Laser nitriding of niobium for application to superconducting radio-frequency accelerator cavities

Abstract

Particle accelerators are a key tool for scientific research ranging from fundamental studies of matter to analytical studies at light sources. Cost-for-performance is critical, both in terms of initial capital outlay and ongoing operating expense, especially for electricity. The major factor is the niobium superconducting radio frequency (SRF) accelerator cavities at the heart of many of these machines. Presently, niobium SRF cavities operate near 1.9 K, well below the 4.2 K atmospheric boiling point of liquid helium to obtain sufficient performance. The consequent electric power costs are the most significant limit to operate the SRF cavities at 1.9 K. Transforming the cavity interior surface from niobium to δ niobium nitride (δNbN) with a critical temperature (Tc) ≅ 17 K instead of 9.2 K, appears to be a promising approach to raising the operating temperature. The traditional furnace method has nitrided niobium, but apparently have not been able to obtain δNbN.1 Moreover, furnace nitriding requires exposing the complete SRF cavity to an aggressive time-temperature history, risking mechanical distortion. As an alternative, laser gas nitriding has been applied successfully to a number of metals.2 A very recent review is available.3 The beam dimensions and thermal diffusion length permit modeling in one dimension to predict the time course of the surface temperature for a range of per-pulse energy densities. As with the earlier work,2 we chose conditions just sufficient for boiling of the niobium surface as a reference point. The treated materials were examined by scanning electron microscopy (SEM), electron probe microanalysis and x ray diffraction (XRD). The SEM images show a sharp transition with fluence from a smooth, undulating topography to significant roughening, interpreted here as the onset of ablation. Electron probe microanalysis measurements found a constant value of the nitrogen/niobium atom ratio to depths greater than the SRF active layer thickness. Certain irradiation conditions resulted in atomic ratio values consistent with formation of δNbN, and XRD data indicated only δNbN on top of the niobium metal.