Vertical electropolishing for 1.3 GHz single- and nine-cell superconducting niobium cavities: A parametric optimization and rf performance

Abstract

Electropolishing (EP) is a standard process for the treatment of the interior surface of niobium (Nb) superconducting radio-frequency (SRF) cavities used in particle accelerators. We previously conducted a study on vertical electropolishing (VEP), in which the cavity was set vertically during EP of a 1.3 GHz Nb single-cell cavity. In that study, we showed that the major cause of asymmetric removal in VEP is the accumulation of hydrogen gas bubbles, generated during the EP process, on the upper half cell of the cavity. Three types of unique rotating cathodes were tested with the aim to resolve the issues of asymmetric removal and rough surface. Appropriate cathode design and parameters reduced the asymmetric removal in the cell and yielded a smooth surface of the cavity. In this study, we have performed an extensive investigation on VEP of both single-and nine-cell Nb 1.3 GHz cavities with a modified cathode. The VEP parameters with single-and nine-cell coupon cavities were investigated, which facilitate an in situ study of the coupon currents at various positions of the cavities and observation from the viewports located near the iris and equator positions of the cavities. The modified cathode and optimized VEP process for the singlecell cavity yielded uniform removal in the cavity cell and a smooth surface of the interior of the cavity. A novel acid flow method that allows the separation of the acid flow in the cavity and cathode housing was employed in the nine-cell cavity to efficiently remove the gas bubbles from the cavity. The optimized acid flow rates in the cavity and cathode housing, along with other adequate parameters including temperature, cathode rotation speed, and applied voltage, made the surface smooth and significantly reduced the removal nonuniformity in the nine-cell cavity. The single-and nine-cell cavities after an average removal of 46 and 130 μm, respectively, were tested in a vertical cryostat. The single-and nine-cell cavities achieved 31 MV=m at a Q 0 value of 9 × 10 9 and 28.3 MV=m at a Q 0 value of 6.7 × 10 9 , respectively, in the rf tests at the temperature of 2 K. The achieved SRF performances were as good as the baseline performances achieved after the horizontal electropolishing process of the same cavities.

Latest development of electropolishing optimization for 650 MHz Niobium cavity

“…Consequently, the average I-V curve in EP might reach a plateau at a lower voltage than that required for the equator surface. An EP study conducted on a 9-cell cavity demonstrated notable differences between the average I-V curve and measurements obtained solely from isolated coupons located at the equator, as described in my paper [2].…”

“…Electropolishing (EP) is a widely utilized technique for treating metal surfaces. It is commonly employed to process niobium made superconducting radiofrequency (SRF) cavities [1][2][3], which are key components in particle accelerators. These cavities are operated at high frequencies, typically ranging from several hundred megahertz to a few gigahertz, in order to accelerate charged particles.…”

An Innovative Approach of Surface Polishing for SRF Cavity Applications

“…Standard mechanical polishing can provide very smooth surfaces but a damaged layer (about 100 µm on Nb) is still present at the surface. Buffered chemical polishing (BCP) and electropolishing (EP) are conventional methods used to remove the damaged layer and prepare cavities [9,10]. BCP is composed of acids such as HNO 3 , H 3 PO 4 , and HF at a typical 1:2:1 ratio.…”

Development and Understanding of Nb3Sn films for radiofrequency applications through a sample-host 9-cell cavity

“…The cavity could be used for parameter optimization in conjunction with RF tests of actual cavities with the corresponding geometry, to help provide an understanding of how the parameters are affecting the local microstructure and connect that to the overall RF performance. Sample-host cavities have been used in the past to study other SRF processing techniques (Cooper and Cooley 2013, Lee et al 2017, Chouhan et al 2019 From a materials science perspective, the Nb 3 Sn vapor diffusion process is not well understood. There are a number of empirical observations that have been made in the past that have led to the coating procedures currently used.…”