Plasma chemical vapor deposition apparatus

A plasma chemical vapor deposition apparatus for deposition of one or more layers of silica onto an interior wall of an elongated hollow glass substrate tube, the apparatus comprising:

• a microwave generator;
• a plasma generator receiving microwaves from the microwave generator in use;
• a cylindrical cavity extending through the plasma generator;
• and
• a cylindrical liner that is transparent to microwave radiation and that is positioned in the cavity, a central axis of the liner being coaxial with a central axis of the cavity, wherein the substrate tube passes through the liner in use, wherein, over a part of the length of the liner, the liner has at least one section having a reduced inner diameter with respect to an inner diameter of a remaining part of the liner, said at least one section having a reduced inner diameter providing a contact zone for the substrate tube;
• wherein the microwave generator is configured to generate microwaves having a wavelength Lw in the range of 40 millimeters to 400 millimeters;
• and
• wherein said at least one section having the reduced inner diameter has a length of at least 1 millimeter and at most 0.1×Lw.