The CERN ISR Control Scheme for Acceleration by Phase Displacement

In the CERN ISR high-intensity coasting beams are accelerated routinely from 26.6 to 31.4 GeV/c using the technique of phase displacement. The constraints imposed by this technique dictate a rather unusual control scheme. The total acceleration is performed in around two hundred steps; each incremental step in momentum is produced by sweeping empty RF buckets through the beam. The RF parameters are calculated and set by the control computer for each sweep. During each sweep the computer controls all magnetic elements synchronously so as to compensate for normal variations in the working line, the closed orbits and the beam position. This control is based on linear interpolation on the values held in a number of intermediate files between 26.6 and 31.4 GeV/c. The differences in the power supply settings contained in these files produce the magnetic changes required to maintain the correct space charge tune shift and to compensate for magnetic saturation of the poles of the main bending magnets. The intermediate files are generated from the results of measurements made during the acceleration of a bunched beam using a different mode of the acceleration program.