The Closed Orbit Observation System of the CERN Antiproton Accumulator

The Antiproton Accumulator (AA) works at 3.5 GeV/c central orbit momentum, has 5O¿ m circumference, Q-values around 2 ¿ and a large dispersion, up to 10 m, over most of its circumference (see ref. 1 for description). Closed orbits have to be measured over the whole range of the very large momentum acceptance (6.5%) and at beam intensities as low as 5 × 109 particles (1.5 mA). Furthermore, the pick-up electrodes have to serve as clearing field electrodes and no straight section space was available for them. They have to be compatible with an UHV of 10-11 Torr and bakeable to 300°C. The AA closed orbit observation system therefore has several unusual features: a wide range of dimensions of the pick-up electrodes, adapted to beam size, up to 70 cm horizontal width; location inside the main magnets and quadrupoles; high input impedance head amplifiers, mounted directly on the vacuum chamber feedthroughs; electronic normalizers to provide the ratio of difference/sum signals; high precision for measuring orbits far from centre. Acquisition, calibration, data treatment and display are performed with the single, stand-alone, controls computer.