The variable power coupler for the LHC superconducting cavity

Variable input couplers, providing a remotely controlled change of external Q by an order of magnitude under power, are required for the 400 MHz LHC superconducting cavities. These couplers must handle a forward power of 120 kW average and 180 kW pulsed with a large variety of load conditions up to full reflection. A summary of the LHC prototype coupler design (using d.c. bias on the main coupler transmission line to suppress multipactor) and of the RF power tests on a normal conducting test cavity will be given. The same couplers now have been RF power tested on a prototype superconducting LHC bi-module. During both of these RF tests, multipactor events have also been observed in the variable coupler part - outside the main coupler line - which cannot be suppressed by the actual d.c. bias. An improved design with a second d.c. bias will therefore be implemented. Nevertheless, after the usual RF conditioning, these prototype couplers have successfully passed all RF tests at power levels well above the LHC requirements.