Impedance-Induced Beam Instabilities

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Physics. Please check back later for the full article. Modern particle accelerators require ever higher currents to meet user demands, both for high energy physics experiments and for other applications, as with, for example, FLASH therapy, an innovation in radiation therapy where short pulses of electrons at very high dose rates are required. These high currents interacting with the accelerators’ environment produce strong self-induced electromagnetic fields that perturb the external fields used to guide and accelerate the charged particles. Under certain conditions, these perturbations can be so large as to limit the accelerators’ performances giving rise to unwanted effects, such as uncontrolled beam oscillations or even instabilities. The beam self-induced fields are described in terms of the so-called wakefields and coupling impedances, two quantities that are used to evaluate their impact on beam dynamics and on instabilities’ thresholds. It is therefore very important, in particular for high currents, to determine both wakefields and coupling impedances generated by the interaction of the beam with the different machine devices and of the corresponding induced instabilities. This is carried out with analytical approaches using simplified models, or in a more rigorous and realistic way, through simulation codes. A first step in this type of study is generally represented by a complete electromagnetic characterization of the different accelerator devices and by the search for possible minimization of wakefields and coupling impedances. Once these quantities are known, their effect on beam dynamics must be evaluated, and a proper machine working point, far away from any impedance-induced beam instabilities, needs to be determined. Nowadays, since the machine performances are pushed higher and higher, new effects produced by wakefields and coupling impedances are found, and are related, in many cases, to the interference between different mechanisms that can no longer be studied separately. Finally, mitigation solutions, such as feedback systems, use of nonlinearities, and other techniques must also be investigated to have different tools able to counteract possible unwanted beam-induced instabilities.