THE LHC LOW LEVEL RF

The LHC RF consists of eight 400 MHz superconducting cavities per ring, with each cavity independently powered by a 300 kW klystron, via a circulator. The challenge for the Low Level is to cope with very high beam current (more than 1 A RF component) and achieve excellent beam lifetime (emittance growth time in excess of 25 hours). Each cavity has an associated Cavity Controller rack consisting of two VME crates which implement high gain RF Feedback, a Tuner Loop with a new algorithm, a Klystron Ripple Loop and a Conditioning system. In addition each ring has a Beam Control system (four VME crates) which includes a Frequency Program, Phase Loop, Radial Loop and Synchronization Loop. A Longitudinal Damper (dipole and quadrupole mode) acting via the 400 MHz cavities is included to reduce emittance blow-up due to filamentation from phase and energy errors at injection. Finally an RF Synchronization system implements the bunch into bucket transfer from the SPS into each LHC ring. When fully installed in 2007, the whole system will count over three hundred home-designed VME cards of twenty-three different models installed in forty-five VME crates. The paper presents the various loops: it outlines the expected performances, summarizes the algorithms and the implementation. Thanks to a full scale test set-up including klystron and cavity we have measured the response of the RF Feedback and Tuner Loop; and these will be presented and compared to the expectations. LAYOUT The sixteen cavities (eight cavities per ring) are installed in the Intersection Point 4 (IP4) of the LHC. A huge cavern, 150 m below ground level, houses the sixteen klystrons (one per cavity) and the Cavity Controller part of the Low Level RF (LLRF), responsible for the RF feedback and Tuner loop around each individual klystron-cavity pair. On the surface, the Beam Controllers (one per ring) are responsible for the generation of the Master 400 MHz sent to the Cavity Controllers via fiber links. They implement the classic Low Level loops using three reference signals per ring