Designing and Constructing a Controlled-Flow Apparatus to Study the Effect of Surface Flow Velocity on the Quality of Electropolishing of Niobium

The focus of this project was to design and construct a model controlled-flow apparatus to study electropolishing of one cm 2 niobium coupons at surface flow rates typical of cavity processing. A simulation of the apparatus was constructed using CFDesign, a flow and thermal simulation software, to ensure that the selected dimensions may be expected to provide steady-state, laminar flow across the surface of the niobium coupon. Based on these dimensions, a sample system and apparatus was produced to determine the correct reservoir elevation heights for the desired flow rates for fluid viscosities represented in the mixed acid electrolyte. From the CFDesign simulations, it was found that the flow channel supplied laminar flow rates when the center of the niobium coupon was located 40 mm downstream from the inlet stream. The corresponding system, based on the CFDesign simulations, showed that the reservoir elevation heights for flow rates of 0 cm/s to 5 cm/s ranged from 0 to 1.27 cm. The correlation between pressure heads and flow rates has been analyzed and an equation for flow rate was determined using experimental results. The detailed dimensions regarding the flow channel and information regarding the respective pressure heads serve as resources to finding the optimal flow rate for electropolishing the niobium cavities. Although previous research has found a correlation between the quality of electropolishing and internal surface flow rates, research facilities, including Jefferson Lab, did not have the equipment to pursue further analysis. Each nine-cell niobium cavity costs over $50,000, so it is cost prohibitive to use real cavities to conduct early stage research. The prototype built through this research work provides a cost effective alternative. It can be used to validate some of the theoretical results obtained through simulation. In addition, the device allows for easy variable measurements that are either difficult or impractical with an enclosed niobium cavity, as sensors can be embedded into the device in the construction stage. Moreover, the data collected through our experiment furthers superconducting radiofrequency (SRF) technology by allowing Jefferson Lab to design a more effective electropolishing process.