Nitrogen infusion R&D at DESY a case study on cavity cut-outs

Abstract

A first series of nitrogen infusion runs of 1.3 GHz single-cell cavities at DESY resulted in an unexpected and severe deterioration observed during the vertical cold test. To investigate the origin of the deterioration, one of the cavities underwent extensive radio-frequency measurements and a temperature- and magnetic field-mapping was performed in collaboration with the Helmholtz-Zentrum Berlin. After combining all results, regions of interests were identified and cut-out of the cavity. Subsequent surface analysis techniques (EBSD, PALS, PIXE, SEM/EDX, SIMS, XPS) were applied in order to identify the microscopic origin of the deterioration and especially the differences between hot and cold spots as well as quench spots. An excess of niobium carbides, reducing the thermal conductivity, was identified as the probable cause for the deterioration, and the size- and density-distributions were observed to be crucial for the resulting performance reduction. The origin for the local differences in the niobium carbide formation between hot and cold spots is an effect of preexisting variations of the crystal structure.

Systematic study of niobium thermal treatments for superconducting radio frequency cavities employing x-ray photoelectron spectroscopy

“…An extensive study of the origin of the deteriorated cavity performance upon 120 • C-nitrogen infusion was performed at DESY [17]. The structural changes of niobium surface have been verified both on cavity cut-outs and reference samples treated in the same experiment run.…”

“…Thus, as compared to traditional cavity processing, nitrogen gas is added during the final heat treatment and is considered necessary for the improvement of cavity performance characteristics. It should be pointed out that successful infusion has proved hard to achieve by some research institutes [15][16][17].…”

“…By now, several investigations exploring the niobium treated with nitrogen infusion have been published [12,[16][17][18][19]. In [12], the pioneering work for nitrogen infusion, secondary electron mass spectroscopy (SIMS) was used to compare the infusion conducted at 120 • C and 160 • C. According SIMS, at 160 • C less Nb 2 O 5 and NbO 2 oxides formed, but more carbon and nitrogen reacted with niobium as compared to 120 • C. It was suggested that the infusion step with N 2 supply may decrease the size or amount of carbide precipitates that were observed with the transmission electron microscopy.…”

RF surface resistance tuning of superconducting niobium via thermal diffusion of native oxide

“…Interestingly, the role of adventitious oxygen in RF surfaces exposed to nitrogen at modest (200-400°C) temperatures (sometimes termed "nitrogen infusion") may have been underestimated. Near surface oxygen is observed to be nearly two orders of magnitude greater than bulk background [68][69][70][71][72]. Future analysis will seek to clarify if indeed oxygen is the dominant agent in such circumstances.…”

Influence of Furnace Baking on Q-E Behavior of Superconducting Accelerating Cavities

“…The nitrogen doping process has been applied in the Linac Coherent Light Source (LCLS−II) cavity fabrication process 11,12 because it is highly reproducible and has resulted in high Q 0 and anti-Q slope in several studies 7 . In the nitrogen infusion technique, the Q-E behavior does not change significantly, and both Q 0 and E acc are improved compared with those values obtained using the standard surface treatment methods [13][14][15] ; however, the reproducibility has been limited to a few laboratories. The two-step baking process developed at Fermi National Accelerator Laboratory (FNAL) can produce cavities with a maximum E acc of approximately 50 MV/m 9,16 , and other laboratories are currently verifying the effectiveness of two-step baking.…”