  The RF system for the SNS superconducting linac consists of a superconducting
cavity, a klystron, and a low-level RF (LLRF) control system. For a proton
linac like SNS, the field in each individual cavity needs to be controlled to
meet the overall system requirements. The purpose of the LLRF control system is
to maintain the RF cavity field to a desired magnitude and phase by controlling
the klystron driver signal. The Lorentz force detuning causes the shift of the
resonant frequency during the normal operation in the order of a few hundreds
hertz. In order to compensate the Lorentz force detuning effects, the cavity is
pre-tuned into the middle of the expected frequency shift caused by the Lorentz
force detuning. Meanwhile, to reduce the overshoot in the transient response, a
feed-forward algorithm, a linear parameter varying gain scheduling (LPV-GS)
controller, is proposed to get away a repetitive noised caused by the pulsed
operation as well as the Lorentz force detuning effects.
