ESS, Lund, Sweden
CEA/IRFU, Gif-sur-Yvette, France
IPN, Orsay, France
IPN, Orsay, France
CERN, Geneva, Switzerland
Fermilab, Batavia, USA
CEA/DSM/IRFU, France
SCK•CEN, Mol, Belgium
The goal of the MYRRHA project is to demonstrate the technical feasibility of transmutation in a 100MWth Accelerator Driven System (ADS) by building a new flexible irradiation complex in Mol (Belgium). The MYRRHA facility requires a 600 MeV accelerator delivering a maximum proton flux of 4 mA in continuous operation, with an additional requirement for exceptional reliability. This paper will briefly describe the beam dynamics design of the main superconducting linac section which covers the 17 to 600 MeV energy range and requires enhanced fault-tolerance capabilities.
IIT, Chicago, USA
Fermilab, Batavia, USA
MSU, East Lansing, USA
Michigan State University, East Lansing, USA
FRIB, East Lansing, USA
(MSU), East Lansing, USA
The thermal conductivity k of Nb at less than 3 K is dominated by phonon transport. In Nb with sufficiently few lattice imperfections, a maximum in k occurs at 1.8 K, called the phonon peak (PP). A large PP is desired to reduce potential local hot spots and contributes to an increased Q factor. The magnitude of the PP is sensitive to SRF cavity manufacturing processes. The effect of interstitial hydrogen on the magnitude of the PP is examined by subjecting two bicrystal Nb specimens to 300 C for 1 h in a 75% H2, 25% N2 atmosphere at 0.5 atm. Prior to hydrogen infusion, specimen 1 was heated to 800 C for 2 h, while specimen 2 was heated to 1100 C for 4 h. Both specimens displayed a 25% reduction in the PP due to the additional hydrogen, independent of their crystal orientations and heat treatment histories. An 800 C vacuum heating for 2 h was found to be sufficient to recover the PP in specimen 1, while an 1100 C heating for 4 h was required to recover the PP in one of the grains of specimen 2. The results suggest that hydrogen trapped in the Nb lattice will degas when the Nb is heated to at least the temperature to which it was heated at prior to the hydrogen infusion step.
JLab, Newport News, Virginia, USA
AASC, San Leandro, California, USA
JLAB, Newport News, Virginia, USA
NSU, Newport News, Virginia, USA
In this study, we used a differential-depth X-Ray diffraction Reciprocal Spacing Mapping (XRD RSM) technique to investigate the crystal quality of a variety of SRF-relevant Nb film and bulk materials. By choosing different X-ray probing depths, the RSM study successfully revealed the materials’ microstructure evolutions after different materials processes, such as energetic condensation or surface polishing. The RSM data clearly measured the materials’ crystal quality at different thickness. Through a novel differential-depth RSM technique, this study found: I. for a heteroepitaxy Nb film Nb(100)/MgO(100), the film thickening process, via a cathodic arc-discharge Nb ion deposition, created a near-perfect single crystal Nb on the surface’s top-layer; II. for a mechanic polished single-crystal bulk Nb material, the microstructure on the top surface layer is more disordered than that in-grain.
JLAB, Newport News, Virginia, USA
NCSU AIF, Raleigh, North Carolina, USA
The College of William and Mary, Williamsburg, USA
JLab, Newport News, Virginia, USA
NSU, Norfolk, USA
For the past three decades, bulk niobium has been the material of choice for SRF cavities applications. RF cavity performance is now approaching the theoretical limit for bulk niobium. For further improvement of RF cavity performance for future accelerator projects, Superconductor-Insulator-Superconductor (SIS) multilayer structures (as recently proposed by Alex Gurevich) present the theoretical prospect to reach RF performance beyond bulk Nb, using thinly layered higher-Tc superconductors with enhanced Hc1. Jefferson Lab (JLab) is pursuing this approach with the development of NbTiN and AlN based multilayer SIS structures via magnetron sputtering and High Power Impulse Magnetron Sputtering (HiPIMS). This paper presents the results on the characteristics of NbTiN and insulator films and the first RF measurements on NbTiN-based multilayer structure on thick Nb films.
AASC, San Leandro, California, USA
Energetic Condensation refers to thinfilm growth on a surface using ~100eV ions, versus lower energy deposition using sputtering (~1-10eV with no substrate bias) or still lower energy thermal evaporation. The relatively high incident energy of energetic condensation creates defects and vacancies within the first few atomic layers and enables diffusion to lower free-energy sites in the lattice. Shallow defects migrate to the heated surface and are annihilated, leading to low-defect crystal growth. It has been shown [1] that the purer the film, the closer are its superconducting parameters to those of the bulk metal. Use of cathodic arc plasmas was proposed in 2000 by Langner [TESLA Rep. 2000-15, Ed. D. Proch, DESY 2000], followed by detailed development of the process [2]. AASC picked up from the European Community-Research Infrastructure Activity and has demonstrated very high RRR=541 in Nb films grown on crystal substrates [3]. Ongoing work to coat 1.3GHz copper cavities using cathodic arc plasmas, as well as growth of higher temperature films such as NbTiN, Nb3Sn and MgB2 are described. A related technique for energetic condensation using an ECR plasma source is also described.
1. C. Benvenuti et al, IEEE Trans. Appl. Supercond. 9 (1999) 900
2. R. Russo et al, Supercond. Sci. Technol. 18 (2005) L41-L44
3. M. Krishnan et al, Supercond. Sci. Technol. 24, 115002 (2011)
