INFLUENCE OF SURFACE-ROUGHNESS AND PREPARATION, BULK PURITY AND HEAT-TREATMENT ON ELECTRON FIELD-EMISSION FROM NB AND NB3SN

DC field emission studies on cm(2)-sized Nb samples are presented in d ependence on wet surface preparation, properties of bulk and surface, heat treatments and after coating with Nb3Sn. A strong reduction of en hanced field emission (EFE) from Nb up to 200 MV/m was achieved by an improved chemical surface treatment. The density of emitters was 2-3 t imes higher on chemically polished Nb of low purity (RRR 30) than of h igh purity (RRR 250) material. Also the activation of emission from fo rmerly inactive particles by heat treatments at 400 degrees C in UHV d epended strongly on the bulk purity. First studies on Nb3Sn coatings o n Nb, which would allow a reduction of the operational costs of accele rating structures, showed encouraging low emitter densities up to 70 M V/m. A statistical evaluation of the emission parameters of a large nu mber of emitters resulted in a new correlation of the Fowler-Nordheim parameters beta and S. The data could not be explained by pure geometr ical field enhancement alone, but suggest a more complex process of en hanced field emission.