Comparative studies on enhanced field emission from mechanically and chemically polished broad-area Nb, Cu, and Al cathodes

For an understanding of enhanced electron field emission (EFE) from metallic cathodes and their reduction via improved preparation techniques, the distribution of emission sites, of onset field strengths Eon and of the Fowler-Nordheim parameters β and S on mechanically and chemically polished Nb, Cu, and Al cathodes have been investigated systematically. Very low emission densities at field strengths of 200 MV/m have been achieved on chemically polished niobium and mechanically polished copper cathodes with 16.6 and 10.3 emitters/cm2, respectively. Five different emitter types have been localised: particles, surface defects, sulphur-contaminated etched defects, foreign-element inclusions with damaged morphology and molten craters after discharges. No dependence of emission behaviour on cathode material was observed within these emitter categories. Intrinsic emission from the metal or its oxides was never detected up to 200 MV/m. The determined values of the Fowler-Nordheim parameter S showed a surprisingly large spread (1 × 10−17 cm2 to 6 × 10+15 cm2). After a first “switch on ” in a high electrical field, many emitters were already active at ∼ 70% of that field.