Electrochemical Polishing of Chemical Vapor Deposited Niobium Thin Films

Combining chemical vapor deposition (CVD) with electrochemical polish (EP) operations is a promising route to producing performance-capable superconducting films for use in the fabrication of cost-effective components for superconducting radiofrequency (SRF) particle accelerators and superconducting quantum computers. The post-deposition EP process enables a critically necessary reduction in surface roughness of niobium thin films to promote optimal superconducting surface conditions. In this work, surface morphology, roughness, and crystal orientation of the CVD-grown and EP-polished niobium films were investigated. The grain growth and polishing mechanisms were analyzed. The CVD films were found to comprise steps, kinks, and pyramidal features, resulting in undesirable large peak-to-valley distances. The electrochemical polish was demonstrated to significantly diminish the height of pyramids and effectively minimize the overall surface roughness. In contrast to buffered chemical polishing (BCP), EP results showed a probable dependence on crystal orientation, suggesting this process was influenced by locally enhanced current density and thickness variations of oxide dielectrics. These understandings identify the EP principles tied to CVD-grown Nb films that allow further refinement of surface profiles for film-based SRF application

COMPARISON OF BUFFERED CHEMICAL POLISHED AND ELECTROPOLISHED 3.9 GHZ CAVITIES*

Abstract Five 3.9 GHz 9 cell cavities have been measured for the DESY FLASH module. These cavities were BCP processed and reached gradients of typically about 25 MV/m with Q drop starting at about 20 MV/m. Recently a few one cell cavities have been processed with EP and at least one has tested to a gradient of 30 MV/m with Q drop starting at about 25 MV/m. We will compare the results and give an update to the thermal analysis in relation to global thermal breakdown at 3.9 GHz

Comparison of buffered chemical polished and electropolished 3.9 GHz cavities

Five 3.9 GHz 9 cell cavities have been measured for the DESY FLASH module. These cavities were BCP processed and reached gradients of typically about 25 MV/m with Q drop starting at about 20 MV/m. Recently a few one cell cavities have been processed with EP and at least one has tested to a gradient of 30 MV/m with Q drop starting at about 25 MV/m. We will compare the results and give an update to the thermal analysis in relation to global thermal breakdown at 3.9 GHz

Study of the Surface and Performance of Single-Cell Nb Cavities After Vertical EP Using Ninja Cathodes

International audienceA 1.3 GHz single-cell niobium (Nb) coupon cavity was vertically electropolished (VEPed) with three different Ninja cathodes which were specially designed for VEP of 1.3 GHz superconducting RF elliptical (ILC/Tesla type) cavities. The cathodes were fabricated to have different surface areas and different distances between cathode surface and the equator. The Ninja cathode prepared with an enhanced cathode surface area was covered with a meshed shield to avoid bubble attack on the surface of the cavity cell. It has been turned out that the anode-cathode distance and the cathode area affect surface morphology of the equator. A smooth equator surface was obtained in the cases in which the cathode surface was geometrically close to the equator or instead the cathode surface area was sufficiently larger. Two 1.3 GHz ILC/Tesla type single-cell cavities VEPed with the Ninja cathodes and using optimized conditions showed good performance in vertical tests