Investigations and technology developments for a final cooling scheme in muon colliders

This thesis investigates the beam dynamics of the final cooling stage in a multi-TeV muon collider, where ionization cooling reduces the normalized transverse emittance to its target value. Achieving this enables the collider to reach peak luminosities necessary for precision studies of the Higgs boson, leptonic parton distribution functions, and other fundamental phenomena. As a transformative tool for high-energy physics, the muon collider drives advancements in accelerator technologies.

Ionization cooling is a fast-acting technique that reduces muon beam emittance within the muons’ short lifetime, primarily through interactions with material combined with high-field solenoids. This work incorporates a semi-Gaussian scattering model, parameterized with the Bethe-Wentzel model, into the RF-Track code. The implementation is compared to established tracking software, which confirms the suitability of RF-Track to simulate ionization cooling and allows future studies of collective beam effects.

The Bethe-Wentzel model is further useful for the analytic evolution of the transverse emittance reduction, showing unprecedented agreement with the simulation. This innovation allows optimizing initial beam parameters in final cooling cells without relying on time-intensive macro-particle simulations. Furthermore, an analytical calculation for energy deposition estimates pressure increases in hydrogen absorbers, revealing that earlier assumptions underestimated the pressure increase within hydrogen. A new design with density-adjusted liquid and vapor hydrogen is proposed to mitigate excessive pressure
buildup.

A critical contribution of this thesis is the development of an adiabatic ramping method to match the beam to a 40 T solenoid. This ensures smooth beam transport from low-field to high-field solenoids incorporated with hydrogen. A refined RF-solenoid beamline layout is introduced, improving compactness and realism over previous designs. The proposed final cooling lattice comprises nine uniquely configured cells, each with adjusted hydrogen properties and beam windows, offering a more practical and effective solution of the final cooling in a muon collider.