Magnesium Diboride thin Films, multilayers, and coatings for SRF cavities

Technical Report · Thu Aug 17 00:00:00 EDT 2017

DOI:https://doi.org/10.2172/1375332· OSTI ID:1375332

Xi, Xiaoxing ^[1]

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1. Temple Univ., Philadelphia, PA (United States)

Superconducting radio frequency (SRF) cavities currently use low-temperature superconductor niobium, and the Nb SRF cavities have approached the performance levels predicted theoretically. Compared to Nb, MgB₂ becomes superconducting at a much higher temperature and promises a better RF performance in terms of higher quality factor Q and higher acceleration capability. An MgB₂ SRF technology can significantly reduce the operating costs of particle accelerators when these potentials are realized. This project aimed to advance the development of an MgB₂ SRF technology. It had two main objectives: (1) materials issues of MgB₂ thin films and multilayers related to their applications in SRF cavities; and (2) coating single-cell cavities for testing at RF frequencies. The key technical thrust of the project is the deposition of high quality clean MgB₂ films and coatings by the hybrid physical-chemical vapor deposition (HPCVD) technique, which was developed in my group. We have achieved technical progress in each of the two areas. For the first objective, we have confirmed that MgB₂ thin film coatings can be used to effectively enhance the vortex penetration field of an SRF cavity. A vortex is a normal region in the shape of spaghetti that threads through a superconductor. Its existence is due to an applied magnetic field that is greater than a so-called lower critical field, H_c1. Once a vortex enters the superconductor, its movement leads to loss. This has been shown to be the reason for an SRF cavity to break down. Thus, enhancing the magnetic field for a vortex to enter the superconductor that forms the SRF cavity has be a goal of intense research. To this end, Gurevich proposed that a coating of thin superconductor layer can impede the vortex entrance. In this project, we have done two important experiment to test this concept. One, we showed that the enhancement of H_c1 can be achieved by using in both epitaxial and polycrystalline MgB₂ films. Although H_c1 is low for bulk MgB₂ samples, about 600 Oe at 5 K, it increases with decreasing film thickness, reaching 1880 Oe when the film thickness is 100 nm. Two, we coated Nb ellipsoids with MgB₂ films to achieve an “inverse cavity” configuration, mimicking the coating of an actual RF cavity. Our results demonstrate that it is indeed possible to increase the vortex penetration field of a cavity by a substantial amount (~600 Oe) by coating it with a thin MgB₂ film. For the second objective, we modified the existing HPCVD system to be able to coat a 3.9 GHz SRF cavity, and using a stainless steel mock cavity showed that a uniform film with good superconducting property can be grown across the cavity interior. Further, we successfully deposited MgB₂ on Cu disc. The two results combined demonstrate that it is possible to coat Cu cavities with high quality MgB₂ films using HPCVD. MgB₂ coated Cu could open up a possibility of using SRF cavities at 20–25 K with cryocoolers.

• Research Organization: Temple Univ., Philadelphia, PA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), High Energy Physics (HEP)
• DOE Contract Number: SC0011616
• OSTI ID: 1375332
• Report Number(s): Final Report: DOE-SRF-Temple-Final
• Country of Publication: United States
• Language: English

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36 MATERIALS SCIENCE