Abstract: The present invention relates to an apparatus for carrying out a PCVD process in which one or more doped or undoped glass layers are coated onto the interior of a glass substrate tube. The apparatus comprises an applicator having an inner wall and an outer wall and a microwave guide that opens into the applicator. The applicator extends around a cylindrical axis and which is provided with a passage adjacent to the inner wall, through which the microwaves supplied via the microwave guide can exit, over which cylindrical axis the substrate tube can be positioned, while the applicator is fully surrounded by a furnace that extends over the cylindrical axis.