Wakefield computations at extreme-scale for ultra-short bunches using parallel hp-refinement

The computation of wakefield induced by an ultra-short charged particle beam is one of the most computationally challenging problems in high-performance computing for accelerator. With SciDAC collaborations in computational science, a novel moving-window technique is developed for the finite element time domain (FETD) code T3P to tackle this problem through the reduction of computing needs. The window moving along with the beam in the computational domain adopts high-order finite-element basis functions through p refinement and/or a high-resolution mesh through h refinement so that a sufficient accuracy is attained with substantially reduced computational costs. Algorithms to transfer discretized fields from one mesh to another, which are the key to implementing a moving window in a finite-element unstructured mesh, are presented. Numerical experiments are carried out at DOE flagship computers using the proposed technique to compute short-range wakefields in long accelerator structures. The results are compared with those obtained from the normal FETD method and the advantages of using the moving window technique are discussed. The technique has been successfully used in evaluating wakefield effects due to ultra-short bunches for accelerator components in proposed DOE accelerators.