Improved impedance reduction in the CERN SPS superconducting cavities for high intensity proton operation

Two four cell superconducting cavities are installed in one bimodule in the SPS accelerator to provide accelerating voltage for leptons. The four 3,5 GeV-20 GeV lepton cycles are interleaved with a 14 GeV-450 GeV fixed target high intensity proton cycle, the composite supercycle being 14.4 s long. The extremely high impedance of the superconducting cavities is unacceptable for the high intensity proton beam and to maintain beam stability the resonances in the main cavity passband must be heavily damped. This damping is produced by an RF feedback system. The limitations for this feedback system are due mainly to the proximity in frequency of the two upper resonances in the cavity and the loop delay. Typically a final, operationally reliable, impedance of ≈ 400 KΩ/cavity can be obtained. Measurements on the proton beams at high intensity have shown that with two cavities in the machine this impedance produces coupled bunch instabilities when ≈ 3x10¹³ protons/pulse are accelerated. As there are plans to increase the proton intensity beyond this value, and since the possibility also exists of installing a second bimodule for lepton acceleration, it has become very important to study possible methods of further reducing the total impedance. Two methods have been studied, built and tested in machine development periods, both of these acting as a supplement to the existing RF feedback system. The first consists of a feedforward loop injecting a beam current signal into the feedback loop and the second consists of an additional feedback loop around the first, acting only at harmonics of the revolution frequency