Performance of the LEP2 SRF system

In 1996, LEP2 operated at an energy of up to 86 GeV per beam using, in addition to the conventional RF system, 176 newly installed superconducting (SC) cavities, which together provide up to 2 GV per turn at 352 MHz. Almost all SC cavities are of the niobium film on copper type; they ran at an average operating gradient of 6 MV/m with a total beam current of up to 5.5 mA. The behaviour of SC cavities and couplers has been very satisfactory: only two cavities out of 176 were field limited; they have however been recovered in situ since. Small modifications in the liquid helium distribution reduced to a large extent turbulent phenomena and the associated microphonic effects. However the intrinsic electroacoustic instabilities (ponderomotive oscillations) remain the major concern for the operation of the RF system at high intensity. Their effect on the beam is minimized using RF feedback (on the vector sum of individual cavity signals); this also suppresses the intensity limitations due to beam loading instabilities. Installation, commissioning and operation of a huge new RF power plant (24 MW installed RF power) went as expected, despite a few weak points which are now being fixed