LHC MAGNET QUENCH PROTECTION SYSTEM

The quench protection system for the superconducting magnets of the CE RN Large Hadron Collider (LHC) is described. The system is based on th e so called ''cold diode'' concept. In a group of series connected mag nets if one magnet quenches then the magnetic energy of all the magnet s will be dissipated in the quenched magnet so destroying it. This is avoided by by-passing the quenched magnet and then rapidly de-exciting the unquenched magnets. For the LHC machine it is foreseen to use sil icon diodes situated inside the cryostat as by-pass elements - so call ed ''cold diodes''. The diodes are exposed to some 50 kGray of radiati on during a 10 year operation life-time. The high energy density of th e LHC magnets (500 kJ/m) coupled with the relatively slow propagation speed of a ''natural'' quench (10 to 20 m/s) can lead to excessive hea ting of the zone where the quench started and to high internal voltage s. It is therefore necessary to detect quickly the incipient quench an d fire strip heaters which spread the quench out more quickly over a l arge volume of the magnet. After a quench the magnet chain must be de- excited rapidly to avoid spreading the quench to other magnets and ove r-heating the by-pass diode. This is done by switching high-power ener gy-dump resistors in series with the magnets. The LHC main ring magnet will be divided into 16 electrically separated units which has import ant advantages.