Nitrogen-doped niobium for superconducting RF cavities

Niobium is the standard material for superconducting RF (SRF) cavities. Superconducting materials with higher critical temperature or higher critical magnetic field allow cavities to work at higher operating temperatures or higher accelerating fields, respectively. Enhancing the surface properties of the superconducting material in the range of the penetration depth is also beneficial. One direction of search for new materials with better properties is the modification of bulk niobium by nitrogen doping. In the Nb-N phase diagram the cubic δ-phase of NbN has the highest critical temperature (16 K). Already slight nitrogen doping of the α-Nb phase results in higher quality factors. In this contribution the status of the refurbished UHV-furnace, located at the Institute of Nuclear Physics in Darmstadt, and the results of the N-doping experiments will be presented. Nb-sheets will be used to optimize the recipe to obtain the lowest RF resistance. The structural and superconducting properties of the samples will be investigated at the department of Materials Research.