Summary form only given. The transient 3D electromagnetic Particle-In-Cell [1,2] code HALO3D operates on unstructured meshes. It uses a high order discontinuous Galerkin approach to discretize the full set of the Maxwell equations in time domain. HALO3D is designed to be highly scalable, being able to simulate even high frequency particle-wave interactions and field propagation in state-of-the-art gyrotrons. Very recently, this solver was used to simulate the resonant cavity and the large-scale mode converter of a TE 34,19 gyrotron. To enable such computations, the coupled solver had to be optimized to run efficiently on more than 1000 CPU cores. The parallelization of the explicit scheme is base on a domain decomposition approach.