Review of heavy-ion induced desorption studies for particle accelerators

Edgar Mahner

doi:10.1103/physrevstab.11.104801

Abstract

During high-intensity heavy-ion operation of several particle accelerators worldwide, large dynamic pressure rises of orders of magnitude were caused by lost beam ions that impacted under grazing angle onto the vacuum chamber walls. This ion-induced desorption, observed, for example, at CERN, GSI, and BNL, can seriously limit the ion intensity, luminosity, and beam lifetime of the accelerator. For the heavy-ion program at CERN's Large Hadron Collider collisions between beams of fully stripped lead ($^{208}\mathrm{Pb}^{82+}$) ions with a beam energy of $2.76\text{ }\text{ }\mathrm{TeV}/\mathrm{u}$ and a nominal luminosity of ${10}^{27}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ are foreseen. The GSI future project FAIR (Facility for Antiproton and Ion Research) aims at a beam intensity of ${10}^{12}$ uranium ($^{238}\mathrm{U}^{28+}$) ions per second to be extracted from the synchrotron SIS18. Over the past years an experimental effort has been made to study the observed dynamic vacuum degradations, which are important to understand and overcome for present and future particle accelerators. The paper reviews the results obtained in several laboratories using dedicated test setups, the mitigation techniques found, and their implementation in accelerators.

Highlights

The effect of ions on the dynamic vacuum of a particle accelerator dates back to 1973 where a so-called ioninduced vacuum instability was first observed in the Intersecting Storage Rings (ISR) at CERN [1]
The circulating proton beam ionized residual gas molecules, which were accelerated by the beam potential, bombarded the vacuum chamber walls and released gas adsorbed on the surface
The motivation to study the molecular desorption induced by heavy ions has been triggered about one decade ago by the observation of dynamic vacuum effects in heavy-ion accelerators strongly affecting the machine operation; this includes, for example, very strong pressure rises, limited beam intensities, and reduced beam lifetimes

Summary

Introduction

The effect of ions on the dynamic vacuum of a particle accelerator dates back to 1973 where a so-called ioninduced vacuum instability was first observed in the Intersecting Storage Rings (ISR) at CERN [1]. A critical beam current the vacuum system became unstable and limited the ISR beam intensity. A different type of vacuum instability, induced by the loss of heavy ions, was first observed in 1997 during Pb54þ accumulation and cooling tests in the Low Energy Antiproton Ring (LEAR) at CERN [3]. It was found that the beam lifetime was no longer independent of the injected ion current and that injection losses, either at the vacuum chamber wall or at aperture limiting devices, were responsible for large pressure ex-

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