Very fine pumps

In Building 181, half the total number of vacuum chambers required for the LHC Long Straight Sections (LSS) have now been coated with getter material by sputtering. This innovative technique, invented at CERN, will be used on all the LHC 's "warm" vacuum chambers.

Everyone seems to agree that a vacuum is a space with nothing inside it. But from the physicist 's viewpoint that is not entirely true, as certain particles, tiny grains of matter, are to be found everywhere. Furthermore, at the Earth's surface a cubic centimetre of air contains some 25 million million million ( 10¹⁸ ) gas molecules. In such conditions protons would never be able to circulate around the LHC ring as they would constantly interact with gas molecules and rapidly be lost. In accelerators, particles therefore have to be circulated inside a vacuum chamber where the number of atoms or gas molecules has been reduced to the absolute minimum.

The LHC will have two types of vacuum chamber - one for the cold sections that contain the superconducting magnets, and one for the warm, non-cryogenic sections, mainly situated either side of the beam interaction points.
A near-perfect vacuum is obtained in the cold sections using the technique of cryo-pumping. In the warm sections, a different technique is used, namely coating the chamber walls with a thin layer of getter material, which acts like flypaper, capturing the residual gas particles and thereby improving the vacuum.
The getter layer is produced by introducing a cathode into the centre of the vacuum chamber, extracting air from the chamber using turbomolecular pumps, injecting krypton gas into the chamber at a pressure of approximately 10^-2 Torr and applying a negative voltage of around -500V to polarise the cathode and produce a discharge. The ions produced by this discharge bombard the cathode and cause it to eject atoms that adhere to the vacuum chamber wall. This process continues until a getter layer of micrometric thickness is obtained.
Since the cathode is made up of three intertwined wires - one of zirconium, one of titanium and one of vanadium - the getter layer contains these three metals in nearly identical quantities. The getter enables the vacuum chamber to act as its own vacuum pump.
Once this process has been completed, the chambers are filled with dry air and stored in nitrogen pending their installation in the machine.
Getters were successfully implemented at LEP but in a much less sophisticated fashion, with a strip of getter material being applied directly to the walls of the vacuum chamber. Both techniques were developed by Cristoforo Benvenuti, now Leader of the TS-MME Group, which is responsible for preparing the warm vacuum chambers for the LHC.
Sputtering is the last in a long sequence of treatment processes which the vacuum chambers undergo at CERN prior to storage. Before arriving at Building 181, they go to the mechanical workshop for brazing and welding, after which they are moved to Building 927 for leak detection and then on to the chemical workshop for degreasing and pickling. In this context special mention should be made of the Transport and Handling Service, which is responsible for moving the vacuum chambers between the various buildings where the treatment process are performed.
Today, over half the vacuum chambers for the Long Straight Sections, 380 out of 662, have received their getter coating. Once all the chambers have been completed, the TS-MME Group will set about applying the same process to the vacuum chambers that will be installed directly in the LHC detectors and inside the warm magnets of the LHC machine.
The novel technique of sputtering, developed entirely at CERN, has been patented and is a shining example of technology transfer. It has been successfully tested in the SPS and at other facilities, including ESRF in France and ELETTRA in Italy.