Fabrication and Radio Frequency Properties of 3-GHz SRF Cavities Coated with MgB 2
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ORIGINAL PAPER
Fabrication and Radio Frequency Properties of 3-GHz SRF Cavities Coated with MgB2 Xin Guo 1,2 & Wenura K. Withanage 1 & Jay R. Paudel 1 & Grigory Eremeev 3 & Fay Hannon 3 & Robert Rimmer 3 & Alireza Nassiri 4,5 & Xiaoxing Xi 1 Received: 20 June 2020 / Accepted: 7 September 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Magnesium diboride (MgB2) is considered a potential material for superconducting radio frequency cavities. MgB2-coated Cu cavity will allow for a higher operational temperature than a bulk Nb cavity because of the higher transition temperature of MgB2 and the high thermal conductivity of Cu. Using the hybrid physical chemical vapor deposition technique, MgB2 coatings were successfully achieved on the inner wall of 3-GHz Cu cavities. The surface and superconducting properties of the coatings were characterized using small samples on Cu plugs mounted at different locations of mock cavities. RF measurement of a MgB2coated single-cell 3-GHz test cavity was carried out, and it showed superconducting transition at 36 K. The quality factor of this test cavity was lower than expected due to poor connectivity and inclusion of Mg–Cu alloy in the MgB2 coating. Keywords SRF cavity . MgB2 film . HPCVD
1 Introduction Most modern particle accelerators currently employ superconducting radio frequency (SRF) cavities made from niobium [1–4]. Over the decades, niobium cavities have overcome several limitations such as multipacting, thermal breakdown, field emission, and high field Q-slope, and the cavity performance has approached the fundamental limits of the Nb material [1, 2]. To allow even larger accelerating gradient, higher quality factor, and higher operation temperature, new superconducting materials beyond niobium have been explored [5–7]. Superconductor MgB2 shows potential as an alternative to * Xiaoxing Xi [email protected] 1
Department of Physics, Temple University, Philadelphia, PA 19122, USA
2
School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, People’s Republic of China
3
Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA
4
Argonne National Laboratory, Argonne, Lemont, IL 60439, USA
5
College of Engineering, Michigan State University, East Lansing, MI 48824, USA
niobium for SRF applications, with a potentially low BCS surface resistance as a result of the higher transition temperature of 39 K and small normal resistivity [5, 8, 9], a high critical field [9, 10], and no weak-link behavior across grain boundaries [11, 12]. Among all the MgB2 thin film fabrication techniques, hybrid physical-chemical vapor deposition (HPCVD) has a unique advantage of being compatible to coating SRF cavities, requiring only slight modifications to the standard HPCVD setup [13]. To demonstrate the ability of coating MgB2 films on the interior of cylindrical and curved objects by HPCVD technique, 6-GHz dummy stainless steel and niobium cavities [14] and 3.9-GHz mock niobium ca
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