Evaluation of niobium as candidate electrode material for dc high voltage photoelectron guns

Abstract

The field emission characteristics of niobium electrodes were compared to those of stainless steel electrodes using a DC high voltage field emission test apparatus. A total of eight electrodes were evaluated: two 304 stainless steel electrodes polished to mirrorlike finish with diamond grit and six niobium electrodes (two single-crystal, two large-grain, and two fine-grain) that were chemically polished using a buffered-chemical acid solution. Upon the first application of high voltage, the best large-grain and single-crystal niobium electrodes performed better than the best stainless steel electrodes, exhibiting less field emission at comparable voltage and field strength. In all cases, field emission from electrodes (stainless steel and/or niobium) could be significantly reduced and sometimes completely eliminated, by introducing krypton gas into the vacuum chamber while the electrode was biased at high voltage. Of all the electrodes tested, a large-grain niobium electrode performed the best, exhibiting no measurable field emission (< 10 pA) at 225 kV with 20 mm cathode/anode gap, corresponding to a field strength of 18:7 MV=m.

TiN coated aluminum electrodes for DC high voltage electron guns

“…The evaluation of seven test electrodes at high voltage required several months of effort. Similar to past studies, 5,20 it was not uncommon to observe completely different field emission behavior from nominally identical test samples, upon the first application of high voltage. The variability in results associated with initial high voltage processing of electrodes of the same material and treatment was attributed to undetectable levels of contamination and/or subsurface variations from sample to sample.…”

“…This performance is comparable to that of traditional diamond-paste polished stainless steel. 5 Ti-alloy electrode sample TA1 was subjected to helium gas conditioning, which actually resulted in degraded performance. The post-helium gas conditioning data are not included in Fig.…”

“…Details of the test apparatus can be found in Ref. 5, an abbreviated description is provided here. Cathode electrodes were suspended from a tapered conical insulator that extended into each test chamber (Fig.…”

“…Further details of the cathode electrodes geometry, gap separation, and maximum field strength can be found in Ref. 5.…”

“…18,19 Niobium electrodes perform well at a high voltage when surfaces are etched with a mixture of strong acids-a technique known as buffered chemical polishing (BCP). 5 The BCP process significantly reduces electrode preparation time but not everyone has access to BCP facilities.…”