| AVS 64th International Symposium & Exhibition | |
| Vacuum Technology Division | Thursday Sessions |
| Session VT-ThA |
| Session: | Surface Science for Accelerators |
| Presenter: | Anne-Laure Lamure, CERN, Switzerland |
| Authors: | A.-L. Lamure, CERN, Switzerland V. Baglin, CERN, Switzerland P. Chiggiato, CERN, Switzerland B. Henrist, CERN, Switzerland |
| Correspondent: | Click to Email |
The CERN Large Hadron Collider (LHC) is the world’s biggest particle storage ring. Particles circulate in a 27km pipe, under vacuum. One of the main vacuum limitations is the electron cloud.
Photoelectrons are produced when the synchrotron radiation from the proton beam hits the wall. They are then accelerated toward the beam, gain energy and extract new electrons by secondary electron emission. The avalanche phenomenon which is observed is called multipacting.
Electron cloud is deleterious as it interacts with the beam, induces gas desorption and produces additional heat load on the cryogenic system of the magnets.
In order to mitigate the multipacting effect for the upgraded LHC (HL-LHC), amorphous carbon, with a low secondary electron yield, will be coated in some cryogenic magnets.
In this context, it is important to know the behaviour of the usual residual gas (H2, CO, CH4, CO2) on amorphous carbon coating held at cryogenic temperature, in order to know how to operate the vacuum in the accelerator. The quantity of gas that can be stored on the surfaces and the binding energy of adsorption are two highly interesting information.
The results of two types of experiments will be presented. Adsorption isotherms give the vapor pressure depending on the coverage of gas on the surface. Isotherms of H2 at 4.2K and of CO and CH4 at 77K have been measured.
Thermal Desorption Spectroscopy, that allow to determine the average binding energy between the gas and the surface, have been carried out for the four gases, for different initial coverages.
It has been measured that amorphous carbon is a porous material which can store more gas at cryogenic temperature than usual technical surfaces such as copper or stainless steel. The consequences for the accelerator will be discussed. A model to compute the pressure rise in the vacuum pipe, depending on the temperature variation and on the initial coverage, is under development.