COMPENSATION OF TIME-DEPENDENT PERSISTENT CURRENT EFFECTS IN SUPERCONDUCTING SYNCHROTRONS

Persistent currents in superconducting accelerator magnets are caused by the magnetization of the superconducting filaments in the field of the magnet itself. The magnetized filaments create additional field di stortions which can have an important effect on beam dynamics. During the initial operation of the Tevatron as a colliding beam accelerator, the chromaticities at the injection energy were found to be time depe ndent, leading to instabilities and particle loss during injection and at the start of acceleration. Laboratory measurements on single Tevat ron dipole magnets indicated that these effects were due to time-depen dent persistent current phenomena. Using additional laboratory measure ments and beam observations, we have developed a set of procedures to compensate the time-dependent chromaticities due to the persistent cur rents. The application of these procedures has eliminated all problems caused by time-dependent persistent current effects. We will discuss the general problem of persistent current distortions in superconducti ng accelerators, and, then, the laboratory measurements, the beam obse rvations, and the successful implementation of the correction schemes. While these procedures have worked well, they have limitations which will be discussed, as well as possible future improvements and implica tions for future projects.