Electron-cloud simulation and theory for high-current heavy-ion beams

Abstract

Stray electrons can arise in positive-ion accelerators for heavy-ion fusion or other applications as a result of ionization of ambient gas or gas released from walls due to halo-ion impact, or as a result of secondary-electron emission. We summarize the distinguishing features of electron-cloud issues in heavy-ion-fusion accelerators and a plan for developing a self-consistent simulation capability for heavy-ion beams and electron clouds (also applicable to other accelerators). We also present results from several ingredients in this capability. (1) We calculate the electron cloud produced by electron desorption from computed beam-ion loss, which illustrates the importance of retaining ion reflection at the walls. (2) We simulate the effect of specified electron-cloud distributions on ion beam dynamics. We consider here electron distributions with axially varying density, centroid location, or radial shape, and examine both random and sinusoidally varying perturbations. We find that amplitude variations are most effective in spoiling ion beam quality, though for sinusoidal variations which match the natural ion beam centroid oscillation or breathing-mode frequencies, the centroid and shape perturbations can also have significant impact. We identify an instability associated with a resonance between the beamenvelope ''breathing'' mode and the electron perturbation. We estimate its growth rate, which is moderate (compared to the reciprocal of a typical pulse duration). One conclusion from this study is that heavy-ion beams are surprisingly robust to electron clouds, compared to a priori expectations. (3) We report first results from a long-time-step algorithm for electron dynamics, which holds promise for efficient simultaneous solution of electron and ion dynamics.

Absolute measurement of electron cloud density

“…The electron line charge is obtained dividing the electron current by the average drift velocity Clearing electrode measurements show that, when the suppressor and clearing electrodes are off, electrons produced at the end wall structures drift through each of the quadrupole magnets at a velocity of 0.60 + 0.11 m/µs. This value was obtained by dividing the time that the electrons need to reach the successive clearing electrodes by the effective magnetic field length of the quadrupole (~32 cm) [15] and it is in excellent agreement with the value of 0.66 m/µs [16] predicted by the simulations. …”

The WARP Code: Modeling High Intensity Ion Beams

“…For electrons, an advanced integrator is being developed that allows large time -steps compared to the electron cyclotron frequency. [3] The lattice description allows a range of field descriptions, from uniform, pure multipole components, to axially varying, mixed components, to gridded field data.…”

Research and development toward heavy ion driven inertial fusion energy

“…Possible causative mechanisms are envelope mismatch, imperfect applied fields, electron clouds [43], and beam-gas interactions. Collective effects of space-charge waves are expected to relax after several plasma oscillations [44].…”