Electron field emission (FE) from broad-area metal surfaces is known to occur at a much lower electric field than predicted by the Fowler-Nordheim law. This enhanced field emission (EFE) presents a major impediment to high electric field operation in a variety of applications, e.g., in superconducting niobium radio-frequency cavities for particle accelerators, klystrons, and a wide range of high voltage vacuum devices. Therefore EFE has been the subject of wide fundamental research for years. Although micron or submicron particles are often observed at such EFE sites, the strength and number of emitting sites and the causes of EFE strongly depend on surface preparation and handling, and the physical mechanism of EFE remains unknown. To systematically investigate the sources of this emission and to evaluate the best available surface preparation techniques with respect to resulting field emission, a DC scanning field emission microscope (SFEM) has been built at Thomas Jefferson National Accelerator Facility (Jefferson Lab). Broad-area samples can be moved laterally in a raster pattern (2.5 mu-m step resolution) under a high voltage micro-tip for EFE detection and localization in the SFEM. The emitting sites can then be characterized by SEM and EDX without breaking ultra high vacuum. EFE sources from planar Nb have been studied after preparation by chemical etching and electropolishing combined with ultrasonic deionized water rinse (UWR). Emitters have been identified and analyzed, and the preparation process has been refined and improved based on scan results. With the improved preparation process, field-emission-free or near field-emission-free surfaces at -140 MV/m have been achieved consistently on a number of samples.

Authors: Tong Wang, Charles E. Reece, Ron Sundelin

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