Abstract
It was measured that the dielectric permittivity of sapphire window changed with temperature rise at the frequencies of 110.0 GHz and 170.0 GHz. The permittivity ε varied from 9.4 to 10.0 at 170.0 GHz with temperature rise from 300 K to 630 K, which can cause a significant rf reflection at the surface of the window and degrade gyrotron oscillation. The permittivity change should be considered for the window design of high power, long pulse gyrotrons and transmission.