Phasing schemes for the CEBAF cavities

The CEBAF accelerator requires accurate phasing of the 338 superconducting cavities to achieve the design rms energy spread of 2.5·10^-5. The rms phase error along each linac, with 160 cavities over a length of 200 m, may not exceed 2.6°, assuming that the whole linac is operated on crest. The common procedure is to maximize the energy gain with a spectrometer. At CEBAF, however, phase-dependent cavity steering effects cause deflections of the beam of several mrad, requiring steering corrections in the linac, which makes this method very time consuming. Beam-induced transients can also be used in pulsed operation to determine the zero-energy-gain phase with high accuracy. Better than 2° accuracy is achieved when the signal-to-noise ratio is improved by signal averaging. These and other approaches for the phasing of cavities are compared and accuracy and feasibility are discussed