Development of vertical electropolishing process applied on 1300 and 704 MHz superconducting niobium resonators

Abstract

An advanced setup for vertical electropolishing of superconducting radio-frequency niobium elliptical cavities has been installed at CEA Saclay. Cavities are vertically electropolished with circulating standard HF-HF-H 2 SO 4 electrolytes. Parameters such as voltage, cathode shape, acid flow, and temperature have been investigated. A low voltage (between 6 and 10 V depending on the cavity geometry), a high acid flow (25 L= min), and a low acid temperature (20°C) are considered as promising parameters. Such a recipe has been tested on single-cell and nine-cell International Linear Collider (ILC) as well as 704 MHz five-cell Super Proton Linac (SPL) cavities. Single-cell cavities showed similar performances at 1.6 K being either vertically or horizontally electropolished. The applied baking process provides similar benefit. An asymmetric removal is observed with faster removal in the upper half-cells. Multicell cavities (nine-cell ILC and five-cell SPL cavities) exhibit a standard Q 0 value at low and medium accelerating fields though limited by power losses due to field emitted electrons.

Effort towards symmetric removal and surface smoothening of 1.3-GHz niobium single-cell cavity in vertical electropolishing using a unique cathode

“…Although a higher flow of the electrolyte on an Nb surface enhances the EP rate, we believe that a higher electrolyte flow on the top iris owing to electrolyte circulation [18,23] and gravity as stated in Ref. [22] might not be the main reasons for the higher removal at the top iris.…”

“…The removal thickness trends clearly show that Nb removal was the highest at the top iris and the lowest at the bottom iris, as was also reported in Refs. [22,23].…”

“…In contrast to the advantages, a VEPtreated cavity is usually found to be inhomogeneously polished along its length [23], where the upper half cell always shows the highest removal of Nb. It was reported that a higher flow rate of electrolyte on the upper half cell [18,23] or gravity [22] might result in a thin viscous layer, and hence higher removal at the upper half cell. The top iris surface usually suffers from bubble traces as well, since hydrogen (H 2 ) gas bubbles generating on an Al cathode reach the iris surface.…”

“…Research on VEP of the cavities is being carried out at different laboratories [17][18][19][20][21][22]. In contrast to the advantages, a VEPtreated cavity is usually found to be inhomogeneously polished along its length [23], where the upper half cell always shows the highest removal of Nb. It was reported that a higher flow rate of electrolyte on the upper half cell [18,23] or gravity [22] might result in a thin viscous layer, and hence higher removal at the upper half cell.…”

Challenges and opportunities in next generation of electropolishing surfaces

“…On the other hand, projects such as, Continuous Electron Beam Accelerator Facility upgrade, X-ray Free Electron Laser linac, and International Linear Collider, need high performance on Nb cavities. EP has become the reference process to reach the surface requirements of these cavities [5].…”