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By the end of October 2022, the US HL-LHC Accelerator Upgrade Project (AUP) had completed fabrication of ten MQXFA magnets and tested eight of them. The MQXFA magnets are the low-beta quadrupole magnets to be used in the Q1 and Q3 Inner Triplet elements of the High Luminosity LHC. This AUP effort is shared by BNL, Fermilab, and LBNL, with strand verification tests at NHMFL. An important step of the AUP QA plan is the testing of MQXFA magnets in a vertical cryostat at BNL. The acceptance criteria that could be tested at BNL were all met by the first four production magnets (MQXFA03-MQXFA06). Subsequently, two magnets (MQXFA07 and MQXFA08) did not meet some of the criteria and were disassembled. Lessons learned during the disassembly of MQXFA07 caused a revision to the assembly specifications that were used for MQXFA10 and subsequent magnets. In this article, we present a summary of: 1) the fabrication and test data for all the MQXFA magnets; 2) the analysis of MQXFA07/A08 test results with characterization of the limiting mechanism; 3) the outcome of the investigation, including the lessons learned during MQXFA07 disassembly; and 4) the finite element analysis correlating observations with test performance. By the end of October 2022, the US HL-LHC Accelerator Upgrade Project (AUP) had completed fabrication of ten MQXFA magnets and tested eight of them. The MQXFA magnets are the low beta quadrupole magnets to be used in the Q1 and Q3 Inner Triplet elements of the High Luminosity LHC. This AUP effort is shared by BNL, Fermilab, and LBNL, with strand verification tests at NHMFL. An important step of the AUP QA plan is the testing of MQXFA magnets in a vertical cryostat at BNL. The acceptance criteria that could be tested at BNL were all met by the first four production magnets (MQXFA03-MQXFA06). Subsequently, two magnets (MQXFA07 and MQXFA08) did not meet some criteria and were disassembled. Lessons learned during the disassembly of MQXFA07 caused a revision to the assembly specifications that were used for MQXFA10 and subsequent magnets. In this paper, we present a summary of: 1) the fabrication and test data of all the MQXFA magnets; 2) the analysis of MQXFA07/A08 test results with characterization of the limiting mechanism; 3) the outcome of the investigation, including the lessons learned during MQXFA07 disassembly; and 4) the finite element analysis correlating observations with test performance.

The EN-MME Main Workshop holds more than fifty years of experience in fabrication of diverse and complex mechanical equipment for the CERN Accelerators and its Experiments. State of the art processes in machining, sheet-metal forming and joining are exploited daily. Fabrication and related expertise are ensured for a wide range of needs, such as prototyping and feasibility developments, multi-activity, and small-to-large-size projects, also integrating on-site facilities and sub-contracting. The following contribution will highlight the engineering competencies and the portfolio of technologies employed for manufacturing in a demanding domain such as the one of accelerator technology. Specific examples of components and fabrication solutions will also be given.

During operation the LINAC4 RFQ (Radio-Frequency-Quadrupole) is exposed to high electric fields which can lead to vacuum breakdown. It is also subject to beam loss that can cause surface modification, including blistering, which can result in reduced electric field handling and an increased breakdown rate. An experimental study has been made to identify materials with high electric field capability and robustness to low-energy irradiation. In this paper we briefly discuss the selection criteria and we analyze these materials investigating their metallurgical properties using advanced microscopic techniques such as Scanning Electron Microscope, Electron Back Scattered Diffraction, Energy-dispersive X-ray Spectroscopy and conventional optical microscopy. These allow to observe and characterize the different materials on aμand a nano-scale, allowing us to compare results before and after irradiation and breakdown testing., Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK

Beam loss occurs in Radio Frequency Quadrupoles (RFQ), and has been observed in the H⁻ linear accelerator Linac4 (L4) at CERN. To determine if beam loss can induce breakdowns, and to compare the robustness of different materials, tests have been done using pulsed high-voltage DC systems. Electrical breakdown phenomena and conditioning processes have been studied using these systems. Cathodes of different materials were irradiated with 1.2x10¹⁹ H⁻ p/cm², the estimated beam loss of the L4 RFQ over 10 days. The irradiated electrodes were installed in a system to observe if the irradiated area coincided with the breakdown locations, with pulsing parameters similar to the RFQ. Tests of irradiated and non-irradiated electrodes of the same material were done for comparison. The main difference observed was an increase in the number of breakdowns during the initial conditioning that returned to non-irradiated sample values with further running. Visual observations after irradiation show the beam centre and a halo the same diameter of the beam pipe. Breakdown clusters occur in the centre and halo regions, suggesting irradiation is not the only factor determining the breakdown probability., Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK

In modern accelerators, stringent requirements are placed on the materials used for vacuum systems. Their physical and mechanical properties, machinability, weldability and brazeability are key parameters. Adequate strength, ductility, magnetic properties at room as well as low temperatures are important factors for vacuum systems of accelerators working at cryogenic temperatures. In addition, components undergoing baking or activation of Non-Evaporable Getters (NEG) or directly exposed to the beam impose specific choices of material grades for suitable outgassing and mechanical properties in a large temperature range. Today, stainless steels are the dominant materials of vacuum systems. The reasons for specific requirements in terms of metallurgical processes are detailed for obtaining adequate purity, inclusion cleanliness and fineness of the microstructure. In many cases these requirements are crucial to guarantee the final leak tightness of the vacuum components. Innovative manufacturing and material examination technologies are also treated., Comment: 30 pages

The future CERN Super Proton Synchrotron (SPS) internal dump (Target Internal Dump Vertical Graphite, known as TIDVG#5), to be installed during CERN's Long Shutdown 2 (2019-2020), will be required to intercept beam dumps from 26 to 450 GeV, with increased intensity and repetition rates with respect to its predecessor (TIDVG#4). The beam power to be managed by the dump will increase by approximately a factor of four; resulting in new challenges in terms of design in order to fulfil the highly demanding specification, which is based on guaranteeing a good performance of the machine with little or no limitations imposed by this device. This paper presents the proposed design, including material selection, manufacturing techniques and thermo-mechanical simulations under different operational scenarios expected during the lifetime of the device., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

A third-generation neutron spallation target for the neutron time-of-flight facility at CERN (n_{TOF}) is currently undergoing the design and prototyping stage. The new design aims at improving reliability, increasing beam intensity on target and avoiding issues encountered in the current generation target, in particular the contamination of the cooling system water with radioactive spallation products coming from washing out lead. After a preliminary design and an initial prototyping stage*, a baseline solution has been defined consisting in a pure lead target core contained in a Ti-6Al-4V cladding and embedded in a massive Pb block. A backup solution has also been defined, consisting in a Ta-cladded W core embedded in a Pb block. Both solutions are currently undergoing the detailed design stage. This contribution details the prototyping activity, the robustness studies for accidental scenarios and the design of a beam irradiation test on prototypes of the target core., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

A new spallation target for the CERN neutron time-of-flight facility will be installed during Long Shutdown 2 (2019-2020), with the objective of improving operational reliability, avoiding water contamination of spallation products, corrosion/erosion and creep phenomena, as well as optimizing it for the 20 m distant vertical experimental area 2, whilst keeping the same physics performances of the current target at the 200 m far experimental area 1. Several solutions have been studied with FLUKA Monte Carlo simulations in order to find the optimal solution with respect to neutron fluence, photon background, resolution function, energy deposition and radiation damage. Thermo-mechanical studies (including CFD simulations) have been performed in order to evaluate and optimize the target ability to withstand the beam loads in terms of maximum temperatures reached, cooling system efficiency, maximum stresses, creep and fatigue behaviour of the target materials, leading to a preliminary mechanical design of the target. This paper also covers the further prototyping and material characterization activities carried out in order to validate the feasibility of the investigated solutions., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

CERN's Super Proton Synchrotron (SPS) high-energy internal dump (Target Internal Dump Vertical Graphite, known as TIDVG) is required to intercept beams from 10² to 450 GeV. The equipment installed in 2014 (TIDVG#3) featured an absorbing core composed of different materials surrounded by a water-cooled copper jacket, which hold the UHV of the machine. An inspection of a previous equipment (TIDVG#2) in 2013 revealed significant beam induced damage to the aluminium section of the dump, which required imposing operational limitations to minimise the risk of reproducing this phenomenon. Additionally, in 2016 a vacuum leak was detected in the dump assembly, which imposed further limitations, i.e. a reduction of the beam intensity that could be dumped per SPS supercycle. This paper presents a new design (TIDVG#4), which focuses on improving the operational robustness of the device. Moreover, thanks to the added instrumentation, a careful analysis of its performance (both experimentally and during operation) will be possible. These studies will help validating technical solutions for the design of the future SPS dump to be installed during CERN's Long Shutdown 2 in 2020 (TIDVG#5)., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

The manufacturing of the Linac4 Drift Tube Linac (DTL) components has been completed and the assembly of the structures is in its final stages. 3 tanks of 3.9m, 7.3m, and 7.3m, designed to accelerate a 40mA average pulse current H–beam from 3 to 50MeV, are being assembled from 2, 4 and 4 segments of about 2.0m length, containing each from 22 drift tubes at the low energy end, down to only 6 at the high energy end. Due to its peculiar design avoiding adjustment mechanisms on the drift tube, tight tolerances have to be maintained in the production. This paper discusses the assembly stages that are used to achieve the tolerances over the full length of the structures. Metrology results on the assembled DTL Tank1 confirm the required precision.

The fabrication of the Linac4 Drift Tube Linac (DTL) required the welding assembly of 10⁸ drift tubes (DT) which has been undertaken at the CERN workshop. The design of the DTL is particular in that it was purposely simplified to avoid any position adjustment mechanism for drift tubes in the tank. In consequence, drift tubes have been designed with tight tolerances and parts have been assembled with an optimised welding procedure. Two re-machining stages have been introduced in order to compensate for welding distortions. This paper discusses the various assembly stages with a view on the final precision that has been achieved.

Following the analysis of the September 2008 LHC incident, the assembly process and the quality assurance of the main 13 kA interconnection splices were improved, with new measurement and diagnostics methods introduced. During the 2008-2009 shutdown ~5% of these 10 000 splices were newly assembled with these improvements implemented, but essentially maintaining the original design. It is known today that a limiting factor towards 7 TeV operation is the normal conducting resistance of ~15% of the original main 13 kA interconnection splices, associated to the electrical continuity of the copper stabiliser. A "Splices Task Force" has been set up at CERN to evaluate the need for, develop and test design improvements and prepare the implementation of a consolidation campaign. Important issues of splice design, process choice, resources and time requirements are considered., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

The room temperature elastic and plastic properties under uniaxial tensile loading of the different phases of an un-reacted, internal-tin process, Nb$_{3}$Sn wire have been determined by tensile tests of whole wires and of extracted Ta, Nb and Nb alloy filaments, as well as by indentation hardness measurements in metallographic wire cross sections.

In modern accelerators stringent requirements are placed on materials of vacuum systems. Their physical and mechanical properties, machinability, weldability or brazeability are key parameters. Adequate strength, ductility, magnetic properties at room as well as low temperatures are important factors for vacuum systems of accelerators working at cryogenic temperatures, such as the Large Hadron Collider (LHC) under construction at CERN. In addition, baking or activation of Non-Evaporable Getters (NEG) at high temperatures impose specific choices of material grades of suitable tensile and creep properties in a large temperature range. Today, stainless steels are the dominant materials of vacuum constructions. Their metallurgy is extensively treated. The reasons for specific requirements in terms of metallurgical processes are detailed, in view of obtaining adequate purity, inclusion cleanliness, and fineness of the microstructure. In many cases these requirements are crucial to guarantee the final leak tightness of the vacuum components.

The Large Hadron Collider to be built at CERN will require 1232 superconducting dipole magnets operating at 1.9 K. By virtue of their mechanical properties, weldability and improved austenite stability, nitrogen enriched austenitic stainless steels have been chosen as the material for several of the structural components of these magnets. Powder Metallurgy (PM) could represent an attractive production technique for components of complex shape for which dimension tolerances, dimensional stability, weldability are key issues during fabrication, and mechanical properties, ductility and leak tightness have to be guaranteed during operation. PM Hot Isostatic Pressed test plates and prototype components of 316LN-type grade have been produced by Santasalo Powdermet Oy. They have been fully characterized and mechanically tested down to 4.2 K at CERN. The fine grained structure, the absence of residual stresses, the full isotropy of mechanical properties associated to the low level of Prior Particle Boundaries oxides resulted in superior mechanical properties and high ductility down to liquid helium temperature. The ready weldability and the leak tightness of the alloy have been demonstrated. The properties measured on test plates are comparable to those found in real components, such as prototype end covers fabricated by the same PM technique.

The aim of this work is to study the austenitic stability against a' martensitic transformation of three non-magnetic austenitic steels : a new stainless steel X2CrMnNiMoN 19-12-11-1 grade, a traditional X8CrMnNiN 19-11-6 grade and a high manganese X8MnCrNi 28-7-1 grade. Measurements of relative magnetic susceptibility at room temperature are performed on strained tensile specimens at 4.2 K. A special extensometer for high precision strain measurements at low temperature has been developed at CERN to test specimens up to various levels of plastic strain. Moreover, the high precision strain recording of the extensometer enables a detailed study of the serrated yield phenomena associated with 4.2 K tensile testing and their influence on the evolution of magnetic susceptibility. The results show that high Mn contents increase the stability of the austenitic structure against a' martensitic transformation, while keeping high strength at cryogenic temperature. Moreover, proper elaboration through primary and possibly secondary melting maintains high levels of low temperature ductility.