FCC-ee Radiation Challenges and Mitigation Measures

Abstract

A lepton collider, like the Future Circular Collider lepton machine (FCC-ee), faces various radiation challenges in different parts of the accelerator complex. Radiation load studies are valuable in finding potential showstoppers and mitigating issues, aiming to implement feasible solutions. This thesis studies two parts of the FCC-ee accelerator complex for different operation modes. First, the radiation load for a possible option of the positron production source is studied for the $Z$-operation mode (45.6 GeV). This solution features a superconducting solenoid made of high-temperature superconductor (HTS) coils as an adiabatic matching device, which captures positrons produced in the target before they enter the capture linac. Compared to normal-conducting coils, superconducting coils produce a solenoid field with a higher magnetic field strength, improving the positron yield. In this setup, novel HTS coils are implemented that can resist higher radiation loads than common superconducting materials. Nevertheless, these coils still are prone to a high radiation load, which requires a detailed analysis of the power load on the coils and their surroundings. The considered target is made out of amorphous tungsten, which is optimal for positron production due to its high atomic number. Bremsstrahlung and pair production generate a significant electromagnetic shower in the target, implying a high radiation load in the target itself, the superconducting coils, and the capture linac downstream. From this electromagnetic shower, the positrons are then accelerated and at a later stage extracted. The capture linac downstream is embedded in normal conducting solenoids that focus the positrons in the forward direction. Energy deposition calculations with FLUKA are carried out to assess the feasibility of such a setup, studying the heat load and long-term radiation damage on the structure. The second part of the thesis studies the synchrotron radiation (SR) emitted by electrons and positron beams in the FCC-ee arcs for the ttbar-operation mode at 182.5 GeV. At this beam energy, SR includes a vast amount of photons with energies higher than 1.25 MeV ($E_c$) that heavily impact the entire machine. Dedicated photon absorbers are envisaged as a mitigation strategy to reduce the radiation load on the tunnel. Different materials for the absorbers are investigated and compared to a layout resembling the LEP SR mitigation strategy, which comprises a continuous shielding along the arcs. The study assesses the heat load, time-integrated dose, and particle fluence distribution in the magnets and the surrounding tunnel environment. Furthermore, shielding options for the electronics in the tunnel and considerations for the booster placement are presented. This part is complemented with beam-gas bremsstrahlung studies for the $Z$-operation mode, where the beam current is significantly higher and hence the impact of interactions with residual gas atoms in the vacuum chamber is more relevant. The various topics in this thesis show that radiation is significantly impacting the machine, but to an extent where mitigation measures can be implemented efficiently. It is demonstrated that the radiation load on the HTS coils of the positron production target coils is most likely sustainable, which is also true for the target, considering an elaborate cooling scheme. The radiation levels in the collider tunnel are challenging, even if photon absorbers are used. Considering further mitigation strategies, like additional localized shielding of critical components or optimised positioning of components in the tunnel, can effectively reduce the risk of failure