COMMISSIONING OF THE 400 MHz LHC RF SYSTEM

The installation of the 400 MHz superconducting RF system in LHC is finished and commissioning is under way. The final RF system comprises four cryo-modules each with four cavities in the LHC tunnel straight section round IP4. Also underground in an adjacent cavern shielded from the main tunnel are the sixteen 300 kW klystron RF power sources with their high voltage bunkers, two Faraday cages containing RF feedback and beam control electronics, and racks containing all the slow controls. The system and the experience gained during commissioning will be described. In particular, results from conditioning the cavities and their movable main power couplers and the setting up of the low level RF feedbacks will be presented. OVERVIEW OF THE 400 MHZ RF SYSTEM The system consists of eight single-cell SC cavities per beam. Four cavities are housed in a single cryo-module, operating at 4.5 K. Each cavity has a high-power variable RF coupler to optimize the requirements from injection to top energy. Each cavity supplies 1 MV at injection and 2 MV at top energy. The four cryo-modules, two per beam, were extensively tested and RF conditioned to 1.5 times their nominal operating voltage and the couplers to full power prior to installation in LHC. [1, 2] Figure 1: RF Cryo-modules in the LHC tunnel at IP4. Each cavity is powered by its own 300 kW klystron. The klystrons are housed in the nearby UX45 cavern and each is connected by waveguides via a circulator and load to its cavity coupler. Four klystrons share a high-voltage power supply installed on the surface. The klystrons have a modulating anode to set optimum DC current. Tetrodes driving the modulators, fast protection (crowbar) and other high-voltage equipment are located in fireproof concrete bunkers near the klystrons, one per group of four klystrons. Figure 2: Klystrons, Power Equipment and Control Racks in the UX45 Cavern. The control system is based on commercial PLCs, but with dedicated hardware for functions such as fast RF interlocks and RF power measurement. The control system interface is provided by CERN’s “Front-End Software Environment” (FESA) and applications for operation and expert use are written in JAVA or LabView. Figure 3: Layout of Klystrons, HV Equipment, Cavities and Power Converter. Sophisticated low level RF and beam control systems are needed in a high intensity proton storage ring like LHC. Each cavity has two VME based Cavity Controllers, containing cavity tuner loop, RF feedback loop, klystron feedbacks to minimise noise and ripple, as well as ramp function generator and timing interfaces [3]. These systems are housed in Faraday Cages in UX45, close to the klystrons. Additional beam control loops and fast timing and synchronization systems are housed in a Proceedings of EPAC08, Genoa, Italy MOPP124 07 Accelerator Technology Main Systems T07 Superconducting RF