MD 1856 - Landau Damping: Beam Transfer Functions and diffusion mechanisms

In the 2012, 2015 and 2016 several instabilities were developing during the betatron squeeze where beam-beam interactions become stronger modifying the tune spread provided by the octupoles magnets. Studies of the stability area computed by evaluating the dispersion integral for different tune spread couldn’t explain the 2012 observed instabilities during the squeeze. The size of the stability area given by the computed dispersion integral depends on the transverse tune spread but its shape is defined by the particle distribution in the beams. Therefore any change of the particle distribution due to for instance a diffusion from excited resonances can lead to a deterioration of the Landau stability area. The Beam Transfer Functions (BTF) measurements are direct measurement of the Stability Diagrams (SD). They are sensitive to the particle distribution and contain information about the transverse tune spread in the beams. In this MD we wanted to verify the findings of MD 1407 and try to explain observed instabilities. A scan of the beam-beam separation and crossing angles have been performed to quantify the tune shift measured during the MD 1429.