Luminosity leveling with angle

Abstract

The very high luminosity foreseen for the LHC luminosity upgrade entails in all cases a significant luminosity decrease during a few hours run. We present in this note a new method of luminosity leveling, based on the on-line adjustment of the crossing angle, while keeping the optics unchanged. It is implemented using the D0 dipole of a possible Early Separation Scheme and an orbit corrector. The whole bump is confined in the experimental drift space. It should be operationally simple as it avoids most complicated side effects that other leveling principles would produce. INTRODUCTION AND CONCEPT The LHC luminosity upgrade aims at increasing significantly the peak and average LHC luminosity [1]. In all scenarios, the decay of the luminosity due to the beam-beam interaction becomes dominant over other mechanisms and very significant as compared to the nominal LHC parameters. This is particularly true for the most efficient and economical scenarios where the luminosity increase is obtained by other means than a beam current increase. A large variation of the luminosity over a few hours run shows many drawbacks, both for the detectors and the machine components. From the machine point of view the main issues are the peak and average power deposition in the superconducting triplets and ancillary magnets. To prevent a quench, it has to be designed for the maximum instantaneous luminosity. The present knowledge shows that the capability of Nb-Ti appears significantly exceeded while the Nb3Sn technology could face it though with additional improvements of the shielding efficiency. For the experiment itself, the high initial peak luminosity produces a higher multiplicity and a stronger background. To cope with it, either the detector has to be designed for the peak multiplicity that is significantly above the design goals of the present detectors or a fraction of the running time will not be used efficiently for data taking. An answer to this challenge is luminosity leveling. It is traditionally proposed to adjust in real time the beam size at the crossing point to obtain this result. The authors ignore whether this was ever made operational in practice. While a modulation of the focusing is indeed a priori simple in principle, it shows a large potential of side effects that is bound to make it delicate in practice: when the focusing is modified, its chromatic correction has to be adjusted. As it is not locally corrected, all the lattice sextupoles have to be ramped, with unwanted feed-down effects on the betatron tunes and closed orbit all around the machine, includθcσz 2σ∗ G eo m et ri ca ll os s fa ct or F Nominal LHC working point