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A novel system for real-time tune measurement during deceleration of a
low-intensity particle beam is presented. The CERN Antiproton Decelerator
decelerates low intensity (2×107) antiproton beams from 3.5 GeV/c
to 100 MeV/c. Because of the eddy-currents in the magnets, a
tune-measurement during a pause in the deceleration would not be
representative. One must thus be able to measure the tune in real time
during the deceleration. The low intensity of the antiproton beam
prevents the use of standard Schottky techniques, and swept Beam Transfer
Function (BTF) measurements are too slow. A system was therefore
developed which uses an M-shaped power spectrum, exciting the beam in a
band around the expected frequency of a betatron side-band. Excitation at
the betatron frequency, where beam response is highest, is thus minimized
and measurements of BTF, and therefore the tune, can be made with much
reduced emittance blowup.
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