IMPACT is a parallel particle-in-cell code suite for modeling high intensity, high brightness beams in rf proton linacs, electron linacs and photoinjectors. It consists of two parallel particle-in-cell tracking codes. IMPACT-Z uses longitudinal position as the independent variable, and allows for efficient particle advance over large distances, as in an rf linac. IMPACT-T uses time as the independent variable, and is needed to accurately model systems with strong space charge, as in photoinjectors. It incorporates an rf linac lattice design code, an envelope matching and analysis code, and a number of pre- and post-processing codes.

Both of the IMPACT codes assume a quasi-electrostatic model of the beam (i.e., electrostatic self-fields in the beam frame, possibly with energy binning for a beam with large energy spread) and compute space-charge effects self-consistently at each time step together with the external acceleration and focusing fields. The 3D Poisson equation is solved in the beam frame at each step of the calculation. The resulting electrostatic fields are Lorentz-transformed back to the laboratory frame to obtain the electric and magnetic self-forces acting on the beam.

There are six Poisson solvers in the IMPACT suite, corresponding to transverse open or closed boundary conditions with round or rectangular shape, and longitudinal open or periodic boundary conditions. These solvers use either a spectral method for closed transverse boundary conditions, or a convolution-based Green function method for open transverse boundary conditions. The convolution for the most widely used open boundary condition Poisson solver is calculated using an FFT with a doubled computational domain. The computing time of this solver scales like N × log(N), where N is the number of grid points. The parallel implementation includes both a 2D domain decomposition approach for the 3D computational domain and a particle-field decomposition approach to provide the optimal parallel performance for different applications on modern supercomputers.

Besides the fully 3D space-charge capability, the IMPACT suite also includes detailed modeling of beam dynamics in rf cavities (via field maps or z-dependent transfer maps including rf focusing/defocusing), various magnetic focusing elements (solenoid, dipole, quadrupole, etc), allowance of arbitrary overlap of external fields (3D and 2D), structure and CSR wake fields, tracking multiple charge states, tracking multiple bin/bunches, Monte-Carlo simulation of gas ionization, an analytical model for laser-electron interactions inside an undulator, and capabilities for machine error studies and correction.

## Uses

The IMPACT code suite has been applied to studies of halo formation and coupling resonance in high intensity beams, microbunching instability in high brightness electron linac, beam dynamics in photoinjectors and streak cameras, and beam dynamics. It has been applied to a variety of accelerators, including

- The Spallation Neutron Source (SNS) linac
- The J-PARC driver linac in the Japan Proton Accelerator Research Complex
- The Rare Isotope Accelerator (RIA) driver linac for the Facility for Rare Isotope Beams (FRIB) at Michigan State University
- The CERN superconducting linac
- The Low Energy Demonstration Accelerator (LEDA) halo experiment at LANL
- KoRIA, a planned rare-isotope facility in the Republic of Korea
- The Chinese Accelerator Driven System (C-ADS), a future accelerator-based subcritical system for waste transmutation
- The Proton Synchrotron at CERN, which is being used in the Large Hadron Accelerator injection chain

## To Learn More…

Visit Ji Qiang’s IMPACT-T and IMPACT-Z page in the Accelerator Modeling and Advanced Computation Group. Ji is the lead developer of the IMPACT suite and handles technical correspondence regarding it.

J. Qiang, R. D. Ryne, M. Venturini, A. A. Zholents, I. V. Pogorelov, “High resolution simulation of beam dynamics in electron linacs for x-ray free electron lasers,” Physical Review Special Topics: Accelerators and Beams ** 12**, 100702 (2009).

Qiang, Lidia, Ryne, and Limborg-Deprey, “Three-dimensional quasistatic model for high brightness beam dynamics simulation,” Physical Review Special Topics: Accelerators and Beams ** 9**, 044204 (2006).

J. Qiang, R. Ryne, S. Habib, V. Decyk, “An Object-Oriented Parallel Particle-In-Cell Code for Beam Dynamics Simulation in Linear Accelerators,” Journal of Computational Physics ** 163** (2000), pp. 434-451.