Beam Transfer Function measurements and transverse beam stability studies for the Large Hadron Collider and its High Luminosity upgrade

The motion of high energetic particle beams in accelerators is influenced by their interactions with the accelerator environment through electromagnetic fields induced by the particle passages. Traveling with a speed close to the speed of light, the particles induce image charge and currents in the surroundings generating wake-fields that act back on the beams. Destabilizing effects may arise from the coupled motion between the circulating particles and the induced wake fields compromising the accelerator performances. The stability of the beams is ensured by the Landau damping of coherent motions generated by the diversity of oscillation frequencies of the particles in the beams. Under the effect of non linear forces produced by machine non linearities or beam-beam interactions the particles oscillate with slightly different frequencies depending on their amplitudes in the beams (tune spread). At the Large Hadron Collider (LHC) transverse instability thresholds are evaluated via the computation of the dispersion integral that depends on the tune spread in the beams as well as on the particle distribution. A large tune spread is beneficial for the Landau damping as long as no diffusive mechanism is present. In the presence of diffusive mechanisms, caused by resonance excitations or noise, the stability diagram can be deformed due to the modification of the particle distribution inside the beam leading to a possible lack of Landau damping of the impedance coherent modes previously damped by lying within the unperturbed stability area. This work aims to experimentally explore the transverse stability of the beams by means of Beam Transfer Function measurements at the LHC. They provide direct measurements of the stability diagrams and are sensitive to particle distribution changes. First measurements of the Landau stability diagrams at the LHC are presented and compared to model expectations. Experimental studies have been carried out in the presence of different sources of non linear effects such as octupole magnets and beam-beam interactions and compared to the model expectations. Limitations deriving from transverse coherent instabilities in the LHC are analysed and possible explanations for the observed LHC instabilities are discussed. In the perspective of the High-Luminosity LHC upgrade (HL-LHC), transverse stability studies for different beam-beam interactions and machine configurations are presented together with possible solutions to compensate reductions of the Landau damping during the operational phases of the HL-LHC.

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