RF Voltage Calibration Using Phase Space Tomography in the CERN SPS

Voltage calibration using longitudinal phase-space tomography is a purely beam-based technique to determine the effective RF voltage experienced by a bunch. It was applied in the SPS, separately to each of its six accelerating travelling wave structures. A low spread in voltage errors was obtained by carefully optimizing the number of acquired bunch profiles. The technique moreover provided the relative phases of the cavities, which allowed their alignment to be checked. Pairs of cavities were measured as well to validate the consistency of the single-cavity voltages. The beam measurements were repeated after several months to confirm the reproducibility of the results. Longitudinal beam dynamics simulations, including the full SPS impedance model, were performed as a benchmark. The aim was to verify that the effect of the cable transfer-function on the bunch profiles can be neglected, as well as collective effects and small errors in the accelerator parameters.