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

Abstract

Articles you may be interested inThe external Q factor of a dual-feed coupling for superconducting radio frequency cavities: Theoretical and experimental studies Rev. Sci. Instrum. 84, 113304 (2013); 10.1063/1.4828790 Design and performance of a new induction furnace for heat treatment of superconducting radiofrequency niobium cavities Rev. Sci. Instrum. 83, 065105 (2012); 10.1063/1.4725589 Low temperature laser scanning microscopy of a superconducting radio-frequency cavity Rev. Sci. Instrum. 83, 034704 (2012); 10.1063/1.3694570 The Planning of the Cryogenic Supply Infrastructure for the Superconducting Cavities of the European XFEL Linear Accelerator AIP Conf.Effect of low-temperature baking on the radio-frequency properties of niobium superconducting cavities for particle accelerators 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 d niobium nitride (dNbN) with a critical temperature (T c ) % 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 dNbN. 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 dNbN, and XRD data indicated only dNbN on top of the niobium metal.

Laser polishing of niobium for superconducting radio-frequency accelerator applications

“…5. The BCP treated surface shows the The polished surfaces show a concave curve where the PSD values at high frequencies are lower than the BCP curve [22]. Laser treatment at 0.68 and 0.90 J=cm 2 produced a smoothened surface, while 1.13 J=cm 2 laser treatment produced a surface having critical topographic features similar to the starting BCP surface.…”

“…Here a simplified model was used to simulate laser polishing as described in [22]. A one-dimensional conduction heat transfer equation [Eq.…”

Laser polishing of niobium for application to superconducting radio frequency cavities

“…As before, 19 the modeling uses the optical constants for niobium at 1064 nm for calculating the absorption. The pulse duration of our laser (s p ¼ 15 ns) is much longer than the electron collision frequency of the niobium (s ep ffi 60 ps), 20 so that a thermal Fermi distribution can be assumed for the electrons.…”

“…Further, our laser spot (ffi80 lm) is much larger than the thermal diffusion length (% nm), justifying use of a 1-D diffusion equation. 19 To gain additional insight into the effect of workpiece preheat, calculations were carried out assuming the initial surface temperature to be 300, 473, or 673 K. Figure 1 shows the calculated surface temperature of niobium versus fluence, for a single pulse at the three initial surface temperatures. 21 Further details have been reported previously.…”

“…19 Processing is carried out in a vacuum chamber evacuated to 10 À7 Torr by a rotary backed turbo pump. Experiment setup and procedure A PVD Products Inc. PLD 5000 system was used for the experiments as before.…”