Automated path length and M₅₆ measurements at Jefferson Lab

Accurate measurement of path length and path length changes versus momentum (M₅₆) are critical for maintaining minimum beam energy spread in the CEBAF (Continuous Electron Beam Accelerator Facility) accelerator at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). The relative path length for each circuit of the beam (1256 m) must be equal within 1.5 degrees of 1497 MHz RF phase. A relative path length measurement is made by measuring the relative phases of RF signals from a cavity that is separately excited for each pass of a 4.2 μs pulsed beam. This method distinguishes the path length to less than 0.5 degrees of RF (<280 μm) and gives the direction of the path length error. The development of a VME based automated measurement system for path length and M₅₆ has contributed to faster machine setup time and has the potential for use as a feedback parameter for automated control.