Aging of theHF−H2SO4electrolyte used for the electropolishing of niobium superconducting radio frequency cavities: Origins and cure

Abstract

Electropolishing (EP) in the HF-H 2 SO 4 electrolyte is the most desirable surface treatment for niobium superconducting radio frequency cavities yet demonstrated, in terms of performance and surface finish. However, the efficiency of the electrolyte declines quickly with time (decrease in removal rate, deterioration of the niobium surface, increased sulfur generation). Previous studies at CEA Saclay have highlighted the impact of the water content in EP mixtures rather than the content of dissolved niobium. Knowledge of the electrochemical system was improved thanks to studies using a rotating disk electrode (RDE). Measurements with a RDE give precious information concerning mass transport of the different ionic groups present in the solution. The performed measurements prove that EP is controlled by the diffusion of fluorine ions and the value of the related diffusion coefficient D FÀ was estimated for different mixtures. Electrochemical impedance spectroscopy (EIS) measurements were also performed with different EP mixtures. Both volt ampere metric and EIS measurements prove the central role of fluorine during EP and show that EP mechanisms evolve with the aging of the bath. Another major problem related to electrolytes is the formation of impurities such as sulfur. We have proved that working at a reduced voltage of 5 V does not alter cavity performance and makes it possible to reduce the undesirable particulate contamination in electrolytes and to increase their lifetime.

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

“…Nitrogen is also blown in the acid tank and at the top end group of the cavity in order to prevent any explosion risk and to favor the removal of the gas generated during the process. Previous EP investigations showed that a reduced electropolishing voltage (down to 5 V) has no influence on cavity performance (a process called "low-voltage electropolishing") [14]. We decided to apply low-voltage VEP with the following expectations: (i) a decreased joule heating and decreased temperature gradient in the cavity and (ii) reduced parasitic electrochemical reactions as sulfur forms [14][15][16][17].…”

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

“…Experiments on samples and on cavities were carried out in different institutes for precise understanding of the niobium electropolishing process [10][11][12][13].…”

“…The studies using rotating disk electrodes (RDE) and electrochemical impedance spectroscopy measurements [11][12][13] have demonstrated the dominant role of fluorine diffusion as a limiting electrochemical step in the presence of a surface film considered as a salt film [11] or porous oxide [13]. As a result, current density will be dominated by the concentration gradient in the F À ion through the diffusion layer once the niobium surface temperature is controlled.…”

“…Electropolishing at 5 V was successfully tested with horizontal setup [25]. In fact sulfur is known to be generated during EP [26][27][28] and a lower voltage is more desirable to avoid sulfur generation at high overpotential due to cathodic reaction [13]. Vertical EP was carried out at 10 V. Brightening of the surface has been achieved after 10 m removal.…”

Superconducting Radio-Frequency Technology R&D for Future Accelerator Applications

“…The typical physical arrangement mounted first by KEK for the TRISTAN cavities has the cavity and collinear cathode horizontal with the electrolyte filling approximately 60% of the cavity [72]. Recent involvement of electrochemical insights and methods in Nb EP research has yielded a clarified understanding of the polishing mechanism and in turn provided pointers toward process optimization [73][74][75]. Identification of the importance of managing process temperature control for both uniform removal and optimum surface leveling, together with recognition of the source and methods for reducing and removing precipitating sulfur have borne fruit in increased cavity performance for the ILC R&D effort [17], the JLab 12 GeV Upgrade cavities [76,77], the ANL QWR cavities [19], and the high-grad cavity set for the XFEL [18], among others.…”