Shape Evolution of C75 Large-Grain Niobium Half-Cells During Cavity Fabrication

Abstract

The largely anisotropic deformation of large-grain Nb discs during deep drawing into half-cells poses a challenge for achieving a desired shape accuracy. Two 5-cell cavities for the C75 CEBAF cryomodule rework program have been fabricated at Jefferson Lab from large-grain Nb discs directly sliced from an ingot. The shape of the inner surface of eight half-cells has been inspected using a FARO Edge laser scanner during the fabrication process and compared to the reference shape. On average, approximately 63% of the half-cell inner surface was found to be within 0.1 mm of the reference shape and ~90% to be within 0.2 mm, after the final equator machining. Several 5-cell C75 cavities have also been fabricated at Research Instruments, Germany, and measurements of the shape accuracy using a Zeiss 3D coordinate measuring machine gave similar results. One half-cell was measured both at Research Instruments and Jefferson Lab for comparison.

Characterization of dissipative regions of a N-doped superconducting radio-frequency cavity

“…The losses at the hot-spot locations can be characterized with a power law, ΔT ∝ B n p , where the exponent n obtained by the least square fit to the data was in the range of 2.1 − 2.4 for Test 1 and 2.5 − 3.7 for Test 2. Values of n = 2.5 − 3.5 were reported at hot-spot locations known to have trapped flux in an earlier study (Ciovati and Gurevich, 2007).…”

First attempts in automated defect recognition in superconducting radio-frequency cavities

“…The different loss models discussed in the literature predict different correlations between the RF performance and the surface properties, but all of them assume the grain boundaries as a major potential for limitations or losses [41][42][43][44][45][46][47]. Using the above mentioned variables obtained by the image processing algorithm, a set of 24 parameters for each cell were obtained.…”

Investigation of the effect of structural defects from hydride precipitation on superconducting properties of high purity SRF cavity Nb using magneto-optical and electron imaging methods

“…Chemical removal of the surface damage introduces hydrogen that can form normal conducting hydrides when cooled to cryogenic temperatures. Dislocations, GBs, and normal conducting hydrides can facilitate magnetic flux trapping and perturb the superconducting currents and increase residual resistance [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Defects can be flux traps in a superconductor as it cooled below T c , where an ideal superconductor would be diamagnetic and flux free [14].…”

“…This study focuses on three kinds of defects: dislocations, GBs and hydrogen. Dislocations can facilitate flux entry and are capable of trapping magnetic flux, resulting in radio frequency (RF) losses due to oscillatory motion of trapped flux under RF fields [6,7,15,21]. Previous work shows that GBs are weak links and show lower critical current density than bulk material [22].…”