Capital "S" when referring to CERN's first particle accelerator. Otherwise, lower case.
A synchrocyclotron is a type of cyclotron in which the frequency of the driving radiofrequency-electric-field is varied to compensate for relativistic effects as particle velocity approaches the speed of light. This is in contrast to the classical cyclotron, where this frequency is constant.
CERN's first accelerator, the Synchrocyclotron, first started up on 11 May 1957. It was one of the few synchrocyclotrons in the world at the time.