Method and an apparatus for performing a plasma chemical vapour deposition process and a method

A method for performing a plasma chemical vapour deposition process, comprising the steps of:

• providing an apparatus comprising a mainly cylindrical resonator being provided with an outer cylindrical wall and an inner coaxial cylindrical wall defining therebetween a resonant cavity operable at an operating frequency and extending in a circumferential direction around a cylindrical axis of the inner and outer cylindrical wall, wherein the outer cylindrical wall includes an input port connectable to an input waveguide, wherein the inner cylindrical wall includes slit sections extending in a circumferential direction around the cylindrical axis;
• and operating the apparatus for performing a plasma chemical vapour deposition process at an operating frequency such that a greatest dimension defining the aperture of the slit sections is smaller than half the wavelength of the operating frequency, propagating modes are not allowed to radiate through the aperture of the slit sections, and non-propagating modes are allowed to radiate through the aperture of the slit sections.