The LHC Beam Presence Flag System

Be­fore in­ject­ing any high in­ten­si­ty bunch­es into the LHC a circulating low in­ten­si­ty pilot bunch must be pre­sent to con­firm the cor­rect set­tings of the main ma­chine pa­ram­e­ters. For the 2010 LHC run the de­tec­tion of this pilot beam was done with the beam cur­rent transformer sys­tem. To in­crease re­dun­dan­cy of this im­por­tant safe­ty func­tion a ded­i­cat­ed beam pres­ence flag sys­tem was de­signed, built and test­ed with beam to be used op­er­a­tional­ly in the 2011 run. In this sys­tem sig­nals from four elec­trodes of a beam po­si­tion mon­i­tor (BPM) are pro­cessed with sep­a­rate chan­nels, re­sult­ing in a quadru­ple sys­tem re­dun­dan­cy for ei­ther beam. Each sys­tem chan­nel con­sists of an ana­logue front-end con­vert­ing the BPM sig­nals into two logic states, which are then trans­mit­ted op­ti­cal­ly to the ma­chine pro­tec­tion and inter­lock sys­tems. For safe­ty rea­sons the sys­tem does not have any remote con­trol or ad­justable el­e­ments and its only in­puts are the beam sig­nals. This paper de­scribes the new LHC beam pres­ence flag system, in par­tic­u­lar the ana­logue front-end based on diode peak detec­tors.