▁a ▁high ▁voltage ▁source ▁( typ ically ▁~ 1 0 0 – 3 0 0 ▁k V ) ▁and , ▁given ▁sufficient ▁current , ▁the ▁gun ▁will ▁begin ▁to ▁emit ▁electrons ▁either ▁by ▁therm ionic ▁or ▁field ▁electron ▁emission ▁into ▁the ▁vac u um . ▁In ▁the ▁case ▁of ▁a ▁therm ionic ▁source , ▁the ▁electron ▁source ▁is ▁typically ▁mounted ▁in ▁a ▁We hn elt ▁cyl inder ▁to ▁provide ▁pre lim inary ▁focus ▁of ▁the ▁em itted ▁electrons ▁into ▁a ▁beam ▁while ▁also ▁stabil izing ▁the ▁current ▁using ▁a ▁pass ive ▁feedback ▁circuit . ▁A ▁field ▁emission ▁source ▁uses ▁instead ▁electro static ▁electro des ▁called ▁an ▁extract or , ▁a ▁suppress or , ▁and ▁a ▁gun ▁l ens , ▁with ▁different ▁volt ages ▁on ▁each , ▁to ▁control ▁the ▁electric ▁field ▁shape ▁and ▁intensity ▁near ▁the ▁sharp ▁tip . ▁The ▁combination ▁of ▁the ▁c ath ode ▁and ▁these ▁first ▁electro static ▁l ens ▁elements ▁is ▁often ▁collect ively ▁called ▁the ▁" elect ron ▁gun ." ▁After ▁it ▁leaves ▁the ▁gun , ▁the ▁beam ▁is ▁typically ▁acceler ated ▁by ▁a ▁series ▁of ▁electro static ▁pl ates ▁until ▁it ▁reaches ▁its ▁final ▁voltage ▁and ▁enters ▁the ▁next ▁part ▁of ▁the ▁mic ros cope : ▁The ▁cond enser ▁l ens ▁system . ▁These ▁upper ▁l enses ▁of ▁the ▁T EM ▁then ▁further ▁focus ▁the ▁electron ▁beam ▁to ▁the ▁desired ▁size ▁and ▁location ▁on ▁the ▁sample . ▁ ▁Man ip ulation ▁of ▁the ▁electron ▁beam ▁is ▁performed ▁using ▁two ▁physical ▁effects . ▁The ▁interaction ▁of ▁electrons ▁with