Fundamental Study of Micro-Defects in Electropolished EB-Welded and Hydroformed SRF Accelerating Structures

[摘要] In the area of niobium elecropolishing fundamentals, we focused on understanding the influence of the surface topology, and geometry (with effects from gravity included. The formation of a viscous film is essential for the electropolishing process to take place. The exact nature and composition of the film formed on niobium is still unknown because of its solubility in the electrolyte. Extensive pitting may take place at surface where a stable film cannot form. This has to be taken into consideration while determining the speed with which the SRF cavities are rotated while EP. Hydrodynamic aspects must be taken into consideration while optimizing the polishing parameters. There is improvement in surface finish with polishing time. There is a huge change in surface quality when the EP time is increased from 2 hours to 4 hours but not much change takes place when the time is further increased to 6 hours. So keeping the economic points in view, about 100 um defect layer removal may be sufficient to get the desired performance. In the area of Electropolishing of untreated and treated niobium with Weld Joints we studied untreated and treated Nb, especially for the heat affected areas next to welded bumps, electropolished for different durations. The electropolishing of the untreated Nb caused the formation of pits on the surface at about 15 min but they disappeared when the electropolishing duration was more than 15 min. Electropolishing for 120 min smoothened the surface of untreated Nb by levelling the surface, but the severe formation of pits on the whole surface was found after 240 min. The treatment of Nb significantly changed the Nb surface morphology which was covered by grains of different size that looked light or dark in the optical microscope. The treated Nb was susceptible to pitting during the entire electropolishing starting from 15 min and the dark grains had more susceptibility to pitting than the light grains. In addition, electropolishing for 240 min again resulted in severe pit formation. In the area of Bulge test and microstructure studies, we worked to create a useful constitutive relationship for the complex stress state that accompanies SRF cavity formation. To do so, bulge tests were performed on Cu and Nb tube samples that exhibited the greatest degrees of uniaxial elongation after HT. The data from the bulge tests and an accompanying set of tensile tests provided input to a finite-element model that recreated the bulge tests numerically. As expected the model based on the bulge test results fit the experimental data well at least at low stress levels safely below the bursting pressure. Not so for that based on the tensile results. The results of the study emphasize the importance of for bulge testing rather than tensile testing when deriving the constitutional relationships eventually needed for modelling the hydroforming of Nb SRF cavities.

[发布日期] 2014-08-29

[学科分类] 核物理和高能物理

[关键词] Superconducting RF cavities;Niobium;hydroforming;electropolishing