Electron-cloud build-up in hadron machines

Abstract

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][6][7][8][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][11][12][13][14][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. Primary sources of electrons and secondary electron emissionDepending 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. 2 direction perpendicular to the beam; as they strike the walls of the chamber, they generate secondary electrons which add to the existing electron population.Of all hadron machines presently existing or under construction...

Ecloud Build-Up Simulations for the FNAL MI for a Mixed Fill Pattern: Dependence on Peak SEY and Pulse Intensity During the Ramp

“…This behavior has also been noticed in simulations for the proposed PS2 and for the SPS [16][17][18], although not yet experimentally verified. The non-monotonicity can likely be explained by the fact that, as N t grows, the average electron-wall impact energy crosses E max ~ 293 eV (where the SEY is maximum) when N t -3 x 10 13 .…”

Electron Clouds in the Relativistic Heavy Ion Collider

“…The multiplication depends on beam parameters (like bunch spacing, bunch population, bunch length, etc), and chamber characteristics (beam pipe radius, SEY, reflection probability at low energy, et cetera). It should be stressed that for RHIC, and many other machines with electron clouds, the secondary electron emission process has a more significant effect on the overall electron density than the primary source mechanism [8,32].…”

“…The pressure rises in Fig. 2.6 are sensitive to the bunch intensity and spacing [34], a common electron multipacting signature [2,32] (as will be seen in Chapters 3 and 10). A second mechanism related to molecular desorption induced by beam loss is also suggested in Ref.…”