Continuous Tune Measurements Using the Schottky Detector

Abstract

A continuous measurement of the betatron tune has been implemented in the SPS pp/sup -/ collider with hardware normally used for the detection of Schottky noise. Suitable values for the tunes and chromaticities are thus more easily achieved in a machine where the strong space charge effects of the two counterrotating beams severely reduce the available space in the resonance diagram. The basic idea is to monitor the frequency of one of the betatron lines together with the revolution frequency and to deduce the tune according to a formula provided in this paper. The betatron lines which naturally appear as noise signals adding incoherently are strongly enhanced by exciting the beam at the required frequency with a kicker. The whole system then forms a closed loop in which an oscillator excites the beam producing a coherent signal at the Schottky detector, this signal providing the reference to lock the oscillator frequency. The excitation, 50 W at 10.7 MHz causes particle loss (about 3%/cycle) and emittance blow-up (about 10%/cycle) restricting the use of the apparatus to setting-up periods.