Operational experience of beam stability control

The CERN accelerator complex, as well as other accelerator facilities world-wide, produce a large variety of particle beams for use in experiments. The beam quality is of prime importance and can be characterised by many parameters, such as beam or bunch intensity, longitudinal and transverse beam size, time structure, etc. Certain combinations of these beam parameters, along with the machine characteristics such as impedance, can lead to challenging configurations that can drive the particle beams unstable, potentially leading to degradation of the beam quality and particle losses around the circumference of the accelerator. These losses result in activation of the accelerator components and therefore prolong the cooldown time to allow for hands-on preventive and corrective maintenance, hence reducing the beam time available for physics. Beam stability control is therefore of major importance for the operation of accelerators and can be obtained through distinct means for different accelerators and beam characteristics. This paper outlines the principal operational instability observations and mitigations applied for the CERN accelerator complex, complemented with approaches used in some other accelerator laboratories around the world. An attempt is made to illustrate the interplay between the beam dynamics experts and the operations teams.