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 H-P 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 H-P. It is observed that the pinning properties and H-P can be altered significantly with the chemical treatment of bulk niobium. A correlation is proposed between H-P of the niobium materials and the anomalous high-field Q drop observed in the superconducting RF cavities fabricated using such niobium material.