Engineering Design and Prototype Fabrication of HOM Couplers for HL-LHC Crab Cavities

The High-Luminosity upgrade for the LHC relies on a set of RF Crab Cavities for reaching its goals. Two parallel concepts, the Double Quarter Wave (DQW) and the RF Dipole (RFD), are going through a comprehensive design process along with preparation of fabrication in view of extensive tests with beam in SPS. High Order Modes (HOM) couplers are critical in providing damping in RF cavities for operation in accelerators. HOM prototyping and fabrication have recently started at CERN. In this paper, an overview of the final shape is provided along with an insight in the mechanical and thermal analyses performed to validate the design of these critical components. Emphasis is also given to test campaigns, material selection, prototyping and initial fabrication that are aimed at fulfilling the highly demanding tolerances of the couplers

Deformability Tests of Pure Niobium

research team at the University of Miskolc's Faculty of Materials Science and Technology has signed a cooperation agreement with the Geneva-based European Organization for Nuclear Research (CERN) for testing of the materials employed in the Crab Cavities will be installed in the next generation of the LHC (the so-called High Luminosity Large Hadron Collider – HL-LHC). At the University of Miskolc, high purity niobium rolling experiments were carried out in conventional (unidirectional) and cross-rolled manners in order to increase the deep drawability of the final sheet. The deformability of niobium was measured by Watts- Ford and compression tests. The microstructure and anisotropy (texture) results of the initial material and the straight-rolled products are reported

Design of Dressed Crab Cavities for the HL-LHC Upgrade

The HL-LHC upgrade relies on a set of RF crab cavities for reaching its goals. Two parallel concepts, the Double Quarter Wave (DQW) and the RF Dipole (RFD), are going through a comprehensive design process along with preparation of fabrication in view of extensive tests with beam in SPS. High Order Modes (HOM) couplers are critical in providing damping in RF cavities for operation in accelerators. HOM prototyping and fabrication have recently started at CERN. In this paper, an overview of the final geometry is provided along with an insight in the mechanical and thermal analyses performed to validate the design of this critical component. Emphasis is also given to material selection, prototyping, initial fabrication and test campaigns that are aimed at fulfilling the highly demanding tolerances of the couplers

Design of load-to-failure tests of high-voltage insulation breaks for ITER's cryogenic network

The development of new generation superconducting magnets for fusion research, such as the ITER experiment, is largely based on coils wound with so-called cable-in-conduit conductors. The concept of the cable-in-conduit conductor is based on a direct cooling principle, by supercritical helium, flowing through the central region of the conductor, in close contact with the superconducting strands. Consequently, a direct connection exists between the electrically grounded helium coolant supply line and the highly energised magnet windings. Various insulated regions, constructed out of high-voltage insulation breaks, are put in place to isolate sectors with different electrical potential. In addition to high voltages and significant internal helium pressure, the insulation breaks will experience various mechanical forces resulting from differential thermal contraction phenomena and electro-magnetic loads. Special test equipment was designed, prepared and employed to assess the mechanical reliability of the insulation breaks. A binary test setup is proposed, where mechanical failure is assumed when leak rate of gaseous helium exceeds 10-9centerdotPacenterdotm3/s. The test consists of a load-to-failure insulation break charging, in tension, while immersed in liquid nitrogen at the temperature of 77 K. Leak tightness during the test is monitored by measuring the leak rate of the gaseous helium, directly surrounding the insulation break, with respect to the existing vacuum inside the insulation break. The experimental setup is proven effective, and various insulation breaks performed beyond expectations

Post-mortem analysis of ITER CS helium inlets fatigue tested at cryogenic temperature

In the ITER Magnet System, ten thousand tonnes of superconducting Cable In Conduit Conductor (CICC) arecooled down by a forcedflow of supercritical helium, which is supplied from helium inlets. For the ITER CentralSolenoid (CS), consisting of six independent pancake wound modules, the He inlets consist of three overlappingholes covered by an oblong shaped boss, welded to the CS jacket through full penetration, multi-pass TungstenInert Gas (TIG) welding. Since they are located in a region of high cyclic tensile stresses, i.e.first turn at the innerdiameter of the pancake, the CS inlets are one of the most critical structural components. Qualification of thedesign is done by analysis and a comprehensive design optimization has been performed byfinite element (FE)simulations. In order to guarantee the required fatigue life at cryogenic temperature of these component, a post–welding process consisting in ultrasonic shot–peening is required. Based on a qualified weld procedure, sixmock–ups including each two He–inlets on the opposite surfaces have been produced to run a mechanicalfatigue testing program at cryogenic temperature to validate thefindings of the FE simulations. Five were peenedand one not peened. The paper describes the results of a comprehensive post–mortem failure analysis whichincludes non–destructive (penetrant testing, leak testing, X-ray computed tomography) as well as destructiveexaminations (microoptical and hardness tests, scanning electron microscopy). The initiation site, propagation ofthe crack as well as the tensile overload region have been identified and studied. An estimation of the cycles frominitiation to failure based on the width of the fatigue striations was performed. The paper also includes a fullassessment of the welds according to the most stringent acceptance levels of the standards in for

Design and fabrication of a cryostat for low temperature mechanical testing for the Mechanical and Materials Engineering group at CERN

Mechanical testing of materials at low temperatures is one of the cornerstones of the Mechanical and Materials Engineering (MME) group at CERN. A long tradition of more than 20 years and a unique know - how of such tests has been developed with an 18 kN double-walled cryostat. Large campaigns of material qualification have been carried out and the mechanical behaviour of materials at 4 K has been vastly studied in sub - size samples for projects like LEP, LHC and its experiments. With the aim of assessing the mechanical properties of materials of higher strength and/or issued from heavy gauge products for which testing standardized specimens of larger cross section might be more adapted, a new 100 kN cryostat capable of hosting different shapes of normalized samples has been carefully designed and fabricated inhouse together with the associated tooling and measurement instrumentation. It has been conceived to be able to adapt to different test frames both dynamic and static, which will be of paramount importance for future studies of fracture mechanics at low temperatures. The cryostat features a double-walled vessel consisting of a central cylindrical section with a convex lower end and a flat top end closure. The transmission of the load is guaranteed by a 4 column system and its precise monitoring is assured by an internal load cell positioned next to the sample in the load train. This innovative approach will be discussed together with other nonconventional instrumentation solutions. A validation of the whole system has been carried out, where bending efforts on instrumented samples have been measured. Additionally, dedicated tooling has been fabricated for the device's optimization. The preliminary results obtained confirm an excellent performance of the system and enhance the analysis of materials under extreme conditions with state of the art instrumentation

Examination of ITER Central Solenoid prototype joints

The ITER Magnet System will be the largest and most challenging integrated superconducting magnet systemever built. For the Central Solenoid (CS), cable–in–conduit - conductors (CICCs) of nearly one kilometre lengthare produced, but still, it will be necessary to connect several lengths together to wind the gigantic 110 tonnescoils. The creation of these superconducting joints is one of the most delicate parts of the assembly. There arethree types of ITER CS joints: splice joints, coaxial joints and twin–box joints. US ITER, the ITER DomesticAgency of the USA produced a prototype containing all three types of joints. The goal is to test the performanceof the joints in the SULTAN facility of the Swiss Plasma Center (SPC), capable of reproducing close–to–serviceconditions: high magneticfield (up to 11 T backgroundfield), high current (up to 100 kA) and high massflowrate of supercritical helium for cooling. The paper describes the results of a comprehensive examination cam-paign aimed at understanding the relation between fabrication and performance of the CS prototype joints. Thetest campaign combines advanced image analysis for the assessment of the void fraction of the conductors withScanning Electron Microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) to evaluate the quality of the contact between the strands and the sleeve / sole of the joints

Mechanical Design and Fabrication Studies for SPL Superconducting RF Cavities

CERN’s R&D programme on the Superconducting Proton Linac’s (SPL) superconducting radio frequency (SRF) elliptical cavities made from niobium sheets explores new mechanical design and consequently new fabrication methods, where several opportunities for improved optimization were identified. A stainless steel helium vessel is under design rather than a titanium helium vessel using an integrated brazed transition between Nb and the SS helium vessel. Different design and fabrication aspects were proposed and the results are discussed hereafter

Friction stir welding of AISI 316LN high strength austenitic stainless steel for cryogenic application

Friction stir welding (FSW) is a solid state joining process that uses the heat generated by the friction of a rotating tool and the base material to join materials together. Due to the fact that the material is never melted, and that extensive plastic deformation is introduced in the weld seam, a unique set of properties is achieved. The technique has been extensively used to join aluminium and aluminium alloys, but very few developments are reported on high strength austenitic stainless steel, which is the material of choice for many high energy physics and fusion magnets. This paper contains a comprehensive microstructural and mechanical characterization, including at cryogenic temperature, of an 8 mm thick high strength austenitic stainless steel plates. The steel grade is the high alloy version of AISI 316LN (identified as 1.4429 or -X2CrNiMoN17-13-3 according to European standards). Special attention was given to cryogenic elastic – plastic J – integral testing of the weld seam. To the authors’ knowledge, this is the first time fracture toughness at cryogenic temperature on friction stir welded 1.4429 has been measured

Characterization of low temperature high voltage axial insulator breaks for the ITER cryogenic supply line

Cable-in-conduit conductors of the ITER magnet system are directly cooled by supercritical helium. Insulation breaks are required in the liquid helium feed pipes to isolate the high voltage system of the magnet windings from the electrically grounded helium coolant supply line. They are submitted to high voltages and significant internal helium pressure and will experience mechanical forces resulting from differential thermal contraction and electro-mechanical loads. Insulation breaks consist essentially of stainless steel tubes overwrapped by an outer glass – fiber reinforced composite and bonded to an inner composite tube at each end of the stainless steel fittings. For some types of insulator breaks Glass – Kapton – Glass insulation layers are interleaved in the outer composite. Following an extensive mechanical testing campaign at cryogenic temperature combined with leak tightness tests, the present paper investigates through non-destructive and destructive techniques the physical and microstructural characteristics of the low temperature high voltage insulation breaks and of their individual components, thus allowing to correlate the structure and properties of the constituents to their overall performance. For all the tests performed, consistent and reproducible results were obtained within the range of the strict acceptance criteria defined for safe operation of the insulation breaks