Titolo della tesi: PS Impedance Model and Related Beam Stability
In the coming years, CERN plans on entering in the High-Luminosity Large Hadron Collider (HL-
LHC) era. The High-Luminosity LHC is characterized by an improved instantaneous luminosity by
a factor of 5 with respect to the LHC design, thus benefiting to the study of rare events occurring
during particle collisions. This ambitious goal is now within reach, in particular thanks to the LHC
Injectors Upgrade (LIU) project. During LIU, the LHC injectors underwent configuration and
hardware changes to deliver a twofold increase in beam intensity and a reduction of the transverse
emittance compared to the nominal LHC parameters. Yet, these goals rely on the transverse stability
of the beam along the injector chain, which typically results in beam intensity losses and transverse
emittance blow-up, and thus degrade beam quality and machine performance. In this thesis, we
study the impact of the LIU project on the CERN Proton Synchrotron (PS) transverse beam
stability. First, a comprehensive list of the changes introduced during the Long Shutdown 2 (LS2)
is compiled, with a particular emphasis on assessing their impact in terms of transverse impedance.
Then, a post-LS2 impedance model is built on the foundations of its pre-LS2 predecessor. In
addition, the updated model main impedance contributors, the vacuum chamber and kicker magnets,
are refined to depict the machine more realistically. On one hand, the vacuum chamber impedance
now accounts for the elliptical geometry of the PS aperture as well as the beam energy. On the other
hand, the kicker magnets’ impedance was simulated with greater precision. Besides, the impact of
their connecting cables and external circuits was calculated analytically.
Then, the emphasis is shifted toward a beam-based validation of the post-LS2 impedance model.
Two measurement campaigns of impedance-inducted beam observables, namely impedance-induced
tune shift with intensity, and instability growth rate, were conducted. Moreover, novel measurement
methods were introduced in the second campaign, during which their performances have been
compared against standard methods, and demonstrated an improved accuracy. The measurements
serve as references for the subsequent benchmarking of the post-LS2 transverse impedance model.
Finally, the reference measurements are reproduced through macroparticle tracking simulations
relying on the updated PS transverse impedance model. A thorough examination of the mechanisms
influencing the impedance-induced observables is carried out. The most relevant mechanisms, which
were missing in previous studies, are the quadrupolar impedance, second-order chromaticity, and
effect of direct space charge. Then, the validity of the post-LS2 impedance model is discussed,
considering past and present impedance-induced observables measurement campaigns.