When do residual gas electrons destabilize an ion beam?

• We study Coulomb collisions of beam particles with electron cloud particles.
• The effect of Coulomb collisions is simulated for realistic beam shapes.
• Beams with similar shapes and energy have equal saturated neutralization degree.
• Equal space-charge tune shifts of beams give equal two-stream instability thresholds.
• Vacuum conditions achievable in modern ion accelerators can suppress the instability.

The stability of coasting ion beams against residual gas electrons accumulating in the beam potential is studied. We analyze the instability threshold intensities for ion beams of different charge state and mass using the linear centroid theory. Numerical simulations are used to study the thresholds for realistic transverse beam distributions. The effect of Coulomb collisions between beam ions and electrons is analyzed analytically and numerically using the Langevin equations. We show that at low residual gas pressures Coulomb collisions limit the neutralization degree of the ion beam to values below the instability threshold.