The low-impedance transit-time oscillator without foils is a new high-power microwave generator. In a previous report, a radiation power of 2.7 GW at 1.64 GHz has been achieved and the corresponding power conversion efficiency is 18.75%. Recently, the further experiments are continued in our laboratory. By increasing the cathode-anode gap properly, the operating voltage of device is enhanced to about 628 kV, and the corresponding radiation power reaches 3.6 GW. The device efficiency approaches 23%. In the newest experiments, because of the higher power level, the radiation power has been obviously influenced by RF breakdown in the vicinity of the dielectric window. By using a plastic bag filled with sulfur-hexafluoride (SF6), such an influence can be minimized.
