Large-timestep mover for particle simulations of arbitrarily magnetized species

Abstract

For self-consistent ion-beam simulations including electron motion, it is desirable to be able to follow electron dynamics accurately without being constrained by the electron cyclotron timescale. To this end, we have developed a particle-advance that interpolates between full particle dynamics and drift motion. By making a proper choice of interpolation parameter, simulation particles experience physically correct parallel dynamics, drift motion, and gyroradius when the timestep is large compared to the cyclotron period, though the effective gyro frequency is artificially low; in the opposite timestep limit, the method approaches a conventional Boris particle push. By combining this scheme with a Poisson solver that includes an interpolated form of the polarization drift in the dielectric response, the movers utility can be extended to higher-density problems where the plasma frequency of the species being advanced exceeds its cyclotron frequency. We describe a series of tests of the mover and its application to simulation of electron clouds in heavy-ion accelerators.

Overview of theory and simulations in the Heavy Ion Fusion Science Virtual National Laboratory

“…It enables time-steps much larger than the electron gyro-period (constrained instead by the next-larger timescale, which in a magnetic quadrupole is the electron bounce time in the electrostatic well), and thereby offers a computer-time reduction of 1-2 orders of magnitude [29,30].…”

“…These capabilities have been applied to experiments on HCX [22,23,30], wherein the ion beam is directed onto a plate at the end of the system, generating copious primary electrons, which then generate secondaries. A set of control electrodes set to potentials that are varied from shot to shot offers a variety of operating conditions.…”

“…More recently, the code has been applied to the UC Berkeley Penning-Malmberg trap, which includes a multipole field (necessary for anti-Hydrogen confinement) [30,54].…”

Critical issues for high-brightness heavy-ion beams --prioritized

“…Multiple developments that increase code operating speeds by several orders of magnitude, making it feasible to perform 3-D simulations with the added effects above. Speed enhancements include parallel operation, adaptive mesh refinement [12], particle timestep subcycling [13], and a drift-Lorentz electron mover tracking charged particles in magnetic fields using large time steps [14,15]. For relativistic interactions of beams, a further few orders of magnitude reduction in computational time is possible, using our recent discovery of a preferred frame of reference [16].…”

“…The generation and transport of all species (beam particles, ions, electrons, and gas molecules) is performed in a self-consistent manner (the electron, ion and gas distributions can also be prescribed -if needed-for special study or convenience). The code runs in parallel and benefits from adaptive mesh refinement [12], particle timestep sub-cycling [13], a new "drift-Lorentz" particle mover for tracking charged particles in magnetic fields using large time steps [14,15], and for relativistic beams, the recent discovery of a preferred frame of reference [16] that reduces computation time by a factor of 2γ 2 where γ = (1-v 2 /c 2 ) -0.5 . These advanced numerical techniques allow for significant speed-up in computing time (orders of magnitude) relative to brute-force integration techniques, allowing for self-consistent simulations of electron-cloud effects and beam dynamics, which were out of reach with previously available tools.…”