A chemical vapor deposition (CVD) reactor includes a resonating cavity configured to receive microwaves. A microwave transparent window positioned in the resonating cavity separates the resonating cavity into an upper zone and a plasma zone. Microwaves entering the upper zone propagate through the microwave transparent window into the plasma zone. A substrate is disposed proximate a bottom of the plasma zone opposite the microwave transparent window. A ring structure, positioned around a perimeter of the substrate in the plasma zone, includes a lower section that extends from the bottom of the resonating cavity toward the microwave transparent window and an upper section on a side of the lower section opposite the bottom of the resonating cavity. The upper section extends radially toward a central axis of the ring structure. An as-grown diamond film on the substrate is also disclosed.