Mesh refinement implementation in QuickPIC

The PWFA has emerged as a promising candidate for the accelerator technology used to build a future linear collider or a light source. In this scheme witness beams are accelerated in the wakefield created by driver beams. The three-dimensional (3D) quasi-static (QS) particle-in-cell (PIC) approach has been shown to provide 2-4 orders of magnitude speedup over 3D fully explicit PIC codes and maintain high fidelity simulation. In some linear collider designs for the electron arm, the matched spot size of the witness beam can be 2 to 3 orders of magnitude smaller than the spot size of the wakefield. To efficiently simulate such a disparity in scales, we implement a mesh refinement scheme into the 3D QS PIC codes, QuickPIC. We use very high resolution in a small spatial region that includes the witness beam and coarse resolution in the simulation domain. The code has been parallelized with both MPI and OpenMP, and the scalability has also been improved by using pipelining. The boundary effect of refined mesh has been studied, based on which we developed a preliminary adaptive mesh refinement for an evolving beam size. Several benchmark cases have been tested and get consistent result as previously published papers and also consistent as a new quasi-3d QS PIC code called QPAD.