Recent Improvements to the IMPACT-T Parallel Particle Tracking Code

Recent Improvements to the IMPACT-T Parallel Particle Tracking Code J. Qiang , I. V. Pogorelov, R. Ryne, LBNL, Berkeley, CA 94547, USA Abstract The IMPACT-T code is a parallel three-dimensional quasi-static beam dynamics code for modeling high bright- ness beams in photoinjectors and RF linacs. Developed under the US DOE Scientific Discovery through Advanced Computing (SciDAC) program, it includes several key fea- tures including a self-consistent calculation of 3D space- charge forces using a shifted and integrated Green function method, multiple energy bins for beams with large energy spread, and models for treating RF standing wave and trav- eling wave structures. In this paper, we report on recent improvements to the IMPACT-T code including modeling traveling wave structures, short-range transverse and longi- tudinal wakefields, and longitudinal coherent synchrotron radiation through bending magnets. PHYSICAL MODELS The general equations of motion used in the IMPACT-T code are: where, with is the speed of light, is the rest mass of a particle, is the charge of a particle. The electric field, , and mag- netic field, , include the contributions from external fo- cusing and accelerating fields and space-charge fields of intra-particle Coulomb interactions. For an RF linac, under proper gauge ( ), the external electromagnetic fields in a cylindrically symmetric accelerating structure can be obtained from [2]: where the vector potential is given by with and where is the electric field on the axis, is the RF an- gular frequency, is the initial phase of the RF field with respect to global time zero, and a superscript prime denotes the derivative with respect to . In the current version of the code, is up to in the above equations summation so that only second order and third order nonlinearity are included for off-axis fields. If the RF cavity does not have cylindri- cal symmetry, the fully three-dimensional electromagnetic fields are read in from external data files. The electromag- netic fields in a traveling wave structure can be viewed as having the form of a standing wave around the entrance and the exit of the structure, and a traveling wave in most cells INTRODUCTION IMPACT-T is a fully three-dimensional parallel pro- gram to track relativistic particles taking into account space charge forces, short-range longitudinal and transverse wakefields, and coherent synchrotron radiation (CSR). It has a comprehensive set of beamline elements, and further- more allows arbitrary overlap of their fields, which gives the IMPACT-T code a capability to model both standing wave and traveling wave structures. It is also unique in its use of space-charge solvers based on an integrated Green function to efficiently and accurately treat beams with large aspect ratio, and a shifted Green function to efficiently treat image charge effects of a cathode [1]. It is also unique in its inclusion of energy binning in the space-charge calculation to model beams with large energy spread. IMPACT-T has a flexible data structure that allows particles to be stored in containers with common characteristics; for photoinjector simulations the containers represent multiple slices, but in other applications they could correspond, e.g., to particles of different species. The IMPACT-T code has been used to study the beam dynamics at a number of photoinjectors such as the Fermilab A0 photoinjector, LCLS normal RF injector, Cornell ERL DC injector, BNL superconducting photoinjector, and others. Work supported by a Scientific Discovery through Advanced Com- puting project, “Advanced Computing for 21st Century Accelerator Sci- ence and Technology,” which is supported by the US DOE/SC Office of High Energy Physics and the Office of Advanced Scientific Computing Research. jqiang@lbl.gov