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 me tallic cathodes and their reduction via improved preparation technique s, the distribution of emission sites, of onset field strengths E(on) and of the Fowler-Nordheim parameters beta and S on mechanically and c hemically polished Nb, Cu, and Al cathodes have been investigated syst ematically, Very low emission densities at field strengths of 200 MV/m have been achieved on chemically polished niobium and mechanically po lished copper cathodes with 16.6 and 10.3 emitters/cm(2), respectively . Five different emitter types have been localised: particles, surface defects, sulphur-contaminated etched defects, foreign-element inclusi ons with damaged morphology and molten craters after discharges. No de pendence of emission behaviour on cathode material was observed within these emitter categories, Intrinsic emission from the metal or its ox ides was never detected up to 200 MV/m. The determined values of the F owler-Nordheim parameter S showed a surprisingly large spread (1 x 10( -17) cm(2) to 6 x 10(+15) cm(2)). After a first ''switch on'' in a hig h electrical field, many emitters were already active at similar to 70 % of that field.