A method for manufacturing a primary preform for optical fibers via an internal plasma chemical vapor deposition (PCVD) process in a hollow silica substrate tube having a supply side and a discharge side includes depositing doped or non-doped silica layers on the inner surface of the hollow substrate tube by supplying glass-forming gasses to the interior of the hollow substrate tube via the supply side thereof, and by creating a plasma reaction zone in the interior of the hollow substrate tube via microwave radiation having a microwave power, wherein the microwave power is decreased during the depositing.