Measurements, Alarms and Interlocks in the Vacuum Control System of the LHC

In the LHC beam pipes and cryostats, the pressure measurement covers a wide range, from 1500 mbar down to 10⁻¹¹ mbar and even lower. If vacuum deteriorates, alarm signals are generated and sent to other systems, e.g. cryogenics, accelerating cavities, kicker magnets, and beam interlock. In addition, an unacceptable pressure rise in beam pipes generates interlocks to close the adjacent sector valves, thus isolating the sector, so that the pressure rise does not propagate. This paper describes the measurement chains, the alarms and interlocks logic used in the vacuum control system of the LHC. We analyze the possible signal degradation caused by ionizing radiation or due to cable length, shielding and grounding. The weaknesses of the existing vacuum measurement system are pointed out, and a prospective for improvement of the conditioning electronics is proposed. During the first LHC long shut down, several corrections were applied; the results of the tests after commissioning are also presented.