Tune Measurement and Control at the CERN-SPS

Two complementary techniques have been developed which allow the betatron tunes to be controlled during energy ramping of the SPS to a precision of better than ¿0.002. By the use of these techniques the setting-up of the SPS as a collider and a fixed target machine is substantially simplified and better physics understanding has been gained. The first technique uses an electrostatic beam deflection every 60 ms. A Fast-Fourier Transformation of the beam response in a dedicated computer yields the betatron tunes with a precision better than 0.01. In the second technique the beam is continuously excited. The frequency of the excitation is measured and fed back to the beam by a Phase-Locked-Loop. This measurement is accurate to better than 0.001 but requires a reasonably well tuned machine. Tune deviations are automatically compensated by acting on the main quadrupoles through a software loop.