A Magnetic Beam Position Detector

In storage rings, the monitoring of the coasting beam positions at the intersections is of prime importance for luminosity optimization and control of beam-beam space charge effects. The magnetic beam position detector, developed for the CERN Intersecting Storage Rings (ISR), is mounted around the standard vacuum chamber near the intersection point. Coils above and below the beam are used to detect the beam's magnetic field. To avoid the influence of external fields, the beam is wobbled vertically at 80 Hz and only field components at this frequency are measured with synchronous detectors. A feedback system maintains a null signal by centring the probe on the beam so that the beam position is known directly from displacement transducers mounted on the probe. The precision is better than 0.05 mmvertically and 0.25 mm horizontally with a 5 A beam modulated by ± 0.1 mm. Special care has been taken to minimize the parasitic currents in the vacuum chamber which limit the precision. The beam position modulation, which is confined to the intersecting region, is obtained by two a.c. vertical bending magnets per ring and it is synchronous and in phase for the two rings. This scheme was found to have a negligible effect on the beams.