Various embodiments of the present invention relate to a method for operating an additive manufacturing apparatus in which a three-dimensional article is formed. Said method comprising the steps of: providing a vacuum chamber having at least a first and a second section, wherein said first and second sections are openly connected to each other, providing a predetermined vacuum level inside said vacuum chamber, providing a layer of powder material on a work table in said first section of said vacuum chamber, directing an electron beam from said at least one electron beam source provided in said second section over said work table to fuse in first selected locations according to said model to form a first cross section of said three-dimensional article, purging said second section with a dry gas when said vacuum chamber is open for prohibiting ambient air into said second section.