Enhanced field emission from chemically etched and electropolished broad-area niobium

Abstract

Electron field emission 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 widely been the subject of 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 depend strongly on surface preparation and handling. Furthermore, 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 the resulting field emission, a dc scanning field emission microscope (SFEM) was built at the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory). Broad-area samples can be moved laterally in a raster pattern (2.5 μm step resolution) under a high-voltage microtip for EFE detection and localization in the SFEM. The emitting sites can then be characterized by scanning electron microscopy and energy dispersive x-ray spectrometry without breaking ultrahigh vacuum. After preparation by chemical etching and electropolishing combined with ultrasonic deionized water rinse, EFE sources from planar Nb have been studied. Emitters have been identified and analyzed, and based on scan results the preparation process has been refined and improved. 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.