The Model and Simulations of the LHC 400 MHz Cavity Controller

The LHC RF consists of eight superconducting single-cell 400.8 MHz cavities per beam, with each cavity driven by a 300 kW klystron via a circulator and waveguide. As the LHC is a high current collider (nominal 0.6 A DC), the challenge is to keep the effective impedance of the cavity at a minimum and to reduce all sources of RF noise that may degrade the beam lifetime (target value in excess of ten hours). In order to achieve this each klystron-cavity chain has a Cavity Controller implementing three Low Level loops: RF feedback, Tuner loop and Klystron loop. This note presents simulation models for the non-linear klystron, the cavity, the RF feedback, the tuner loop and the klystron loop. A model for the tuner mechanics based on measured step responses is also derived. The gains and time constants are optimized for the various loops. For the RF feedback and the tuner loop, the simulations are in excellent agreement with measurements done on a full-size test set-up including klystron and cavity. For the klystron polar loop, the predictions fit with the design values (not tested yet on a real klystron). To evaluate the effect of transient beam loading simulations are done with a beam containing gaps. Again the predictions fit well with design values.