptive time-stepping strategy. Results from simple test problems verify the AP behavior and strict conservation properties. Finally, we report on progress in implementing the new method in an implicit PIC scheme.

This work was performed under the auspices of the U.S. Department of Energy by LLNL and LANL under contracts DE-AC52-07NA27344, DE-AC52-06NA25396, and supported by the Exascale Computing Project (17-SC-20-SC), a collaborative effort of the U.S. Department of Energy Office of Science and the National Nuclear Security Administration, and the DOE Office of Advanced Scientific Computing Research.