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

An apparatus for performing a plasma chemical vapour deposition process, 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, comprising a wavelength, 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, the inner cylindrical wall includes slit sections extending in a circumferential direction around the cylindrical axis, each of the slit sections comprising a respective aperture, a greatest dimension defining the respective aperture of each of the slit sections being smaller than half the wavelength of the operating frequency, a circumferential dimension of each of the slit sections is smaller than half the wavelength of the operating frequency, the slit sections are staggered on two slit lines circumscribing the cylindrical axis, wherein the slit sections are mainly evenly distributed in the circumferential direction, and the apparatus being operable at the operating frequency such that the greatest dimension defining the respective aperture of each of the slit sections is smaller than half the wavelength of the operating frequency, and such that propagating modes are not allowed to radiate through the respective aperture of each of the slit sections, and non-propagating modes are allowed to radiate through the respective aperture of each of the slit sections.