An oversize cylindrical reentrant cavity has been used to excite and support LSA mode oscillations in an

-GaAs epitaxial diode. Peak pulsed power output as high as 3.5 watts with an efficiency of 8 percent has been observed at 15.2 GHZ. Descriptions of the cavity, the experimental circuit and the diode are given along with reasons for believing that the microwave output power is the result of LSA-mode oscillations.