Design and operational results of a 'one-turn-delay feedback' for beam loading compensation of the CERN PS ferrite cavities

A project was launched to lower the transient-beam-induced voltage by a factor of four, with a one-turn-delay feedback system reducing the equivalent cavity impedance on the first three revolution frequency sidebands around the cavity tune frequency. The design is able to cope with a wide frequency range due to particle acceleration (15% velocity variation) and choice of harmonic number (h=10 to 20). Loop gain is above 0 dB in the vicinity of revolution frequency harmonics over an instantaneous bandwidth of three to four times the 3-dB bandwidth of the original RF system. Fast digital electronics are applied extensively, resulting in a reliable and compact implementation. The various functions are described, closed-loop performance of a cavity is shown, and measurements with beam are presented.<
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