Mitigation of persistent current effects in the RHIC superconducting magnets

Persistent currents in superconducting magnet introduce magnetic errors especially at low operating fields. In addition, their decay causes magnetic field variations and therefore drifts of the beam orbits, tunes, and chromaticities. To reduce field errors and suppress magnetic field variations, a new magnetic cycle was proposed for the low-energy beam operation at the Relativistic Heavy Ion Collider (RHIC). In the new magnetic cycle, the magnet current oscillates around the nominal operating current with diminishing amplitude a few times before it settles. The new magnetic cycle has been demonstrated experimentally to reduce field errors and the amplitude of magnetic field variations significantly and is essential for the ongoing RHIC Beam Energy Scan II (BES-II) program. This article will present beam-based experimental studies of the persistent current effects with the new magnetic cycle, and discuss its application in RHIC.