On the reliable determination of the magnetic field for first flux-line penetration in technical niobium material

We present a way to reliably determine the field for first penetration HP in various kinds of bulk samples of niobium material used in technical applications like the fabrication of superconducting RF cavities. Special attention is given to the role of flux-line pinning in the determination of HP. It is observed that the pinning properties and HP can be altered significantly with the chemical treatment of bulk niobium. A correlation is proposed between HP of the niobium materials and the anomalous high-field Q drop observed in the superconducting RF cavities fabricated using such niobium material.