The present invention relates to an apparatus and a method for an electron beam system for manufacturing a three-dimensional object by fusing successive layers of powder, said system having at least one lens for reshaping of said electron beam, an electron source and a powder bed, said method comprising the step: blocking a selected cross section of said electron beam for controlling the electron beam power. By interference between the electron beam and a beam blocking part a portion of the electron beam is prevented from reaching the powder bed.