The technological and metallurgical requirements of material for highgradient
superconducting cavities are described. High-purity niobium, as the preferred
metal for the fabrication of superconducting accelerating cavities, should meet
exact specifications. The content of interstitial impurities such as oxygen,
nitrogen, and carbon must be below 10{\mu}g/g. The hydrogen content should be
kept below 2{\mu}g/g to prevent degradation of the Q-value under certain
cool-down conditions. The material should be free of flaws (foreign material
inclusions or cracks and laminations) that can initiate a thermal breakdown.
Defects may be detected by quality control methods such as eddy current
scanning and identified by a number of special methods. Conventional and
alternative cavity fabrication methods are reviewed. Conventionally, niobium
cavities are fabricated from sheet niobium by the formation of half-cells by
deep drawing, followed by trim machining and Electron-Beam Welding (EBW). The
welding of half-cells is a delicate procedure, requiring intermediate cleaning
steps and a careful choice of weld parameters to achieve full penetration of
the joints. The equator welds are particularly critical. A challenge for a
welded construction is the tight mechanical and electrical tolerances. These
can be maintained by a combination of mechanical and radio-frequency
measurements on halfcells and by careful tracking of weld shrinkage. The
established procedure is suitable for large series production. The main aspects
of quality assurance management are mentioned. Another cavity fabrication
approach is slicing discs from the ingot and producing cavities by deep drawing
and EBW. Accelerating gradients at the level of 35-45 MV.m-1 can be achieved by
applying Electropolishing (EP) treatment....Comment: 37 pages, contribution to the CAS-CERN Accelerator School:
Superconductivity for Accelerators, Erice, Italy, 24 April - 4 May 2013,
edited by R. Baile