WARP: A 3D (+) PIC code for HIF simulations

The authors describe WARP, a particle-in-cell code being developed for heavy ion fusion accelerator studies. Along with a 3d (Cartesian) description, an r, z model, a 1d (axial) model, and an envelope calculation are being incorporated within the code. The self-field is assumed electrostatic in the beam frame; particle positions and momenta are time-advanced in the laboratory frame. They employ several non-standard techniques, including: (1) An isochronous relativistic leapfrog mover, in which positions and velocities are advanced from one integer time level to the next. This facilitates particle loading and diagnostics, and a variable timestep size. (2) Use of only two 3d field arrays (ρ and φ), differencing φ for each particle to get E; use of a single 3d array is feasible. (3) Residence corrections to the applied fields from sharp-edged elements, to assure a constant impulse independent of the number of discrete steps on which a particle stops within each element. A test run which models a beam undergoing drift compression is presented. A new method for modeling bends, under development for WARP, is also described briefly