Synchrotron Radiation, Adsorption Isotherms and the Large Hadron Collider Vacuum System
In circular particle accelerators and storage rings with relativistic beams of particles, e.g. electrons or protons, the vacuum performance of the accelerator is determined by the outgassing from the walls of the beam vacuum chamber. The outgassing is dominated by a photodesorption process where synchrotron radiation from the circulating particle beam hits the wall of the vacuum chamber and induces the desorption of neutral gas molecules, mainly H2, CH4, CO and CO2, from the surface of the vacuum chamber.
Monte Carlo simulations of the synchrotron radiation in the interaction regions, i.e. at the detectors, of the future Large Hadron Collider (LHC) at CERN, Geneva, Switzerland, have been done. The amount of synchrotron radiation decides the amount of pumping needed to keep an acceptably low pressure in the interaction regions with a vacuum system at room temperature. The synchrotron radiation can also disturb the electronics and different sensors in the detectors around the interaction regions and Monte Carlo simulations of the synchrotron radiation emitted in a future experiment with collisions between the protons of the LHC and the electrons of the Large Electron Positron (LEP) accelerator have been done.
The LHC will use superconducting magnets to produce a magnetic field of about 9 T around the arcs of the 27 km long quasi-circular accelerator. The cold vacuum system in the superconducting magnets of the LHC consists of an inner perforated beam screen and a surrounding cold bore vacuum tube. The gas molecules desorbed by the synchrotron radiation in the LHC will be physisorbed on the cold surfaces in the vacuum chamber and they may induce vacuum instabilities in the accelerator. Hence it is of importance to have knowledge of the isotherms of the desorbed gases at low temperatures.
Adsorption isotherms in the pressure range 10-11 to 10-6 Torr have been measured at 4.2 K for H2 and mixtures of H2 and CH4, CO and CO2 on copper plated stainless steel, a surface identical to the beam screen surface. The measurements have focused on the behaviour of the isotherms at low surface coverage, up to a few monolayers of adsorbed gas on the cold surface. The isotherms were measured in a static situation with a small amount of gas injected for each point on the isotherm. Coadsorption measurements of H2 with CH4, CO and CO2 as well as adsorption of H2 on condensates of CH4, CO and CO2 have been made. A cryotrapping effect of H2 is seen when coadsorbing the gas mixtures, especially strong for the mixture of H2 and CO2. The measurements show that CO2 condensed at 4.2 K may have a porous structure that H2 can penetrate, while CO and CH4 have rather dense structures when adsorbed at 4.2K.
An important aspect of the cold vacuum system of the LHC is the detection of He leaks into the vacuum tube. The superconducting dipole magnets are cooled with superfluid liquid He and considering the size of the LHC there is a finite risk of having He leaks into the beam vacuum tube. Due to physisorption of He on the cold walls of the beam vacuum tube the He pressure front propagates slowly along the beam vacuum tube. The time to detect the existence of a He leak into the cold vacuum system of the LHC is then much longer than in a vacuum system at ambient temperature. A model for the propagation of a He pressure front in a cryogenically cooled tube is presented and compared with experimental results for a 75 m long cryogenically cooled tube.
In the model describing a He pressure front it is crucial to have knowledge of the He adsorption isotherm on the surface of the cold tube and a series of measurement of the adsorption isotherms of He on stainless steel at temperatures between 1.9 K, the working temperature of the superconducting magnets, and 4.2 K have been done. It is found that the He isotherms follow the Dubinin Radushkevich Kaganer (DRK) equation down to about 10-9 Torr and at lower temperatures the isotherms have an almost linear relation between pressure and surface coverage of He atoms on the cold stainless steel surface.
photon stimulated desorption
Monte Carlo simulations