Electron-cloud build-up in hadron machines Page: 1 of 8
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LBNL-56109
Aug. 9, 2004
1.1 Electron-cloud build-up in hadron machines (*)
M. A. Furman(*)
mail to: mafurmanglbl.ov
Lawrence Berkeley National Laboratory
Bldg. 71R0259, 1 Cyclotron Rd., Berkeley, CA 94720-8211, USA
1.1.1 Introduction
The first observations of electron-proton coupling effect for coasting beams and for
long-bunch beams were made at the earliest proton storage rings at the Budker Institute
of Nuclear Physics (BINP) in the mid-60's [1]. The effect was mainly a form of the
two-stream instability. This phenomenon reappeared at the CERN ISR in the early 70's,
where it was accompanied by an intense vacuum pressure rise. When the ISR was
operated in bunched-beam mode while testing aluminum vacuum chambers, a resonant
effect was observed in which the electron traversal time across the chamber was
comparable to the bunch spacing [2]. This effect ("beam-induced multipacting"), being
resonant in nature, is a dramatic manifestation of an electron cloud sharing the vacuum
chamber with a positively-charged beam. An electron-cloud-induced instability has
been observed since the mid-80's at the PSR (LANL) [3]; in this case, there is a strong
transverse instability accompanied by fast beam losses when the beam current exceeds a
certain threshold. The effect was observed for the first time for a positron beam in the
early 90's at the Photon Factory (PF) at KEK, where the most prominent manifestation
was a coupled-bunch instability that was absent when the machine was operated with an
electron beam under otherwise identical conditions [4]. Since then, with the advent of
ever more intense positron and hadron beams, and the development and deployment of
specialized electron detectors [5-9], the effect has been observed directly or indirectly,
and sometimes studied systematically, at most lepton and hadron machines when
operated with sufficiently intense beams. The effect is expected in various forms and to
various degrees in accelerators under design or construction.
The electron-cloud effect (ECE) has been the subject of various meetings [10-15].
Two excellent reviews, covering the phenomenology, measurements, simulations and
historical development, have been recently given by Frank Zimmermann [16,17]. In this
article we focus on the mechanisms of electron-cloud buildup and dissipation for
hadronic beams, particularly those with very long, intense, bunches.
1.1.2 Primary sources of electrons and secondary electron emission
Depending upon the type of machine, the EC is seeded by primary electrons from
three main sources: photoelectrons, ionization of residual gas, and electrons generated
when stray beam particles hit the chamber walls. In addition, for HIF drivers, an
important expected source of electrons is a combination of two of the above, namely:
gas will be desorbed by stray ions striking the chamber walls which will subsequently
be ionized by the beam. These electrons get kicked by successive bunches mostly in the
* Published as Sec. 2.9 of the ICFA Beam Dynamics Newsletter No. 33, July 2004, p. 70.
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Furman, M. A. Electron-cloud build-up in hadron machines, report, August 9, 2004; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc782171/m1/1/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.