CCR’s 1.5 MW CW Brewster Window is currently being tested at General Atomics using a gyrotron at the DIII-D tokamak facility. The window is installed in transmission line between the Mirror Optical Unit (MOU) and high power load. The tests are characterizing the performance, including reflection and potential mode conversion and will eventually transmit the highest RF power level available from the gyrotron. Cameras will look for arcing and power deposition in the CVD diamond window. Testing will continue for the next couple of months, depending on availability of the gyrotron and associated power supplies. Test results will be presented at the IR & MM-Wave Conference in Copenhagen in September.