COOLING CELL DESIGN 3D MODEL MILESTONE NO 2

Milestone No 21 Date: WP8: Cooling Cell Integration Grant Agreement 101094300 PUBLIC 1 / 16 Grant Agreement No: 101094300 MuCoL A Design Study for a Muon Collider complex at 10 TeV centre of mass Horizon Europe Framework Programme MILESTONE REPORT COOLING CELL DESIGN 3D MODEL MILESTONE NO 21 Document identifier: MuCol_MS21_CoolCelDesign3DModel_Final DOI: 10.5281/zenodo.17791559 Due date of milestone: 30/11/2025 (End of Month 33) Justification for delay: NO Work package: WP8: Cooling Cell Integration Lead beneficiary: UMIL Report release date: 30/11/2025 Document version: 1.0 Document status: Final Abstract: This milestone report details the comprehensive 3D mechanical model of the Cooling Cell, a critical component for the Muon Collider. It traces the evolution of the design from its initial conceptualization to a fully integrated engineering model, addressing key challenges in magnet-RFabsorber integration, mechanical architecture, and thermal/vacuum boundary development. The introduction of the Inter-Cell Cryostat and the adoption of pillow-seal vacuum interfaces proved pivotal in achieving a compact, robust, and remotely compatible design, confirming the successful achievement of Milestone MS21. MuCol Consortium, 2025 For more information on MuCol, its partners and contributors please see https://mucol.web.cern.ch/ Milestone No 21 Date: 30/11/2025 Grant Agreement 101094300 PUBLIC 2 / 16 Funded by the European Union (EU). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the EU or European Research Executive Agency (REA). Neither the EU nor the REA can be held responsible for them. Delivery Slip Name Partner Date Authored by L. Rossi [WP8 coordinator and task 8.5 leader] [UMIL] 19/11/25 Edited by L. Rossi [WP8 coordinator and task 8.5 leader] [UMIL] 19/11/25 Reviewed by R. Losito [WP8 deputy coordinator and MuCol Technical Coordinator] C. Rogers [WP4 coordinator, Task 8.4 leader] D. Giove [WP6 coordinator and Task 8.3 leader] M. Statera [Tasks 7.4 and 8.2 leader] G. Scarantino (WP8 and WP7 member) M. Castoldi (WP8 and WP7 member) [CERN] [UKRI] [INFN] [INFN] [INFN] [INFN] 21/11/25 Approved by D. Schulte [Mucol Scientific coordinator] Steering Committee 30/11/2025 Milestone No 21 Date: 30/11/2025 Grant Agreement 101094300 PUBLIC 3 / 16 TABLE OF CONTENTS 1. INTRODUCTION ....................................................................................................................................... 4 2. COOLING CELL DESIGN OVERVIEW .................................................................................................. 5 3. EVOLUTION OF THE COOLING CELL CONFIGURATION: THE INTER-CELL CRYOSTAT . 6 4. MAGNET SYSTEM: MAG2.4 .................................................................................................................... 8 5. RF STRUCTURE INTEGRATION ............................................................................................................ 9 6. ABSORBER SYSTEM ............................................................................................................................... 10 7. REMOTE‑HANDLING CONNECTIONS: FROM SMA TO PILLOW SEAL ................................... 11 8. MECHANICAL AND THERMAL ARCHITECTURE .......................................................................... 12 9. VERIFICATION OF MILESTONE ACHIEVEMENT .......................................................................... 13 10. CONCLUSION AND OUTLOOK ............................................................................................................ 13 11. REFERENCES ............................................................................................................................................ 14 ANNEX: GLOSSARY .......................................................................... ERROR! BOOKMARK NOT DEFINED. Milestone No 21 Date: 30/11/2025 Grant Agreement 101094300 PUBLIC 4 / 16 Executive summary This document provides a concise account of the design evolution and engineering achievements culminating in Milestone MS21. It details the progression of the Cooling Cell design from the conceptual B5/S5-like configuration presented in Deliverable D8.1 to a fully integrated 3D engineering model. Between early 2024 and late 2025, the WP8 team conducted a systematic design effort, encompassing multiple iterations of magnet-RF-absorber integration, mechanical architecture definition, and thermal/vacuum boundary development. A key turning point was the successful implementation of the Inter-Cell Cryostat presented at the Fermilab Demonstrator workshop, which resolved crucial integration conflicts and enabled a compact and feasible layout. This new architecture facilitates the complete closure of magnetic forces within the cold mass (with the exception of the cooling section's extremities), thereby decoupling the cryogenic magnet structure from the room-temperature RF cavity, absorber systems, and inter-cell connections. Concurrently, the strategy for remote-handling compatible vacuum connections evolved from early SMA seal concepts to custom-designed, high-TRL pillow-seal technology. This solution offers a reliable and compact interface, compatible with the highly constrained space inside the magnet bore and between RF cavities. These developments, in conjunction with the MAG2.4 HTS coil design, an updated RF mechanical structure, and LiH absorber integration, constitute the foundation of the completed 3D model, which was validated at the Milan Demonstrator workshop (5–6 November 2025). Consequently, the objective of this milestone is deemed successfully attained. 1. INTRODUCTION The Cooling Cell constitutes a fundamental building block in the rectilinear 6D cooling lattice for a future Muon Collider [1,2]. Its purpose is to reduce the transverse and longitudinal emittance of the muon beam through a sequence of absorbers, RF accelerating structures, and focusing solenoids. The efficient integration of all these components within a compact cryo-mechanical environment is essential for achieving the performance predicted by beam dynamics simulations. Following Deliverable D8.1, which established the conceptual layout and selected a B5-type cooling cell for engineering studies, Milestone MS21 endeavours to deliver the first complete 3D mechanical model of such a cell. This model integrates constraints and requirements originating from MuCol-WP4 (Muon Production and Cooling), MuCol-WP6 (RF systems), MuCol-WP7 (Magnet systems), window engineering and cryogenics. The resulting design serves as the reference configuration for the Muon Cooling Demonstrator [3,4,5] being pursued at CERN within the International Muon Collider Collaboration (IMCC). This document provides a technical account of the design evolution, delineating how initial integration challenges—related to magnetic forces, heat loads, waveguide routing, access limitations, and vacuum interfaces—were resolved through the introduction of new design concepts such as the Inter-Cell Cryostat and pillow-seal vacuum interfaces. The resulting 3D model consequently forms the backbone of the demonstrator design and signifies the achievement of MS21. Milestone No 21 Date: 30/11/2025 Grant Agreement 101094300 PUBLIC 5 / 16 2. COOLING CELL DESIGN OVERVIEW The Cooling Cell serves as the fundamental building block of the rectilinear 6D cooling lattice. Within the B5-like configuration selected for the Demonstrator, each cell comprises: • a low-Z absorber to reduce both transverse and lomngitudinal momentum through ionization cooling, • a 3-cell RF accelerating structure to restore longitudinal momentum, • a high-field HTS solenoid pair to provide high gradient in large apertures, • a complete mechanical support and alignment system, and • warm-cold interfaces compatible with remote handling. The MS21 model represents the initial comprehensive incorporation of all these elements into a true engineering configuration, respecting both lattice optics constraints and realistic mechanical/thermal boundaries. The resulting assembly demonstrates the feasibility of a compact (1000 mm) cooling cell featuring a 3 RF cells structure, somewhat longer than the ultra-compact 800 mm long cell preconized in [6,7] shown in Fig. 1. The result was achieved through the organization of subsystems around the Inter-Cell Cryostat architecture. Fig. 1. From MuCol D8.1 report: schematic of a Cooling Cell (for reference parameters for RF, Magnets and absorber see D8.1 report [7]). The target was a cooling cell of 800 mm length. Milestone No 21 Date: 30/11/2025 Grant Agreement 101094300 PUBLIC 6 / 16 3. EVOLUTION OF THE COOLING CELL CONFIGURATION: THE INTER-CELL CRYOSTAT The initial designs endeavored to place the two solenoids completely outside the RF envelope. However, as presented at the Fermilab workshop [4], these solutions yielded very high peak magnetic fields exceeding 25 T, unacceptable e.m. forces among coils more than 50 MN, and excessive heat loads. These figures, unacceptably high, were obtained with an overall cell length above 990 mm and with a field reduced by 25% with respect to the requested one (7 T peak on the beam axis) [7], see Fig. 2, that made the design actually not compatible with the focusing requirements by beam dynamics. Fig. 2. (from Fermilab workshop): first cell design with RF Cavity+power coupler assembly which is independent of Magnet cell assembly The above features made the first design of Fig.2 incompatible with the cooling lattice requirements. During the Fermilab workshop, the concept of the ‘Inter-Cell Cryostat’ was introduced. This innovation permits the closure of magnetic forces within a cold, robust mechanical structure, while the RF and absorber systems are maintained at room temperature. The electromagnetic (e-m) forces exerted on the SC coils are reacted internally within the cold mass, thus obviating the need for transmission to warm supports. This eliminates several critical constraints, including: • A substantial reduction in magnetic forces transmitted to the warm structure. • A consequent reduction in heat load by approximately a factor of 50 compared to the previous solution. • The internal reaction of forces by adjacent elements, which contributes to reducing stress and torques. Milestone No 21 Date: 30/11/2025 Grant Agreement 101094300 PUBLIC 7 / 16 • Independent assembly of RF and absorber systems. • Viable routing of waveguides and couplers. • Enough space for remote handling cell-to-cell beam pipe connection. • Acceptable total cell length (1000 mm), mainly driven by solenoid size and by the space for the RF power wave guide. • Improved maintainability. In summary, this approach reduces the cold mass volume, facilitates proper routing of the RF waveguide, and resolves the coupling issues between magnets and RF structures. The Inter-Cell Cryostat is now regarded as the backbone of the design. However, it is important to acknowledge a specific disadvantage of this solution: • The right half-cell coils of the (n-1)th cooling cell are strongly mechanically coupled with the left half-cell of the nth cooling cell. • In a string or series of cooling cells, the electromagnetic (e.m.) forces on the first and last half-cell solenoids must still be reacted by a warm structure. This latter drawback is less significant for the final MC Cooling Section, with cooling stages 50 to 100 m long but becomes critical for the initial cells and for a limited number of cells. Fig. 3 schematic showing the concept of the inter-cell cryostat. In addition to the Inter-Cell Cryostat, the concept of designing the cell from a global module perspective, e.g., 5 or 6 cells mounted on a robust girder was also introduced, as suggested by the Integration Review [8]. The interference and clearance were inferred by the study for the RFMFTF, the split-coil test facility for the RF studies and tests designed by INFN and Univ. of Milan [9]. The MS21 design represents the first complete engineering implementation of this new architecture. Milestone No 21 Date: 30/11/2025 Grant Agreement 101094300 PUBLIC 8 / 16 Fig. 4. The intercell cryostat applied to the cell design (solenoidal coil configuration in the picture is the MAG2.3, now superseded by MAG2.4; remote handling and RF design are not updated, too, in this sketch). 4. MAGNET SYSTEM: MAG2.4 The MAG2.4 configuration, developed with multi-objective optimisation, uses HTS REBCO noninsulated coils operating at 20 K. The design satisfies field requirements, mechanical integrity, and thermal constraints. Charging transient analyses and quench studies indicate feasible operation, though some quench scenarios still require optimisation, which will continue in WP7. The MAG2.4 solenoid configuration emerges from a multi-objective optimisation, balancing the following requirements while satisfying the cell spatial constraints: - target field along the axis, - current margin limits, - conductor volume, - peak magnetic field, - cooling cell length, - stored energy - coil axial forces, - force symmetry, - torque minimization, - hotspot temperature limit in case of quench. HTS REBCO was selected for its ability to operate at 20 K with a wide temperature margin. This approach reduces cryogenic power consumption by an order of magnitude relative to LTS magnets. Mechanical FEA confirms acceptable stresses in normal operation, while charging simulations show that the magnets can be powered using a 4-hour ramp [10,11]. Quench simulations with active Milestone No 21 Date: 30/11/2025 Grant Agreement 101094300 PUBLIC 9 / 16 protection (induction heaters) limit unbalanced forces to manageable levels. MAG2.4 has been therefore adopted as the reference for MS21 and fully integrated into the 3D geometry. Fig.5. Main parameters of the solenoidal coil configuration MAG2.4, selected for the integration. (Courtesy of G. Scarantino and M. Statera, MuCol WP7) 5. RF STRUCTURE INTEGRATION A 3-cell, 704 MHz RF structure with thin metallic windows has been designed by WP6, [12], to be placed at each IRIS to reduce power requirement while increasing the effective gradient. The RF structure with its waveguide (WG) power feeder and cooling pipes have been integrated into the 3D Model. Aluminium windows (150 µm, 60 mm radius) were selected for the Demonstrator primarily due to their manufacturing feasibility and safety considerations while still satisfying the electromagnetic performance requirements for the Demonstrator. The 3-cell, 704 MHz copper cavity RF system features a number of small channels (grooves) connecting the volume of the cavity to the volume of the absorber chamber, bypassing the thin metallic foil. These channels allows the absorber chamber to be pumped via the RF cavity structure, thus avoiding dangerous differential pressure that might be generated in case the two volumes (RF cavity and absorber chamber) would be kept fully separated. The Inter-Cell Cryostat enables direct integration of power couplers and waveguides, Milestone No 21 Date: 30/11/2025 Grant Agreement 101094300 PUBLIC 16 / 16 Fig.10. 5-cell module on the girder, with a X-section of one and half-cell with quotes. (Courtesy of M. Castoldi, WP8). Fig. 11. Rendering of a 5 cell-module in a techncal building. (Courtesy of M. Castoldi, WP8).