▁and ▁t ung sten . ▁With ▁t ung sten ▁c ath odes , ▁emission ▁current ▁dens ities ▁about ▁ 1 0 0   m A / mm 2 ▁can ▁be ▁achieved , ▁but ▁only ▁a ▁small ▁portion ▁of ▁the ▁em itted ▁electrons ▁takes ▁part ▁in ▁beam ▁formation , ▁depending ▁on ▁the ▁electric ▁field ▁produced ▁by ▁the ▁an ode ▁and ▁control ▁electro de ▁volt ages . ▁The ▁type ▁of ▁c ath ode ▁most ▁frequently ▁used ▁in ▁electron - be am ▁w eld ers ▁is ▁made ▁of ▁a ▁t ung sten ▁strip , ▁about ▁ 0 . 0 5   mm ▁thick , ▁sh aped ▁as ▁shown ▁in ▁Fig . ▁ 1 a . ▁The ▁appropriate ▁width ▁of ▁the ▁strip ▁depends ▁on ▁the ▁highest ▁required ▁value ▁of ▁emission ▁current . ▁For ▁the ▁lower ▁range ▁of ▁beam ▁power , ▁up ▁to ▁about ▁ 2   k W , ▁the ▁width ▁w = 0 . 5   mm ▁is ▁appropriate . ▁ ▁Acc eler ation ▁of ▁electrons , ▁current ▁control ▁ ▁Elect rons ▁em itted ▁from ▁the ▁c ath ode ▁possess ▁very ▁low ▁energy , ▁only ▁a ▁few ▁e V . ▁To ▁give ▁them ▁the ▁required ▁high ▁speed , ▁they ▁are ▁acceler ated ▁by ▁a ▁strong ▁electric ▁field ▁applied ▁between ▁the ▁em itter ▁and ▁another , ▁posit ively ▁charged , ▁electro de , ▁namely ▁the ▁an ode . ▁The ▁acceler ating ▁field ▁must ▁also ▁navigate ▁the ▁electrons ▁to ▁form ▁a ▁narrow ▁conver ging ▁“ bundle ” ▁around ▁the ▁axis . ▁This ▁can ▁be ▁achieved ▁by ▁an ▁electric