  The particle-in-cell (PIC) method is successfully used to study magnetized
plasmas. However, this requires large computational costs and limits
simulations to short physical run-times and often to setups in less than three
spatial dimensions. Traditionally, this is circumvented either via hybrid-PIC
methods (adopting massless electrons) or via magneto-hydrodynamic-PIC methods
(modelling the background plasma as a single charge-neutral
magneto-hydrodynamical fluid). Because both methods preclude modelling
important plasma-kinetic effects, we introduce a new fluid-PIC code that
couples a fully explicit and charge-conservative multi-fluid solver to the PIC
code SHARP through a current-coupling scheme and solve the full set of
Maxwell's equations. This avoids simplifications typically adopted for Ohm's
Law and enables us to fully resolve the electron temporal and spatial scales
while retaining the versatility of initializing any number of ion, electron, or
neutral species with arbitrary velocity distributions. The fluid solver
includes closures emulating Landau damping so that we can account for this
important kinetic process in our fluid species. Our fluid-PIC code is
second-order accurate in space and time. The code is successfully validated
against several test problems, including the stability and accuracy of shocks
and the dispersion relation and damping rates of waves in unmagnetized and
magnetized plasmas. It also matches growth rates and saturation levels of the
gyro-scale and intermediate-scale instabilities driven by drifting charged
particles in magnetized thermal background plasmas in comparison to linear
theory and PIC simulations. This new fluid-SHARP code is specially designed for
studying high-energy cosmic rays interacting with thermal plasmas over
macroscopic timescales.
