Darmstadt 2016 – wissenschaftliches Programm
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AKBP: Arbeitskreis Beschleunigerphysik
AKBP 4: RF, Resonators and Applications II
AKBP 4.2: Vortrag
Montag, 14. März 2016, 16:45–17:00, S1/05 24
Suppression of field emission from Nb and Mo surfaces by insulating oxides — •Stefan Lagotzky and Günter Müller — University of Wuppertal, D-42097 Wuppertal, Germany
Parasitic field emission (FE) from particulates and surface defects is one of the main field limitations of superconducting Nb cavities and leads to dark current (DC) from the Mo substrates of semiconducting photocathodes. The activation field Eact of typical emitters and the emitter number density N is strongly influenced by the surface oxide thickness dox. Reduction of these oxides, e.g. by heat treatments (HT), leads to activation of new emitting sites [1-2] with N ∼ exp(−dox). Since the achievable surface quality, however, is not yet sufficient for future ILC accelerating structures [3] and actual photoinjectors, thicker oxides are promising to reduce FE and DC, respectively. FE measurements of thermally oxidized and dry ice cleaned single crystal Nb samples (dox ∼ 100 nm) revealed a reduction of N by a factor 20 at 70 MV/m, i.e. the intended electric peak field of future ILC cavities. The remaining FE was caused by surface defects and partially molton features with onset fields Eon above 90 MV/m. Furthermore, oxygen exposure of a Mo sample partially weakened or deactivated emission sites, which were previously activated by HT at 400-600∘C with Eon = 20 MV/m.
[1] A. Navitski et al., Phys. Rev. ST-AB 16, 112001 (2013).
[2] S. Lagotzky et al., Eur. Phys. J. Appl. Phys. 70, 21301 (2015).
[3] S. Lagotzky u. G. Müller, Nucl. Instrum. Meth. A 806, 193 (2016).
This work was funded by BMBF projects 05H12PX6 and 05K13PX2.