In the ISR high density proton beams are stabilized against transverse instabilities by a feedback system1l which in the frequency range from 10kHz to

The transverse coupling impedance has been measured in the ISR in the vertical plane and found to be mostly inductive with a phase angle of about 50° for the lowest mode. The resistive part of this impedance is compensated below 1 MHz by a trans­ verse feedback system. However, by applying to the kicker the signals from two pick-ups giving respectively a resistive and an inductive compensation, the phase angle of the feedback can be varied. The signal levels from the two pick-ups are indivi­ dually adjusted with the correct settings obtained from the information given by the beam transfer function. In this way the stability margin is increased without changing the electro­ nic gain of the feedback. A special set-up has been developed for the measurement of beam transfer functions at low frequen­ cies (10 kHz to 1 MHz).