ON-LINE MECHANICAL INSTABILITIES MEASUREMENTS AND TUNER DEVELOPMENT IN SC LOW-BETA RESONATORS

Bulk niobium, 80 MHz low beta resonators with mechanical damper have been installed since many years in the Legnaro superconducting linac ALPI, and similar cavities have been recently installed and commissioned in the PIAVE injector. We have measured, both in ALPI and in PIAVE during operation, the distribution of the low beta cavities frequency deviation induced by mechanical vibrations. We have developed a new, backlash-free high resolution tuner to improve the control system response to detuning induced by helium pressure fluctuations.