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, like cryogenics, accelerating cavities, kicker magnets. 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 pressure interlock is simultaneously forwarded to the beam interlock system. This paper describes the instrumentation, the interlocks and alarms 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. During the first LHC long shut down, several corrections were applied to mitigate radiation effects and improve signal integrity. The tests performed for the vacuum control commissioning and LHC machine checkout are also presented. (author)