| | Appreciable suppression of field emission from
metallic surfaces has been achieved by the use of
improved surface cleaning techniques, and dry ice
cleaning has emerged recently as a very effective tool in
this respect. In order to understand the effects of surface
preparation on field emission, systematic measurements
were performed on five single crystal and three large
grain samples of high purity (RRR > 300) Nb by means of
AFM, XRD, SEM and dc field emission scanning
microscope. The samples were treated with buffered
chemical polishing (BCP), half of those for 30 um and
others for 100 um removal of surface damage layer,
followed by a final high pressure water rinsing. The
samples with longer BCP treatment showed the onset of
field emission at slightly higher fields. A low temperature
(~ 150 degree C) heat treatment in high vacuum (10-6 mbar)
chamber for 14 hours, on a selected large grain Nb
sample, gives the evidence for the grain boundary assisted
FE at very high fields of 250 and 300 MV/m. Finally, an
interesting correlation between sizes of all investigated
emitters derived from SEM images with respect to their
respective onset fields has been found, which might
facilitate the quality control of superconducting radiofrequency
cavities for linear accelerators. | |