Calculations of safe collimator settings andβ*at the CERN Large Hadron Collider

Abstract

The first run of the Large Hadron Collider (LHC) at CERN was very successful and resulted in important physics discoveries. One way of increasing the luminosity in a collider, which gave a very significant contribution to the LHC performance in the first run and can be used even if the beam intensity cannot be increased, is to decrease the transverse beam size at the interaction points by reducing the optical function β Ã . However, when doing so, the beam becomes larger in the final focusing system, which could expose its aperture to beam losses. For the LHC, which is designed to store beams with a total energy of 362 MJ, this is critical, since the loss of even a small fraction of the beam could cause a magnet quench or even damage. Therefore, the machine aperture has to be protected by the collimation system. The settings of the collimators constrain the maximum beam size that can be tolerated and therefore impose a lower limit on β Ã . In this paper, we present calculations to determine safe collimator settings and the resulting limit on β Ã , based on available aperture and operational stability of the machine. Our model was used to determine the LHC configurations in 2011 and 2012 and it was found that β Ã could be decreased significantly compared to the conservative model used in 2010. The gain in luminosity resulting from the decreased margins between collimators was more than a factor 2, and a further contribution from the use of realistic aperture estimates based on measurements was almost as large. This has played an essential role in the rapid and successful accumulation of experimental data in the LHC.

Modeling of beam-induced damage of the LHC tertiary collimators

“…The first physics run at the LHC proved that the collimation hierarchy constrains the performance in terms of minimum achievable β-function at the collision points, β Ã , determined by the minimum normalized machine aperture that can be protected by the collimators [9]. In the present LHC, the aperture bottlenecks are the inner focusing quadrupoles (triplets) close to the experiments that risk being exposed to local beam losses if not sufficiently protected by the TCTs [9].…”

“…The first physics run at the LHC proved that the collimation hierarchy constrains the performance in terms of minimum achievable β-function at the collision points, β Ã , determined by the minimum normalized machine aperture that can be protected by the collimators [9]. In the present LHC, the aperture bottlenecks are the inner focusing quadrupoles (triplets) close to the experiments that risk being exposed to local beam losses if not sufficiently protected by the TCTs [9]. The small regular losses during standard operation at the triplets and TCTs do not pose concerns for the material robustness, but protection must be also guaranteed during fast failures, where high beam losses could occur in a short time with consequent damage of the tungsten collimators.…”

Measured and simulated heavy-ion beam loss patterns at the CERN Large Hadron Collider

“…Compared to the design settings, larger margins between the collimation stages were chosen at the beginning of LHC operation to avoid hierarchy violations due to machine imperfections [19]. With improving operational experience, the settings have been continuously tightened to provide better protection and thus allow for higher luminosities [17,20]. The nominal retractions have not yet shown a good enough longterm stability of the hierarchy against optics and orbit drifts to be used in standard operation.…”

“…Tertiary collimators (TCT) in each IR protect the triplet of quadrupoles around each experiment from the secondary beam halo and abnormal beam losses during failures [17]. They should also minimize experimental background [18].…”

Design, construction, and beam tests of a rotatable collimator prototype for high-intensity and high-energy hadron accelerators

“…Therefore, these rotatable collimators would be installed in these locations in the LHC. This scenario has been revised since then (see e.g., [6][7][8]) by more realistic models, but is still relevant for design specifications of IR7 collimators.…”