The Tuning Algorithm of the LHC 400 MHz Superconducting Cavities

The LHC Superconducting 400 MHz cavities are of the single-cell type, equipped with a movable coupler allowing for a range of loaded Q from 20000 to 180000. Their mechanical tuner has a range of 250 kHz with a 25 Hz resolution. The LHC bunch spacing is 25 ns but the filling pattern exhibits various gaps free of particles and of different length. Consequently the RF beam current at 400 MHz is also modulated. The largest gap has a length of 3s to accommodate for the rise-time of the beam dump kicker. It will always be kept free of particles. With the strong RF feedback the decay time of the cavity field is between one and two s, much smaller than the revolution period (90 s). Hence two steady state situations are encountered during one turn: The klystron power toggles between beam and no-beam values as the RF feedback keeps the field constant. The tuner position is set to keep the klystron power equal in both cases (Half Detuning). This note presents the tuning algorithm that sets and keeps the tuner at this optimal position.