The invention relates to an apparatus for performing a plasma chemical vapour deposition process. The apparatus comprises a mainly cylindrical resonator being provided with an outer cylindrical wall enclosing a resonant cavity extending in a circumferential direction around a cylindrical axis. The resonator is further provided with side wall portions bounding the resonant cavity in the cylindrical direction, and with a slit configuration extending in a circumferential direction around the cylindrical axis providing access from the resonant cavity radially inwardly. Further, the slit configuration includes slit sections that are mutually offset in the cylindrical direction.
