Algorithm implementation and testing to ensure consistency of Gauss’s law in OSIRIS

Electromagnetic particle-in-cell (PIC) simulations compute the trajectories of particles as they interact via fields calculated by numerically solving Maxwell's equations on a grid using currents (and charge densities) from the particles. Within PICKSC, UCLA maintains a variety of open-source and open-access codes. These include OSIRIS---developed in partnership with IST---and UPIC-EMMA. Standard OSIRIS uses a rigorous charge-conserving current deposit to ensure the consistency of Gauss's law together with a finite-difference (FD) solution to Maxwell's equations. It also contains options for spectral (FFT) and hybrid (FFT and FD) field solvers, as well as a customized, higher-order FD field solver to help mitigate the numerical Cerenkov instability. The standard charge conserving current deposit is only valid for second-order accurate FD solvers. Another option for maintaining the consistency of Gauss's law is the Boris correction, where a ``direct'' current deposit is used and the electric field is corrected through the use of a Poisson solve. The Boris correction---with both exact and iterative multigrid Poisson solves---has been implemented into OSIRIS. Preliminary analyses of timing and fluctuations levels will be presented, including the effects of different particle orders.