The paper describes a new approach for vacuum brazing of niobium-316L stainless steel transition joints for application in superconducting radiofrequency cavities. The study exploited good wettability of titanium-activated silver-base brazing alloy (CuSil-ABA®), along with nickel as a diffusion barrier, to suppress brittle Fe-Nb intermetallic formation, which is well reported during the established vacuum brazing practice using pure copper filler. The brazed specimens displayed no brittle intermetallic layers on any of its interfaces, but instead carried well-distributed intermetallic particles in the ductile matrix. The transition joints displayed room temperature tensile and shear strengths of 122-143 MPa and 80-113 MPa, respectively. The joints not only exhibited required hermeticity (helium leak rate <1.1 × 10−10 mbar l/s) for service in ultra-high vacuum but also withstood twelve hour degassing heat treatment at 873 K (suppresses Q-disease in niobium cavities), without any noticeable degradation in the microstructure and the hermeticity. The joints retained their leak tightness even after undergoing ten thermal cycles between the room temperature and the liquid nitrogen temperature, thereby establishing their ability to withstand service-induced low cycle fatigue conditions. The study proposes a new lower temperature brazing route to form niobium-316L stainless steel transition joints, with improved microstructural characteristics and acceptable hermeticity and mechanical properties.

A New Vacuum Brazing Route for Niobium-316L Stainless Steel Transition Joints for Superconducting RF Cavities

650 MHz multicell superconducting radio frequency (SRF) elliptical cavities are proposed for effective acceleration of proton beam in the Chinese initiative Accelerator Driven Subcritical System (CiADS). Two families of such cavities will be used in the driver superconducting linac to accelerate the H + beam from 175 MeV to 500 MeV, with the possibility to upgrade the energy to 1 GeV and higher. Detail designs of a six-cell β o p t = 0.62 elliptical cavity and a five-cell β o p t = 0.82 elliptical cavity will be presented in this paper, including the multi-parameter electromagnetic design and optimization, high order modes analyses, multipacting simulations, mechanical and engineering analyses. Sub-systems of the cavity such as the fundamental power coupler antenna tip design and mechanical design of cavity-helium vessel-tuner system will also be discussed in detail. Three single cell prototype cavities were fabricated and vertical tested to verify the electromagnetic properties, the fabrication and post-processing technologies. Four multi-cell prototype cavities are under manufacturing and expected to be ready in 2021.

650 MHz elliptical superconducting RF cavities for CiADS Project

This paper describes a new brazing recipe for making intermetallic-free and ductile niobium-316L stainless steel transition joints for application in superconducting radio frequency cavities. Brittle Fe-Nb intermetallic compounds have remained a matter of concern in niobium-stainless steel joints made with the global practice of vacuum brazing that uses pure copper as the filler metal. Important measures adopted to suppress brittle intermetallic formation include the use of nickel plating on mating stainless steel surface as a diffusion barrier and wettability enhancer, and lower temperature, titanium-free brazing filler metal BVAg8. The resultant joints were free of brittle intermetallic compounds, which translated into ductile fracture in tensile and shear tests. The brazed joints displayed tensile and shear strengths of 150–191 and 103–118 MPa, respectively. The joints demonstrated a required level of hermeticity for service in ultrahigh vacuum; a capability to withstand degassing heat treatment at 873 K, a mandatory step for superconducting radio frequency cavities; and also low-cycle thermal fatigue between the room temperature and the liquid nitrogen temperature. The new vacuum brazing recipe marks a significant advancement over existing global practice to make sound and ductile niobium-316L stainless steel transition joints with improved microstructural characteristics.

New Brazing Recipe for Ductile Niobium-316L Stainless Steel Joints

The Superconducting Proton Linac (SPL) is an R&D effort conducted 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 SRF technology. Such an accelerator could serve as a driver in new physics facilities for neutrinos and/or radioactive ion beams [1]. Amongst the main objectives of this effort, are the development of 704 MHz bulk niobium beta=1 elliptical cavities (operating at 2 K and providing an accelerating gradient of 25 MV/m) and the test of a string of cavities integrated in a machine-type cryo-module. In an initial phase, only four out of the eight cavities of the SPL cryomodule will be tested in a half-length cryo-module developed for this purpose, which nonetheless preserves the main features of the full size module. This paper presents the final design of the cryo-module and the status of the construction of the main cryostat parts. Preliminary plans for the assembly and testing of the cryo-module at CERN are also presented.

/pdf/status-of-the-superconducting-proton-linac-spl-cryo-module-15wvy0jmsm.pdf

Status of the superconducting proton linac (spl) cryo- module

/pdf/cleanroom-refurbishment-in-sm18-within-the-spl-project-frame-1oaptiawaf.pdf

Cleanroom refurbishment in SM18 within the SPL project frame, feasibility study

The Superconducting Proton Linac (SPL) is an R&D effort coordinated by CERN in partnership with other international laboratories. It is aiming 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 in new physics facilities for neutrinos and/or Radioactive Ion Beam (RIB). Amongst the main objectives of this R&D effort, is the development of 704 MHz bulk niobium beta=1 elliptical cavities, operating at 2 K with a maximum accelerating gradient of 25 MV/m, and the testing of a string of cavities integrated in a machine-type cryomodule. The cavity together with its helium tank had to be carefully designed in coherence with the innovative design of the cryomodule. New fabrication methods have also been explored. Five such niobium cavities and two copper cavities are in fabrication. The key design aspects are discussed, the results of the alternative fabrication methods presented and the status of the cavity manufacturing and surface preparation is detailed.

/pdf/cern-developments-for-704-mhz-superconducting-cavities-42vz0gbivt.pdf

CERN Developments for 704 MHz Superconducting Cavities

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CERN Developments for 704 MHz Superconducting Cavities