Performance of the arc detectors of LHC high power RF system

During operation, the LHC high power RF equipment, such as klystrons, circulators, waveguides and couplers have to be protected from damage caused by electromagnetic discharges. Once ignited these arcs grow over the full height of the waveguide and travel towards the RF source. The burning plasma can cause serious damage to the metal surfaces or ferrite materials. The LHC arc detector system is based on the optical detection of the discharge through small apertures in the waveguide walls. The light is guided by means of an opti-cal fibre from the viewing port to a photo diode. Experience shows that some of the currently used optical fibres suffer from x-ray induced opacity. The sensors are also exposed to the radiation produced by secondary showers coming from the high intensity beams which, if not treated properly, can cause frequent spurious trips. In the second half of the paper we present a number of improvements to the design, measurements with optical parameters from real arcs and a fibre-less version of the detector with redundant detectors for critical environments.