An apparatus and methods for forming a diamond film, are provided. An example of an apparatus for forming a diamond film includes an electrodeless microwave plasma reactor having a microwave plasma chamber configured to contain a substrate and to contain a reactant gas excited by microwaves to generate a microwave plasma discharge. Gas injection ports extend through an outer wall of the plasma chamber at a location upstream of the plasma discharge and above the substrate. Gas jet injection nozzles interface with the gas injection ports and are configured to form a directed gas stream of reactant gas having sufficient kinetic energy to disturb a boundary layer above an operational surface of the substrate to establish a convective transfer of the film material to the operational surface of the substrate.