Stable 1.1 MW oscillation was achieved by a 170 GHz high power gyrotron. Th
e oscillation mode capable of the CW operation is TE31.8. The efficiency wa
s 32%, which was enhanced to 57% by a depressed-col lector operation. The p
arasitic oscillation in a beam tunnel (a beam drifting tube between an elec
tron gun and a cavity) that degraded the oscillation efficiency was suppres
sed by installing an RF absorber. This result has a large impact on the dev
elopment of a 1-MW-long-pulse gyrotron that is required for fusion devices.