Construction and Processing of the Variable RF Power Couplers for the LHC Superconducting Cavities

The Large Hadron Collider (LHC) will accelerate beams of protons with a main RF system of sixteen 400 MHz superconducting cavities. The different requirements at injection and collision together with the heavy beam loading have imposed the use of a variable power coupler with a coupling value varying by a factor 20. The power requirements are high; 250 kW cw forward for several hours and pulses of 1.85 MW local peak power. In order to withstand these levels, copper sealing rings are used on the main ceramic. To avoid multipactor occurring at high powers and beam intensities, a second ceramic is used to provide a second DC polarization of the antenna. Twenty-five couplers have already been fully conditioned using a special dedicated FM+AM conditioning process, with 16 installed in the cavities in the LHC tunnel for first powering tests.