Soft chemical polishing and surface analysis of niobium samples

The Superconducting Darmstadt Linear Accelerator S-DALINAC uses twelve Niobium Cavities with a RRR of 280-300 which are operated at 2 K. The operating frequency is 3 GHz; the design value of the accelerating gradient is 5 MV/m. To achieve the target value of 3 centerdot 109 for the unloaded quality factor Q0, different surface preparation methods were applied and systematically tested using a vertical 2 K cryostat. A well-established technique is the so called Darmstadt Soft Chemical Polishing, which consists of an ultrasonic cleaning of the cavity with ultrapure water (UPW) followed by oxidizing the inner surface with nitric acid (65% HNO3). After rinsing with UPW again, the niobium oxide layer is removed with hydrofluoric acid (40% HF) in a separate second step. Finally, the structure is rinsed with UPW and then dried by a nitrogen flow. Until now each cavity in operation was chemically treated with a proven record of success. In order to understand and to optimize the process on the niobium surface, systematic tests with samples were performed. The samples were analyzed using material science techniques like Scanning Electron Microscopy (SEM), Secondary Ion Mass Spectrometry (SIMS) and Energy Dispersive X-ray Spectroscopy (EDX). This paper will report on the results of our research and we will give a review on our experiences with varied chemical procedures