Tristan-mp stands for TRIdimensional STANford - massively parallel, and is the parallel version [1] of the code originally developed by O. Buneman, K. Nishikawa, and T. Neubert [2]. In its current form, the code is written in a modular format in Fortran 95, and uses the MPI (e.g. see Open MPI) and HDF5 libraries to support parallelism and standardized parallel output files. It is a fully relativistic Particle-In-Cell (PIC) code used for plasma physics computations; it self-consistently solves the full set of Maxwell’s equations, along with the relativistic equations of motion for the charged particles. It follows the general PIC code architecture [3,4]: fields are discretized on a finite 3D or 2D mesh, the computational grid, and this field is then used to advance the velocity of the particles in time via the Lorentz force equation. The charges and currents derived from the particles’ velocities and positions are then used as source terms to re-calculate the electromagnetic fields. The PIC simulation model is described below, along with the details of the numerical implementation of the physical equations.