New Spill Control for the Slow Extraction in the Multi-Cycling SPS

The flux of particles slow extracted with the 1/3 integer resonance from the Super Proton Synchrotron at CERN was previously controlled with a servo-spill feed-back system which acted on the horizontal tune such as to keep the spill rate as constant as possible during the whole extraction time. The current in two servo-quadrupoles was modulated as a function of the difference between the measured and the desired spill rate. With servo quadrupoles at a single location in the SPS ring and the SPS in multi-cycling mode, the trajectory of the slow extracted beam was seen to change from cycle to cycle depending on the current applied by the servo feed-back. Hence this system was replaced by a feedforward tune correction using the main SPS quadrupoles. In this way the spill control can now be guaranteed without changing the trajectory of the extracted beam. This paper presents the algorithm and implementation in the control system and summarizes the advantages of the new approach. The obtained spill characteristics will be discussed. The technique implemented for the additional reduction of the 50 Hz noise on the spill structure will also be briefly outlined.