FCC-ee Radiation Challenges and Mitigation Measures

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

Energy Deposition Studies for the Experimental Insertions of FCC-hh

In order to explore new possibilities for particle physics for the post-LHC era, the Future Circular Collider (FCC) study was launched in 2014 to assess its feasibility. Different machines are considered, including a hadron collider machine, FCC-hh. Similar to LHC, this would be a circular collider with a significantly larger circumference of 100km and colliding protons at a centre of mass energy of 100TeV. Proton collisions at such high energies lead to a large amount of collision debris in the experimental insertion regions (EIR) of the accelerator. The collision debris impacts the elements of the accelerator causing long term damages, like deterioration of organic materials and superconductors, as well as instantaneous effects, like quenching of the superconducting magnets. This thesis studies the impact of the collision debris in the high-luminosity EIR of FCC-hh. For this purpose Monte Carlo simulations have been performed with the FLUKA code. Quantities like linear power distribution, peak power density, integrated dose and displacements per atom are studied for the elements of the EIR for both, horizontal and vertical crossing schemes. Mitigation strategies are developed for decreasing the impact on the magnets. With those mitigation strategies, both the absorbed power and the peak power density, are below the critical values for all the magnets. The displacements per atom benefited as well from the mitigation strategies, but still require further analysis. The cryogenic system could evacuate the deposited power on the cold masses and the magnets are protected against the risk of quenching. The peak power density remains below 8mW/cm$^{3}$, which is reassuring. As for the integrated dose on the insulator, the situation improved significantly but there are still few magnets where the design limit of 30 MGy is exceeded. This rather conservative limit is expected to rise up to 100 MGy with the use of more resistant materials. In this case, the accumulated dose would be acceptable for all the magnets of the EIR, with one exception. Further mitigation measures should be conceived for this magnet

Studien zur energiedeposition in den Experimentsektionen des Future Circular Hadron Colliders

Abweichender Titel nach Übersetzung der Verfasserin/des VerfassersUm neue Möglichkeiten in der Teilchenphysik für die Post-LHC Ära zu erkunden, wurde 2014 die Future Circular Collider (FCC) Studie gestartet. Unterschiedliche Maschinen werden evaluiert, unter anderem ein Proton-Proton Speicherring. Gleich wie beim LHC, handelt es sich hierbei um einen Kreisbeschleuniger, allerdings mit einem signifikant längerem Umfang von 100km und einer Schwerpunktsenergie von 100TeV. Protonenkollisionen bei solch hohen Energien führen zu einer großen Mengen Kollisionsdebris in den "experimental insertion regions" (EIR) des Beschleunigers. Dieser Debris beeinflusst die Elemente im Beschleuniger durch Langzeitschäden, wie Abnutzung von organischen Materialien und Supraleitern, aber auch sofortige Effekte, wie dem Zusammenbruch der Supraleitung in Magneten. Diese Arbeit setzt sich mit dem Einfluss von Kollisionsdebris in den EIR mit hoher Luminosität in FCC-hh auseinander. Für diesen Zweck wurden Monte Carlo Simulationen mit dem Fluka Code durchgeführt. Größen wie lineare Leistungsverteilung, maximale Leistungsdichte, integrierte Dosis und DPA (Displacements per Atom) wurden für beide Kreuzungswinkeloptionen, horizontal und vertikal, untersucht. Optimierungsmaßnahmen um den Einfluss auf die Magnete zu verringern wurden entwickelt. Durch diese Maßnahmen, sind sowohl die absorbierte Leistung als auch die maximale Leistungsdichte unter den kritischen Werten. Die DPA profitiert ebenfalls von der Optimierung, allerdings sind weitere Untersuchungen notwendig. Das Kryogeniksystem ist fähig die Leistung von den supraleitenden Magneten abzutransportieren und diese sind vor Zusammenbruch der Supraleitung gesichert. Die maximale Leistungsdichte bleibt deutlich unter dem Grenzwert von 8mW/cm 3. Die Situation für die integrierte Dosis auf den Insulatoren hat sich signifikant verbessert, allerdings überschreiten noch immer einige Magnete das Designlimit von 30MGy. Dieses Limit wurde sehr konservativ gewählt und durch die Verwendung von widerstandsfähigeren Materialien sollte das Designlimit auf 100MGy anwachsen. In diesem Fall wäre die integrierte Dosis für alle Magnete in der EIR akzeptabel, mit einer Ausnahme. Für diesen Magneten sollten weitere Optimierungsmaßnahmen angedacht werden.In order to explore new possibilities for particle physics for the post-LHC era, the Future Circular Collider (FCC) study was launched in 2014 to assess its feasibility. Different machines are considered, including a hadron collider machine, FCC-hh. Similar to LHC, this would be a circular collider with a significantly larger circumference of 100km and colliding protons at a centre of mass energy of 100TeV. Proton collisions at such high energies lead to a large amount of collision debris in the experimental insertion regions (EIR) of the accelerator. The collision debris impacts the elements of the accelerator causing long term damages, like deterioration of organic materials and superconductors, as well as instantaneous effects, like quenching of the superconducting magnets. This thesis studies the impact of the collision debris in the high-luminosity EIR of FCC-hh. For this purpose Monte Carlo simulations have been performed with the FLUKA code. Quantities like linear power distribution, peak power density, integrated dose and displacements per atom are studied for the elements of the EIR for both, horizontal and vertical crossing schemes. Mitigation strategies are developed for decreasing the impact on the magnets. With those mitigation strategies, both the absorbed power and the peak power density, are below the critical values for all the magnets. The displacements per atom benefited as well from the mitigation strategies, but still require further analysis. The cryogenic system could evacuate the deposited power on the cold masses and the magnets are protected against the risk of quenching. The peak power density remains below 8mW/cm 3, which is reassuring. As for the integrated dose on the insulator, the situation improved significantly but there are still few magnets where the design limit of 30MGy is exceeded. This rather conservative limit is expected to rise up to 100MGy with the use of more resistant materials. In this case, the accumulated dose would be acceptable for all the magnets of the EIR, with one exception. Further mitigation measures should be conceived for this magnet.8

Synchrotron Radiation Impact on the FCC-ee Arcs

Synchrotron radiation (SR) emitted by electron and positrons beams represents a major loss source in high energy circular colliders, such as the lepton version of the Future Circular Collider (FCC-ee) at CERN. In particular, for the operation mode at 182.5 GeV (above the top pair threshold), its spectrum makes it penetrate well beyond the vacuum chamber walls. In order to optimize its containment, dedicated absorbers are envisaged. In this contribution we report the energy deposition studies performed with FLUKA to assess heat load, time-integrated dose, power density and particle fluence distribution in the machine components and the surrounding environment. Different choices for the absorber material were considered and shielding options for electronics were investigated. Furthermore, possible positions for the booster ring were reviewed from the radiation exposure point of view

Radiation Load Studies for the FCC-ee Positron Source with a Superconducting Matching Device

International audienceFor an electron-positron collider like FCC-ee, the production of positrons plays a crucial role. One of the design options considered for the FCC-ee positron source employs a superconducting solenoid made of HTS coils as an adiabatic matching device. The solenoid, which is placed around the production target, is needed to capture positrons before they can be accelerated in a linear accelerator. A superconducting solenoid yields a higher peak field than a conventional-normal conducting magnetic flux concentrator, therefore increasing the achievable positron yield. In order to achieve an acceptable positron production, the considered target is made of tungsten-rhenium, which gives also a significant flux of un-wanted secondary particles, that in turn could generate a too large radiation load on the superconducting coils. In this study, we assess the feasibility of such a positron source by studying the heat load and long-term radiation damage in the superconducting matching device and surrounding structures. Results are presented for different geometric configurations of the superconducting matching device

Implications of the Upgrade II of LHCb on the LHC Insertion Region 8: From Energy Deposition Studies to Mitigation Strategies

Starting from LHC Run3, a first upgrade of the LHCb experiment (Upgrade I) will enable oeration with a significantly increased instantaneous luminosity in the LHC Insertion Region 8 (IR8), up to 2 $\cdot$ 10$^{33}$ cm$^{-2}$ s$^{-1}$. Moreover, the proposed second upgrade of the LHCb experiment (Upgrade II) aims at increasing it by an extra factor 7.5 (up to 1.5 $\cdot$ 10$^{34}$ cm$^{-2}$ s$^{-1}$, as of Run 5) and collecting an integrated luminosity of 400fb$^{-1}$ by the end of Run 6. Such an ambitious goal poses challenges not only for the detector but also for the accelerator components. Monte Carlo simulations represent a valuable tool to predict the implications of the radiation impact on the machine, especially for future operational scenarios. A detailed IR8 model implemented by means of the FLUKA code is presented in this study. With such a model, we calculated the power density and dose distributions in the superconducting coils of the LHC final focusing quadrupoles (Q1-Q3) and separation dipole (D1) and we highlight a few critical issues calling for mitigation measures. Our study addresses also the recombination dipole (D2) and the suitability of the present TANb absorber, as well as the proton losses in the Dispersion Suppressor (DS) and their implications

Radiation Load Studies for Superconducting Dipole Magnets in a 10 TeV Muon Collider

Among the various future lepton colliders under study, muon colliders offer the prospect of reaching the highest collision energies. Despite the promising potential of a multi-TeV muon collider, the short lifetime of muons poses a severe technological challenge for the collider design. In particular, the copious production of decay electrons and positrons along the collider ring requires the integration of continuous radiation absorbers inside superconducting magnets. The absorbers are needed to avoid quenches, reduce the heat dissipation in the cold mass and prevent magnet failures due to long-term radiation damage. In this paper, we present FLUKA shower simulations assessing the shielding requirements for high-field magnets of a 10 TeV muon collider. We quantify in particular the role of synchrotron photon emission by decay electrons and positrons, which helps in dispersing the energy carried by the decay products. For comparison, selected results for a 3 TeV muon collider are also presented

Comparison Between Run 2 TID Measurements and FLUKA Simulations in the CERN LHC Tunnel of the Atlas Insertion Region

In this paper we present a systematic benchmark between the simulated and the measured data for the radiation monitors useful for Radiation to Electronics (R2E) studies at the Large Hadron Collider (LHC) at CERN. For this purpose, the radiation levels in the main LHC tunnel on the right side of the Interaction Point 1 (ATLAS detector) are simulated using the FLUKA Monte Carlo code and compared against Total Ionising Dose (TID) measurements performed with the Beam Loss Monitoring (BLM) system, and 180 m of Distributed Optical Fibre Radiation Sensor (DOFRS). Considering the complexity and the scale of the simulations as well as the variety of the LHC operational parameters, we find a generally good agreement between measured and simulated radiation levels, typically within a factor of 2 or better

Update on the FCC-ee positron source design studies

International audienceThe studies and R&D on the high-intensity positron source for the FCC-ee have been initiated for a while. The positrons are produced by a 6 GeV electron drive-beam incident on a target-converter at 200 Hz. The drive beam comes in 2 bunches spaced by 25 ns with a maximum charge of ~5 nC per bunch. Two scenarios using conventional and hybrid targets are being studied for positron production. According to the FCC CDR, the Flux Concentrator is used as the matching device for the capture system, followed by several accelerating structures embedded in the solenoidal field. Then, the positrons are further accelerated to be injected into the damping ring. Recently, the feasibility study on using a SC solenoid for the positron capture has been started, and the design based on the HTS technology is under investigation. In addition, the large aperture 2 GHz RF structures, which have been specially designed for the FCC-ee positron capture system, are used with the goal of demonstrating accepted positron yield values well beyond the values obtained with state-of-the-art positron sources. The purpose of this paper is to review the current status of the FCC-ee positron source design, highlighting the recent research into the positron production, capture system, primary acceleration, and injection into the damping ring

FCC-hh Experimental Insertion Region Design

The Future Circular Collider study is exploring possible designs of circular colliders for the post-LHC era, as recommended by the European Strategy Group for High Energy Physics. One such option is FCC-hh, a proton-proton collider with a centre-of-mass energy of 100 TeV. The experimental insertion regions are key areas defining the performance of the collider. This paper presents the first insertion region designs with a complete assessment of the main challenges, as collision debris with two orders of magnitude larger power than current colliders, beam-beam interactions in long insertions, dynamic aperture for optics with peak $\beta$ functions one order of magnitude above current colliders, photon background from synchrotron radiation and cross talk between the insertion regions. An alternative design avoiding the use of crab cavities with a small impact on performance is also presented