Compensation of time-dependent persistent current effects in superconducting synchrotrons.

Abstract

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 distortions 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 dependent, leading to instabilities and particle loss during injection and at the start of acceleration. Laboratory measurements on single Tevatron dipole magnets indicated that these effects were due to time-dependent persistent current phenomena. Using additional laboratory measurements and beam observations, we have developed a set of procedures to compensate the time-dependent chromaticities due to the persistent currents. 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 superconducting accelerators, and, then, the laboratory measurements, the beam observations, 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 implications for future projects.