PURPOSE: To restrain generation of unnecessary modes by providing through the wall between a collector electrode and an output window slits such that the angle formed between the longitudinal direction of each slit and a pipe axis is less than that formed by the pipe axis and the direction of high frequency current, and hermetically sealing the outer peripheral portion of each slit with a manifold.

CONSTITUTION: Unnecessary residual gas in a collector electrode 4 is allowed to pass through each slit 10 and is discharged via a manifold 11 and an exhaust pipe 12 at extra high rate and stable oscillation is obtained by this sufficient exhaust. Being provided at a predetermined angle θ to a pipe axis, each slit 10 does not block high frequency current flowing through a pipe wall at whispering/gallarey mode and oscillation mode is not changed to any unnecessary mode. The size, d, l of the opening of each slit is small enough to minimize high frequency leakage. High frequency leakage is damped by a high frequency absorber 14 in the inner wall of the manifold so as to prevent the manifold 11 from becoming a high frequency cavity. This constitution minimizes conversion to unnecessary modes and provides sufficient exhaust conductance and also minimizes high frequency leakage.