Magnetic properties and hydrides precipitation observation of nitrogen doping niobium used for accelerator applications

TL;DR

This study investigates how nitrogen doping affects the magnetic properties and hydrides precipitation in niobium used for superconducting RF cavities, revealing that doping reduces hydrides and alters critical magnetic fields.

Contribution

It provides detailed measurements of magnetic parameters and hydrides precipitation in nitrogen-doped niobium, highlighting the effects of doping and material removal on superconducting properties.

Findings

Superheating field is reduced in doped samples.

Hydrides precipitation decreases with nitrogen doping and material removal.

Optimal doping reduces hydrides effectively.

Abstract

Nitrogen doping study on niobium (Nb) samples used for the fabrication of the superconducting radio frequency (SRF) cavities has been carried out. The samples' surface treatment was attempted to replicate that of the SRF cavities, which includes heavy electropolishing (EP), nitrogen doping and the subsequent EP with different amounts of material removal. The magnetization curves of both the doped and the un-doped samples have been measured, from which the field of the first flux penetration and the upper critical field were extracted. The thermodynamic critical field, the the Ginzburg-Landau parameter, the penetration depth, the coherence length and the superheating field were calculated from the determined reversible magnetization curves. The superheating field of the doped samples is obviously smaller than that of the un-doped samples. Direct observation of hydrides precipitation on both the doped and the un-doped samples has been carried out using Scanning Electron Microscope (SEM) with a cold stand at 80K, which showed that the amount of hydrides precipitation on the doped samples was reduced to varying degrees with different amounts of material removal. Under our doping recipe, the doped sample with subsequent EP material removal of 7-9 microns corresponds to the minimum nitrogen concentration that can effectively reduce the amounts of hydrides precipitation.