Improvement of the CERN SPS electrostatic septa ion traps

Abstract

At CERN, the SPS synchrotron is equipped with a slow extraction channel towards the fixed target beam lines in the North Area This channel includes five consecutive electrostatic septa, where the field free region and the active high field region are separated by an array of tungsten-rhenium wires. The field-free region provides for the circulating beam, while the high field region is used to deflect the extracted beam. Since the residual gas can be ionized by the orbiting beam, low energy ions could cross the wire array and enter the high field region and cause high voltage breakdown when accelerated onto the cathode. To prevent low energy ions from entering this high electric field region, a vertical field is applied to the orbiting beam using so-called ‘ion traps’ for active protection. The vertical field is created by electrodes placed inside the region containing the circulating beam. Due to electromagnetic coupling onto the ion trap electrodes observed with the high frequency LHC beam (25 ns spaced bunches), the efficiency of the ion traps is greatly reduced. This leads to increased vacuum activity (electron cloud related) as well as high spark rates both in the main field and between the ion trap electrodes and their grounded support. In view of the SPS performance increase required for HL-LHC, this paper highlights the upgrades and improvements required to obtain a stable ion trap field and significantly reduce the number of breakdown events observed with the LHC beam in the accelerator.