  Plasmas with varying collisionalities occur in many applications, such as
tokamak edge regions, where the flows are characterized by significant
variations in density and temperature. While a kinetic model is necessary for
weakly-collisional high-temperature plasmas, high collisionality in colder
regions render the equations numerically stiff due to disparate time scales. In
this paper, we propose an implicit-explicit algorithm for such cases, where the
collisional term is integrated implicitly in time, while the advective term is
integrated explicitly in time, thus allowing time step sizes that are
comparable to the advective time scales. This partitioning results in a more
efficient algorithm than those using explicit time integrators, where the time
step sizes are constrained by the stiff collisional time scales. We implement
semi-implicit additive Runge-Kutta methods in COGENT, a finite-volume
gyrokinetic code for mapped, multiblock grids and test the accuracy,
convergence, and computational cost of these semi-implicit methods for test
cases with highly-collisional plasmas.
