Simulating electron clouds in heavy-ion accelerators

Abstract

Contaminating clouds of electrons are a concern for most accelerators of postive-charged particles, but there are some unique aspects of heavy-ion accelerators for fusion and high-energy density physics which make modeling such clouds especially challenging. In particular, self-consistent electron and ion simulation is required, including a particle advance scheme which can follow electrons in regions where electrons are strongly-, wealdy-, and un-magnetized. We describe o w approach to such self-consistency, and in particular a scheme for interpolating between full-orbit (Boris) and dr&kinetic particle pushes that enables electron time steps long compared to the typical gyro period in the magnets. We present tests and applications: simulation of electron clouds produced by t h e e Werent lcinds of sowces indicates the sensitivity of the cloud shape to the nature of the source; first-of-a-kind self-consistent simulation of electron-cloud experiments on the High-Current Experiment (HCX)

Large-timestep mover for particle simulations of arbitrarily magnetized species

“…This test was reported in Ref. [1], though we have since discovered that the code at the time had an inconsistent boundary condition. Correcting that produces quantitative but not qualitative changes.…”

“…The blended mover concept [1] builds upon the observation by Parker and Birdsall [2] that the conventional Boris mover for particles in a magnetic field, when directly applied with timesteps ∆t large compared to the inverse cyclotron frequency ω −1 c , recovers physically correct electric and magnetic drifts, at least for test problems where these drifts are uniform, but with two drawbacks: (1) the particles oscillate about their gyrocenter with an effective "gyroradius" that is larger than the physical gyroradius by a factor of [1 + (ω c ∆t/2) 2 ] 1/2 , and (2) the particles oscillate by nearly π in gyrophase each timestep, with a slow procession period that is O(ω c ∆t 2 ). For simulation of phenomena with frequencies well below the cyclotron frequency, the first difficulty can be overcome by using an interpolated velocity to advance particle positions.…”

“…[1] is the calculation of electron cloud dynamics for a self-consistent simulation which includes both an ion beam and electrons. This is a simulation of an experiment done in the High Current Experiment (HCX) facility [4], in which an ion beam which has passed through is series of quadrupole magnets and a magnetic-field-free end region is allowed to impact an end plate which emits a copious supply of electrons.…”

“…3, taken from Ref. [1], is a plot of the electron distribution in the final quadrupole magnet and field-free region for the blended mover at large timestep and a comparison run with small timesteps. The blended mover does a good job of reproducing the electron distribution calculated with small timesteps, including capturing the spatial oscillations which appear.…”

“…This mover was described briefly, and some tests presented, in a previous publication [1]. In this paper we describe the mover in more detail, present additional test results, and also describe an extension to the original prescription that allows extension to some higher-density systems with plasma frequency ω p exceeding the gyrofrequency (along with a test of the new capability).…”