The CERN SPS Low Beta Insertion Control System

Within the context of the CERN SPS ppbar colliding beam experiments there are 2 × 15 power supplies which have to provide the currents necessary to produce the two low beta insertions. Their control system is described for both the hardware and the software parts. The references use a combination of an 18 bit DAC and an integrator to produce a relatively smooth drive signal to the current controlled power supplies. These 18 bit devices must operate in a hostile environment (electrical noises, temperature changes). Results are given on their performances, especially on the accuracy achieved at the required settings (better than 10-4 at intermediate levels) and, more important, the repeatability (of the order of 10-5). Each set of 15 references is controlled from a dedicated computer. Each 60 msec point of each reference can be defined for a cycle length of up to 21.6 sec. The software driver is laid out such that during the pulsing mode (setting up) one can insert mini-coasts at any desired level or, during the coast proper at 270 GeV, one can adjust simultaneously the level of 1, some or all 30 references. The system also allows the setting up of a high beta in one insertion and a low beta in the other. It has transposed the theoretically calculated values so well that all the desired beta characteristics were achieved without having to add corrections due to inaccuracies in the reference control system.