In the frame of the EUROfusion Work Package Heating and Current Drive (WP HCD) of the Power Plant Physics and Technology (PPPT) program, CVD diamond disk Brewster-angle windows for gyrotron operation at multi-megawatt RF power levels and long pulses are under development. These windows allow for frequency step-tuneable operation. The Brewster-angle of 67.2° for diamond leads to an elliptical connection of the disk to the copper waveguides (WGs), requiring an advanced joining process. For proper transmission of the RF power, the disk consists of low loss CVD diamond of optical grade. The current target for the WG aperture of DEMO is 63.5 mm. It allows for an RF power transmission of 2 MW, but it requires a disk diameter of 180 mm for the 67.2° angle. In addition, a thickness of approximately 2 mm is needed to achieve the proper mechanical stability. State of the art microwave plasma reactors are not capable of growing disks of such size. The maximum available diameter of a polycrystalline CVD diamond disk suited to microwave applications is currently 140 mm. Thus, the industrial partner Diamond Materials GmbH (Freiburg, Germany) is doing extensive diamond growth experiments. A first of its kind, 180 mm thermal grade, crack-free, diamond disk was produced in the microwave plasma reactor with an average unpolished thickness of about 2 mm. First loss tangent measurements have been also performed. This presentation describes the steps and the first results of this non-straightforward path, a challenging new field for diamond manufacturers and a major breakthrough for future frequency step-tuneable operation.