Very intense neutrino beams and large neutrino detectors will be needed to enable the discovery of CP violation in the leptonic sector. The European Spallation Source (ESS), currently under construction in Lund, Sweden, is a research center that will provide, by 2023, the world's most powerful neutron source. The average power will be 5 MW. Pulsing this linac at higher frequency, at the same instantaneous power, will make it possible to raise the average beam power to 10 MW to produce, in parallel with the spallation neutron production, a high performance neutrino Super Beam of about 0.4 GeV mean neutrino energy. The ESS neutrino Super Beam, ESSnuSB, operated with a 2.0 GeV linac proton beam, together with a large underground Water Cherenkov detector located at 540 km from Lund, close to the second oscillation maximum, will make it possible to discover leptonic CP violation at 5 sigma significance level in 56 percent (65 percent for an upgrade to 2.5 GeV beam energy) of the leptonic Dirac CP-violating phase range after 10 years of data taking. The paper gives an overview of the proposed facility and presents the outstanding physics reach possible for CP violation with ESSnuSB.

The opportunity offered by the ESSnuSB project to exploit the larger leptonic CP violation signal at the second oscillation maximum and the requirements of this project on the ESS accelerator complex

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

Higher order modes (HOMs) may affect beam stability and refrigeration requirements of superconducting proton linacs such as the Superconducting Proton Linac, which is studied at CERN. Under certain conditions beam-induced HOMs can accumulate sufficient energy to destabilize the beam or quench the superconducting cavities. In order to limit these effects, CERN considers the use of coaxial HOM couplers on the cutoff tubes of the 5-cell superconducting cavities. These couplers consist of resonant antennas shaped as loops or probes, which are designed to couple to potentially dangerous modes while sufficiently rejecting the fundamental mode. In this paper, the design process is presented and a comparison is made between various designs for the high-beta SPL cavities, which operate at 704.4 MHz. The rf and thermal behavior as well as mechanical aspects are discussed. In order to verify the designs, a rapid prototype for the favored coupler was fabricated and characterized on a low-power test-stand.

/pdf/comparison-of-coaxial-higher-order-mode-couplers-for-the-1genp03z5l.pdf

Comparison of coaxial higher order mode couplers for the CERN Superconducting Proton Linac study

The potential for a superconducting proton linac (SPL) at CERN started to be seriously considered at the end of the 1990s In the first conceptual design report (CDR), published in 2000 [1], most of the 352 MHz RF equipment from LEP was re-used in an 800 m long linac, and the proton beam energy was limited to 22 GeV During the following years, the design was revisited and optimized to better match the needs of a high-power proton driver for neutrino physics The result was a more compact (470 m long) accelerator capable of delivering 5 MW of beam power at 35 GeV, using state-of-the-art superconducting RF cavities at 704 MHz It was described in a second CDR, published in 2006 [2] Soon afterwards, when preparation for increasing the luminosity of the LHC by an order of magnitude beyond nominal became an important concern, a low-power SPL (LP-SPL) was studied as a key component in the renovation of the LHC injector complex The combination of a 4 GeV LP-SPL injecting into a new 50 GeV synchrotron (PS2) was proposed to replace the ageing Linac2, PSB, and PS In a later stage, if necessary for future physics programmes (neutrino production or generation of radioactive ion beams), the linac could be brought up to multimegawatt beam power by upgrading the cooling, the electrical infrastructure, and the power supplies The construction of the low-energy front end of the LP-SPL started in 2008 as the Linac4 project [3], aimed at the replacement of Linac2, with the potential for being adapted later to the needs of a low-power or high-power SPL (HP-SPL) In parallel, the R&D on the superconducting linac continued, initially to refine the design of the LP-SPL and afterwards that of the HP-SPL This report presents the design of the LP-SPL and the work accomplished in preparing a proposal for new LHC injectors with the support of the European Commission under the Framework Programmes 6 and 7 and in collaboration with multiple laboratories and institutions worldwide The status of the R&D towards an HP-SPL is summarized

Conceptual Design of the Low-Power and High-Power SPL : A Superconducting H$^-$ Linac at CERN

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

In the framework of the SPL R&D effort at CERN, development design efforts study the joining of dissimilar metals: bulk niobium for the superconducting RF cavities and stainless steel (316LN) or titanium alloys (Ti-6Al-4V and Nb55Ti) for the cryostats. Joining techniques of electron beam welding (EBW) and vacuum brazing are particularly important for these applications. These processes have been used in the accelerator community and developed into generally accepted “best practice”. Studies were performed to update the existing knowledge, and comprehensively characterise these joints via mechanical and metallurgical investigations using modern available technologies. The developed solutions are described in detail, some currently being applied uniquely at CERN.

Electron Beam Welding and Vacuum Brazing Characterization for SRF Cavities

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Conceptual Design of the Low-Power and High-Power SPL : A Superconducting H$^-$ Linac at CERN

CERN Developments for 704 MHz Superconducting Cavities

Electron Beam Welding and Vacuum Brazing Characterization for SRF Cavities