Abstract
A weak vertical coupled-bunch instability with oscillation amplitude of the order of a few $\ensuremath{\mu}\mathrm{m}$ has been observed in SPEAR3 at nominal vacuum pressure. The instability becomes stronger with increasing neutral gas pressure as observed by turning off vacuum pumps, and becomes weaker when the vertical beam emittance is increased. These observations indicate that the vertical beam motion is driven by ions trapped in the periodic potential of the electron beam. In this paper we present a series of comprehensive beam measurements, impedance-based stability analysis, and numerical simulations of beam-ion interactions in SPEAR3. The effects of vacuum pressure, gas species, beam current, bunch fill pattern, chromaticity, and vertical beam emittance are investigated.
Highlights
In an electron storage ring, ions generated from electron beam collisions with residual gas molecules can be trapped by the periodic electric potential of the bunched beam
The fast ion instability (FII) has been observed at the Advanced Light Source and Pohang Light Source by artificially increasing the neutral gas pressure with helium injection into the vacuum chamber, or by turning off vacuum pumps to induce pressure buildup [2,3,4]
This paper reports on measurement and modeling of beam-ion instabilities in SPEAR3 including single bunch train (FII) and multiple bunch-train instability phenomena
Summary
In an electron storage ring, ions generated from electron beam collisions with residual gas molecules can be trapped by the periodic electric potential of the bunched beam. Optics, nonlinear space charge forces, and chromaticity simultaneously All these factors can provide damping mechanisms which act to suppress growth of beam-ion instabilities. Different from other types of chamber-impedance driven instabilities, the amplitude of a beam-ion instability typically grows quickly in the linear regime and saturates at low amplitude, about one beam radius [13 m rootmean-square (rms) size in our case], due to nonlinear amplitude-dependent space charge effects. Since SPEAR3 has mode-damped cavities and low vacuum chamber impedance, coupled-bunch instabilities driven by resistive wall impedance and high-order cavity modes are normally small or nonexistent. This makes SPEAR3 an ideal machine to isolate and measure beam-ion interactions.
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