vel " ▁only ▁a ▁very ▁small ▁distance ▁below ▁the ▁surface ▁before ▁they ▁transfer ▁all ▁their ▁kin etic ▁energy ▁into ▁heat . ▁This ▁distance ▁is ▁proportional ▁to ▁their ▁initial ▁energy ▁and ▁in vers ely ▁proportional ▁to ▁the ▁density ▁of ▁the ▁solid . ▁Under ▁conditions ▁usual ▁in ▁w eld ing ▁practice ▁the ▁" tra vel ▁distance " ▁is ▁on ▁the ▁order ▁of ▁hundred th s ▁of ▁a ▁millimeter . ▁Just ▁this ▁fact ▁enables , ▁under ▁certain ▁conditions , ▁fast ▁beam ▁pen etr ation . ▁ ▁Pen etr ation ▁of ▁the ▁electron ▁beam ▁The ▁heat ▁contribution ▁of ▁single ▁electrons ▁is ▁very ▁small , ▁but ▁the ▁electrons ▁can ▁be ▁acceler ated ▁by ▁very ▁high ▁volt ages , ▁and ▁by ▁increasing ▁their ▁number ▁( the ▁beam ▁current ) ▁the ▁power ▁of ▁the ▁beam ▁can ▁be ▁increased ▁to ▁any ▁desired ▁value . ▁By ▁foc using ▁the ▁beam ▁onto ▁a ▁small ▁diameter ▁on ▁the ▁surface ▁of ▁a ▁solid ▁object , ▁values ▁of ▁plan ar ▁power ▁density ▁as ▁high ▁as ▁ 1 0 4 ▁up ▁to ▁ 1 0 7   W / mm 2 ▁can ▁be ▁reached . ▁Because ▁electrons ▁transfer ▁their ▁energy ▁into ▁heat ▁in ▁a ▁very ▁thin ▁layer ▁of ▁the ▁solid , ▁as ▁explained ▁above , ▁the ▁power ▁density ▁in ▁this ▁volume ▁can ▁be ▁extremely ▁high . ▁The ▁volume ▁density ▁of ▁power ▁in ▁the ▁small ▁volume ▁in ▁which ▁the ▁kin etic ▁energy ▁of ▁the ▁electrons ▁is ▁transformed ▁into ▁heat ▁can ▁reach ▁values ▁of ▁the ▁order ▁ 1 0 5 ▁– ▁ 1 0 7   W / mm