COLLISION OPERATORS FOR PARTIALLY LINEARIZED PARTICLE SIMULATION CODES

Algorithms are presented for energy- and momentum-conserving like-part icle Coulomb collisions in partially linearized (delta f) particle sim ulations. They are developed and implemented in particular for gyrokin etic simulation models of a strongly magnetized plasma. The collision operators include both drag and diffusion terms, are not restricted to a single or few Fourier modes, and approximately conserve both moment um and energy locally in space in a statistical sense. The first algor ithm is a many-mode generalization of a test-particle-plus-source algo rithm previously proposed. The second is easier to implement and impro ves upon the first significantly by not requiring many time steps for good conservation. Implementations for the case for ion-ion collisions are given and conservation properties are demonstrated, both directly with non-self-consistent test simulation runs and indirectly with sel f-consistent runs. The computational cost of particle pushing and solv ing for fields depends on the relative collisionality and can result i n a tripling of the total computational costs if collisions are done a t each time step, but typically will be a small fraction of the total simulation cost. It is also shown that binary-collision-based algorith ms are unsuitable for partially linearized simulations.