Characterization of materials and welded interfaces for the SPL Project

The SPL project is an R&D e ort coordinated by CERN in partnership with other international laboratories, aimed at developing key technologies for the construction of a multi-megawatt proton linac based on state-of-the-art RF superconducting technology, which would serve as a driver for new physics facilities such as neutrinos and RIB. The Materials and Mechanical Engineering group of CERN aims for 2013 the construction of a string of 4 bulk niobium =1 elliptical cavities, operating at 2 K with a very ambitious accelerating gradient of 25 MV/m. In order to achieve such a high gradient, bulk high purity niobium is required because of its unique properties in terms of critical superconductive temperature, critical magnetic eld and formability. The highly restrictive technical speci cations proposed by CERN can not be ful lled by much companies, so a full material characterization needs to be done with advanced techniques such as RRR measurements and more conventional ones like tensile testing for the determination of the mechanical properties. Therefore, in this work, a full quali cation of a prototype piece from the rm PLANSEE was done to determine if the values satis ed the requirements for the manufacturing and operation of a SCRF cavity with the proposed parameters. It will be also studied the degradation of purity of niobium as a consequence of the electron beam welding processes that the cavity will undergo during its manufacturing and assembly. Additionally, the R&D program conducted by a team of CERN specialists explores new mechanical design and new fabrication methods for the elliptical =1 cavities fabricated from niobium sheets . One of the main points of this program is the de nition of the interfaces of the cavity and the helium tank, and therefore require careful studies and quali cation of applicable joining techniques. Hence, in this work the feasability of an electron beam butt weld of high purity niobium and Ti6Al4V alloy was studied. The main objective was to determine if the joint ful lled the requirements for the designed interfaces, what was done via non-destructive and mechanical testing. In parallel, more tests were carried out to study the aspect of the weld, its microstructure and its composition, what is of great interest from a point of view of understanding the behavior of the joint during the welding process.Ingeniería Industria

A Comparative Study of Fracture Toughness at Cryogenic Temperature of Austenitic Stainless Steel Welds

The ITER magnet system is based on the "cable-in-conduit" conductor (CICC) concept, which consists of stainless steel jackets filled with superconducting strands. The jackets provide high strength, limited fatigue crack growth rate and fracture toughness properties to counteract the high stress imposed by, among others, electromagnetic loads at cryogenic temperature. Austenitic nitrogen-strengthened stainless steels have been chosen as base material for the jackets of the central solenoid and the toroidal field system, for which an extensive set of cryogenic mechanical property data are readily available. However, little is published for their welded joints, and their specific performance when considering different combinations of parent and filler metals. Moreover, the impact of post-weld heat treatments that are required for Nb3Sn formation is not extensively treated. Welds are frequently responsible for cracks initiated and propagated by fatigue during service, causing structural failure. It becomes thus essential to select the most suitable combination of parent and filler material and to assess their performance in terms of strength and crack propagation at operation conditions. An extensive test campaign has been conducted at 7 K comparing tungsten inert gas (TIG) welds using two fillers adapted to cryogenic service, EN 1.4453 and JK2LB, applied to two different base metals, AISI 316L and 316LN.The authors would like to express their thanks to Dr. Arman Nyilas (in memorian) for the contributions made in this work

Assessment of production, materials and welds applicable at cryogenic temperatures to different components of ITER magnets

Mención Internacional en el título de doctorPrograma de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de MadridPresidente: Klaus Peter Weiss.- Secretario: Juan Cornide Arce.- Vocal: Clement Kelle

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

Advances in Cryogenic Engineering ‐ Materials: Proceedings of the International Cryogenic Materials Conference (ICMC) 2015 28 June to 2 July 2015, Tucson, AZ, USAThe 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 10−9P a · m3 /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

Assessment of residual stresses in ITER CS helium inlet welds fatigue tested at cryogenic temperature

Proceeding of: 27th International Cryogenic Engineering Conference and International Cryogenic Materials Conference 2018 (ICEC-ICMC 2018), September 3-7, 2018, Oxford, United KingdomThe ITER Central Solenoid (CS) consists of six independent wound modules. The cooling of the cable-in-conduit conductor is assured by a forced flow of supercritical He at 4.5 K supplied by He inlets located at the innermost radius of the coil. The inlets consist of a racetrack-shaped boss welded to the outer conduit wall through a full penetration Tungsten Inert Gas (TIG) weld. They are critical structural elements submitted to severe cyclic stresses due to the electro-magnetic forces acting on the coils. The weld contour is shape-optimised and locally processed by Ultrasonic Shot Peening (USP), conferring large compressive residual stresses on a subsurface layer of several millimetres thickness to improve fatigue strength. The distribution of the residual stresses and the effect of USP on microstructure and mechanical properties is assessed, with reference to the results of a cryogenic fatigue test campaign, performed on peened and as-welded inlets for comparison