Abstract
In the past decade, high $Q$ values have been achieved in high purity Nb superconducting radio frequency (SRF) cavities. Fundamental understanding of the physical metallurgy of Nb that enables these achievements is beginning to reveal what challenges remain to establish reproducible and cost-effective production of high performance SRF cavities. Recent studies of dislocation substructure development and effects of recrystallization arising from welding and heat treatments and their correlations with cavity performance are considered. With better fundamental understanding of the effects of dislocation substructure evolution and recrystallization on electron and phonon conduction, as well as the interior and surface states, it will be possible to design optimal processing paths for cost-effective performance using approaches such as hydroforming, which minimizes or eliminates welds in a cavity.
Highlights
The continuing efforts to improve cavity performance to attain a high electric field and quality factor, Q, have shown that the theoretical limit for the maximum field of about 42 MV=m is within reach [1]
Some of the variability in cavity performance may be traceable to the variability of grain orientations on the surface, which etch at different rates, and have different work functions
Dislocation content is a significant contributor to the value of k in cavities, and the presence of residual dislocations is generally undesirable. This overview of the mechanical and physical metallurgy associated with the production of superconducting radio frequency (SRF) cavities clearly shows that dislocations are an omnipresent facilitator for and detractor of the performance of cavities
Summary
The continuing efforts to improve cavity performance to attain a high electric field and quality (efficiency) factor, Q, have shown that the theoretical limit for the maximum field of about 42 MV=m is within reach [1]. The primary focus will be on factors that affect formability of Nb and cavity performance based upon classical [3,4,5] mechanical and physical metallurgical knowledge. These topics are insufficient to identify all that accounts for variability in performance, as there are additional factors related to electromagnetic and superconducting states in the few nanometers near the surface of the interior, which are discussed in other papers [6,7,8]. The basic crystal structure is discussed first, followed by physical metallurgical changes that occur with forming, welding, heat-treating, etching, and baking
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