Experimental Study of High-Power Millimeter Wave Transmission of Sapphire Window for Electron Cyclotron Range of Frequencies System

The possibility of use of a sapphire disk as an output window of a high-power and long-pulse gyrotron was investigated using an RF test facility with high-power gyrotrons. The dielectric loss tangents, tan δ=1.8×10 -4 ( T /300) 1.3±0.1 for 110 GHz and tan δ=2.6×10 -4 ( T /300) 1.3±0.1 for 170 GHz ( T : absolute temperature in K), were obtained in the temperature range of 300 K to 600 K. The experimental results showed that RF reflection from the window became large due to the permittivity change with increasing temperature, which limits the applicable power for the sapphire window rather than the thermal runaway. Moreover, it was found that a 170 GHz, 1 MW, CW gyrotron is feasible using a two-port window system consisting of sapphire double disks more than 18 cm in diameter with a flat RF power profile.