High power 170 GHz gyrotron with synthetic diamond window

A large sized synthetic diamond window assembly was installed on a 170 GHz gyrotron. An aperture and a thickness of the window are 83 and 2.23 mm, respectively, whose edge was directly cooled by water. Gyrotron performances of 520 kW at 6.2 s and 450 kW at 8 s were attained. A temperature increase of the window stabilized after ∼5 s, whereas the power had been limited to below 170 kW with conventional windows. This drastic improvement of a deliverable power was obtained from the outstanding properties of the diamond, that is, an extremely high thermal conductivity (∼1800 W/mK) and a low value of loss tangent (tan δ<1.3×10−4). The gyrotron was inspected after the ∼1300 times operation at 520 kW (>1 s), but no damage nor trouble was found both on the gyrotron and the diamond window. These results give a prospect for a multimegawatt power output from the gyrotron, i.e., the diamond window gives a solution for the window problem which has been regarded as the most serious issue on the development of high power long pulse gyrotron.