Impact of Forming, Welding, and Electropolishing on Pitting and the Surface Finish of SRF Cavity Niobium

Abstract

Abstract-A broad range of coupon electropolishing experiments are described to ascertain the mechanism(s) by which large defects are formed near superconducting radiofrequency (SRF) cavity welds. Cold-worked vs. annealed metal, the presence of a weld, and several variations of electropolishing (EP) parameters were considered. Pitting is strongly promoted by cold work and agitation of the EP solution. Welding also promotes pitting, but less so compared with the other factors above. Temperature increase during EP did not strongly affect glossiness or pitting, but the reduced viscosity made the electrolyte more susceptible to agitation. The experiments suggest that several factors that are rather benign alone are combined by the cavity forming, welding, and processing sequence to promote the formation of defects such as pits. Process changes to mitigate these risks are discussed.

Detection of surface carbon and hydrocarbons in hot spot regions of niobium superconducting rf cavities by Raman spectroscopy

“…We might hypothesize therefore that the dissipation is related to different surface structure and chemistry inside or beneath the pits as compared to other regions of the cavity surface. It has been shown previously that pits are found in conjunction with high dislocation density [12,13] and dislocations trap impurities [14]. However, a link between hot spots and the chemistry underlying the etch pits is not yet confirmed.…”

Annealing to mitigate pitting in electropolished niobium coupons and SRF cavities

“…The same niobium coupons as processed in our previous study [9] were used in the present work. In brief, coupons were cut from cavity-grade niobium sheets.…”

“…Thus, it is plausible that an EP process can on occasion produce flaws such as dislocation-assisted pitting. This was the main result of our past study [9]-Cold work makes SRF cavity niobium susceptible to flaws when the EP process deviates from its baseline settings. The next logical question is whether annealing to remove cold work prior to etching should improve the resiliency of the niobium metal against EP process flaws.…”

“…The disturbances evidently supply fluorine ions to the low points of the niobium surface, thereby permitting attack in addition to the diffusion-limited leveling intended by EP. In our past study [9], deliberate agitation of the electrolyte produced surface finishes akin to those made by simple etching. Thus, it is plausible that an EP process can on occasion produce flaws such as dislocation-assisted pitting.…”

“…Deep pits can thus result [7,8]. Indeed, in our past study of niobium coupons [9], significantly higher tendency for pitting was observed for cold-worked coupons than for recrystallized coupons. Electropolishing has been used to circumvent deep material removal by establishing a fluorine ion gradient near the niobium surface [10].…”