▁field ▁in ▁the ▁proxim ity ▁of ▁the ▁em itting ▁c ath ode ▁surface ▁which ▁has , ▁a ▁radial ▁addition ▁as ▁well ▁as ▁an ▁ax ial ▁component , ▁forcing ▁the ▁electrons ▁in ▁the ▁direction ▁of ▁the ▁axis . ▁Due ▁to ▁this ▁effect , ▁the ▁electron ▁beam ▁converges ▁to ▁some ▁minimal ▁diameter ▁in ▁a ▁plane ▁close ▁to ▁the ▁an ode . ▁ ▁For ▁practical ▁applications ▁the ▁power ▁of ▁the ▁electron ▁beam ▁must , ▁of ▁course , ▁be ▁cont roll able . ▁This ▁can ▁be ▁accomplished ▁by ▁another ▁electric ▁field ▁produced ▁by ▁another ▁c ath ode ▁neg atively ▁charged ▁with ▁respect ▁to ▁the ▁first . ▁ ▁At ▁least ▁this ▁part ▁of ▁electron ▁gun ▁must ▁be ▁ev acu ated ▁to ▁" high " ▁vac u um , ▁to ▁prevent ▁" burn ing " ▁the ▁c ath ode ▁and ▁the ▁emer gence ▁of ▁elect rical ▁dis charg es . ▁ ▁F oc using ▁After ▁leaving ▁the ▁an ode , ▁the ▁diver gent ▁electron ▁beam ▁does ▁not ▁have ▁a ▁power ▁density ▁sufficient ▁for ▁w eld ing ▁met als ▁and ▁has ▁to ▁be ▁focused . ▁This ▁can ▁be ▁accomplished ▁by ▁a ▁magnetic ▁field ▁produced ▁by ▁electric ▁current ▁in ▁a ▁cyl ind rical ▁co il . ▁ ▁The ▁foc using ▁effect ▁of ▁a ▁rotation ally ▁symmet rical ▁magnetic ▁field ▁on ▁the ▁traject ory ▁of ▁electrons ▁is ▁the ▁result ▁of ▁the ▁complicated ▁influence ▁of ▁a ▁magnetic ▁field ▁on ▁a ▁moving ▁electron . ▁This ▁effect ▁is ▁a ▁force ▁proportional ▁to ▁the ▁induction ▁B ▁of ▁the ▁field ▁and ▁electron ▁velocity ▁v . ▁The ▁vector