Tuesday, April 4, 2017
A metamaterial (MTM) composed of complementary split ring resonators can have a fundamental mode with a negative group velocity. This feature is promising in building a backward wave oscillator (BWO). In the talk, the design and the experimental test results of a high power, MTM based BWO will be presented. The electron beam propagates through a waveguide loaded with two MTM plates. The experiment is similar to our previous design except that one MTM plate is built as the reverse of the second plate. The fundamental mode has a negative group velocity and the design frequency is 2.4 GHz. CST Particle-in-cell simulations predict megawatt level output power with a helical beam trajectory, indicating the Cherenkov-cyclotron synchronism condition.
In the experiment, megawatt and microsecond long microwave pulses were generated by an electron beam with a voltage of up to 490 kV and current of up to 84 A. The highest power was generated by a 420 kV, 60 A beam. The total power was 2.5 MW and the efficiency was 10%. A pair of steering coils was installed to manipulate the beam in the transverse direction. The coils helped increase the output power to 2.9 MW with a better temporal flattop. These results demonstrate a major extension of the pulse length out to a full microsecond, significantly longer than the 100 to 400 ns pulses seen in our prior experiments with MTM structures.