Pre-LS2 correction of PSB main bending magnet hysteresis effects on the extraction trajectories

The CERN Proton Synchrotron PS Booster is an accelerator composed by 4 superposed rings that have common bending magnets. These magnets have an asymmetry in the magnetic flux distribution between the inner and outer rings. A dedicated power supply, referred to as TRIM1+4, has been designed to provide extra current on the main windings and equalise the flux distribution. During the operation of the PSB it was empirically observed that after a ZERO cycle (without beam and with flat magnetic cycle), the extraction trajectory of the outer rings would differ from the nominal ones, measured after any standard operational cycle at 1.4 GeV. The observed change in trajectory would lead to sub-optimal steering to the downstream facility ISOLDE and the PS accelerator and, in turn, could cause beam losses or deterioration of the beam quality. In order to mitigate this problem, BE-OP requested a multi-ppm control of the voltage of the TRIM1+4 power supply. A dedicated measurement campaign was launched to validate the solution and find an optimal increase of the voltage for the TRIM1+4 for every cycle after a ZERO cycle, which would empirically match the nominal reference extraction trajectory.