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  • Open Access

Convective instabilities of bunched beams with space charge

A. Burov*

  • Fermilab, P.O. Box 500, Batavia, Illinois 60510-5011, USA

  • *burov@fnal.gov
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Phys. Rev. Accel. Beams 22, 034202 – Published 28 March, 2019

DOI: https://doi.org/10.1103/PhysRevAccelBeams.22.034202

Abstract

For a single hadron bunch in a circular accelerator at zero chromaticity, without multiturn wakes and without electron clouds and other beams, only one transverse collective instability is possible, the mode-coupling instability (TMCI). For sufficiently strong space charge (SC), the instability threshold of the wake-driven coherent tune shift normally increases linearly with the SC tune shift, as independently concluded by several authors using different methods. This stability condition has, however, a very strange feature: at strong SC, it is totally insensitive to the number of particles. Thus, were it correct, such a beam with sufficiently strong SC, being stable at some intensity, would remain stable at higher intensity, regardless of how much higher. This paper suggests a resolution of this conundrum: while SC suppresses the TMCI, it introduces head-to-tail convective amplifications, which could make the beam even less stable than without SC, even if all the coherent tunes are real, i.e., all the modes are stable in the conventional absolute meaning of the word. This is done using an effective new method of analysis of the beam’s transverse spectrum for arbitrary space charge and wake fields. Two new types of beam instabilities are introduced: the saturating convective instability and the absolute-convective instability.

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References (26)

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Phys. Rev. Accel. Beams 22, 034202– Published 28 March, 2019
  • Received 12 July 2018
  • Revised 6 December 2018
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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