The Latest from the LHC: Successful switch to ions

The last 2010 proton beam was extracted from the LHC on Thursday, 4 November. The transition from proton to lead ion operation was accomplished remarkably smoothly. The first ion collisions were recorded just after midnight on 6 November.


The LHC1 screen showing the machine running with ions for the first time.

The LHC switched to operation with ions on Thursday, 4 November, with first ion collisions ensuing about 54 hours later, at 00:30 CET on 7 November. Stable running conditions marking the start of physics with heavy ions were achieved at 11:20 CET on 8 November.

The very smooth and fast transition to operation with ions was made possible by very good beam instrumentation performance with a relatively low number of charges per bunch, and magnetic behaviour very similar to operation with protons, as expected. These two factors combined allowed the setting-up operations to be completed very quickly, and stable beam operation, with 2 bunches per beam, was achieved in just a few days. It was then possible, as was the case for proton operation, to steadily increase the number of bunches: 69 bunches per beam had been achieved at the time of going to press, 121 being the target for 2010.

Before switching to ion operation, the machine had delivered 48 inverse picobarns of integrated luminosity to ATLAS and CMS (with a peak luminosity of 2x1032 cm-2 s-1), and ran with 150ns bunch spacing until Friday, 29 October. Then, for about 6 days, the LHC was operated with 50ns bunch spacing in order to explore the scheme planned for 2011. In this configuration, strong beam-induced effects were observed on the vacuum inside the beam pipe. These effects depend on the number of bunches in the train, the spacing between the trains, and the bunch intensity (the observed threshold is around 6-8x1010 protons per bunch). These early studies are very valuable as they allow us to predict the behaviour of the machine with the beam configuration planned for 2011.

How is a beam of lead ions produced at CERN?

The source of lead ions.

Lead ions are produced when lead atoms are stripped of electrons. A highly purified lead sample is heated to a temperature of about 500 °C. The lead vapour is then ionized by an electron current. Many different charge states are produced, with a maximum around Pb29+. These ions are selected and accelerated to 4.2 MeV/u (energy per nucleon) before passing through a carbon foil, which strips most of them to Pb54+. The Pb54+ beam is accumulated, then accelerated to 72 MeV/u in the Low Energy Ion Ring (LEIR), which transfers them to the PS. The PS accelerates the beam to 5.9 GeV/u and sends it to the SPS after first passing it through a second foil where it is fully stripped to Pb82+. The SPS accelerates it to 177 GeV/u then sends it to the LHC, which accelerates it to 2.76 TeV/u.

More information about the transition to lead ion operation is available by consulting the recent Press Releases.

by CERN Bulletin