Development of an advanced electropolishing setup for multicell high gradient niobium cavities

Abstract

Reproducible operation at high performances of superconducting cavities is required for linear accelerators. High beta elliptical cavities are thus of concern and, to achieve required performances for such resonators, surface preparation including electropolishing is recommended. We have designed and operate a setup for electropolishing in the vertical position of multicell cavities in order to: (i) obtain high yield with large elliptical cavities for Superconducting Linac (SPL) or European Spallation Source projects; (ii) develop a reference installation demonstrating that this process is appropriate for the large scale treatment of cavities in industry. The setup described here is the first one able to electropolish vertically multicell cavities with circulating acid and high safety standards. This equipment makes it possible to use a wide range of parameters such as voltage, acid flow rate, temperature, and nitrogen injection with an R&D purpose in mind. Optimization is studied using modeling with COMSOL software for different cavities. As examples, we present some results for the 704 MHz high-beta SPL cavity and the 1300 MHz International Linear Collider cavity and show the influence of cathode shape on both acid flow and electric field distribution during the process. Importance of the size of the cavity and first results achieved on single-cell and nine-cell cavities will be discussed.

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

“…The same set of parameters (6 V − 20 L= min − T < 20°C) was applied on a nine-cell cavity: 50 μm on average were removed from the "TB9R1025" ILC cavity from Fermi Accelerator National Laboratory (FERMILAB), previously horizontally electropolished. A large cathode diameter (50 mm) was used for nine-cell VEP experiments in [5] in order to decrease the anodic current density. The diameter was reduced down to 40 mm for this experiment in order to avoid contact with HOM antennas during insertion.…”

“…Vertical electropolishing at a voltage of 20 V has already been investigated on single-cell cavities [5], and a gradient of 30 MV=m has been achieved. In a second step, single-cell cavities have been electropolished with parameters derived from standard horizontal EP: (i) moderate acid flow (8 L= min); (ii) voltage above 12 V; (iii) temperature around 30°C.The electrolyte is a mixture of HFð40w%Þ − H 2 SO 4 ð96w%Þ in volume proportions of 1 to 9.…”

“…The cathode chosen for these experiments consists in a rod shape with a small protuberance (20 mm in length and 50 mm in diameter). This cathode is used in [5].…”

“…According to the standard process, the cavity is electropolished in a horizontal position, while rotating and half filled with circulating acid. Vertical electropolishing (VEP) is studied in some laboratories as an alternative [4][5][6][7][8][9][10]. The aim is to develop an easier process compared to horizontal electropolishing (HEP), while providing similar performance, in order to achieve a cost-effective production of cavities for upcoming largescale projects in the near future.…”

“…The cavity is filled from the bottom and the acid runs back to the tank by gravity from the top of the setup. The technical characteristics of this setup are detailed elsewhere [5]. It has been designed for the treatment of elliptical cavities, from 1.3 GHz single-cellto 704 MHz five-cell cavities designed for the Superconductive Proton Linac (SPL) [13].…”