Abstract
Baking is necessary to improve high accelerating gradient performances of superconducting niobium cavities. Ten years after this discovery in 1998, the understanding of this effect still resists a lot of theoretical explanations. For the first time, positron annihilation spectroscopy performed on niobium samples reveals the increase after baking of positrons trapped under the Nb surface. Presence of hydrogen-vacancy complexes and their dissociation by baking could both explain rf losses observed at high fields (Q drop) and its cure (baking effect).
Highlights
The ‘‘baking effect’’ refers to the improvement of performances of Nb superconducting radio frequency (SRF) cavities once they have been baked in a narrow range of temperature during a limited time
Results of fine grain (FG) sample analyses by positron annihilation are shown in Figs. 11 and 12; even if the effect is more pronounced for single crystal (SC) samples, we have noted an increase of vacancy sites after baking
Niobium annealing is recommended in cavity preparation to avoid hydride formation at low temperature and prevent rf losses so-called ‘‘Q disease’’ [17]. (ii) Even after this prior thermal treatment of Nb at 800C, new surface contamination by hydrogen is possible during each chemical treatment (BCP or EP)
Summary
The ‘‘baking effect’’ refers to the improvement of performances of Nb superconducting radio frequency (SRF) cavities once they have been baked in a narrow range of temperature during a limited time. An alternative less time consuming process, called ‘‘fast argon baking’’ has been proposed [2,3] and was successfully demonstrated on electropolished single cell cavities [4]. Surface analyses by secondary ion mass spectroscopy (SIMS) [3] and by diffuse x-ray scattering [5] achieved on Nb samples have not shown significant diffusion of interstitial oxygen after baking, as illustrated in Fig. 2 taken from Ref. Even if interstitial oxygen diffusion cannot be involved, other species including hydrogen atoms and vacancies can diffuse at baking process temperatures. 1. ‘‘Fast baking’’ (145C=2 hours) of single cell cavity in oxygen-free atmosphere (the cavity is filled with 1 atm of argon [14]) FIG. 1. ‘‘Fast baking’’ (145C=2 hours) of single cell cavity in oxygen-free atmosphere (the cavity is filled with 1 atm of argon [14])
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