Investigation of the time evolution of the hump frequency observed at the LHC run 2010

Abstract

Since the LHC started operation, some unknown source is causing an interference in the beams, leading to a vertical oscillation of the order of the micrometer. It can be observed in the tune spectrum where a sort of unexpected broad-band structure so-called hump arises. It varies in frequency, and can cause emittance blow-up and luminosity loss when it approaches the beam tunes [1]. The transverse feedback has been used to mitigate this effect but the source of this perturbation has not been identified yet [2]. One of the observed characteristics of the hump is that the time evolution of the hump frequency, normally slowly varying with time, sometimes shows abrupt changes. These sudden variations of the hump frequency temporal evolution have been investigated in order to identify possible correlations with external factors or actions on any system of the machine. The data adquired with the tune measurement system (BBQ) have been systematically analysed for a long period of the LHC operation (from March to September 2010), and the results are presented in this note