Stable field emission from W tips in poor vacuum conditions

We report a stable field emission ~FE! current from sharp tungsten ~W! tips in relatively poor vacuum ~up to 10 mbar) conditions. We use small tip–anode spacing to keep the extraction voltage low. A simple current regulator circuit, with a bandwidth of ;1.6 kHz, was designed, which controls the voltage applied according to the emission current measured. Without a current regulator circuit, an uncleaned W tip in unbaked 6310 mbar system pressure cannot emit stable FE current and short-term fluctuations at a few nA current were found to be more than 300%. The current regulator circuit improves the FE current stability dramatically. It was observed that at current level of ;3.5 nA and regulation voltage of ;120 V the short-term fluctuations in the current were ;5% at 6310 mbar unbaked system pressure. Subsequently, the system pressure was increased in steps up to 10 mbar of argon gas and it was observed that the current regulator circuit worked at almost the same efficiency. Around 10 mbar of Ar gas pressure larger short-term fluctuations started appearing and around 10 mbar of Ar gas pressure the current regulator circuit failed to regulate the current. In another experiment, at 10 mbar of Ar gas pressure, at ;1.5 and 25 nA current levels long-term FE current stability was recorded. Our results show that the FE current stability in our experiments is much better than that reported in the literature with feedback on the tip–anode distance. This study may find applications in focused electron beam systems as well as in electron impact ion sources. © 2003 American Vacuum Society. @DOI: 10.1116/1.1575760#