X-band relativistic backward wave oscillator with two-spiral corrugated Bragg reflector

Backward wave oscillators are typically known to radiate in the TM₀₁ mode. A two-spiral corrugated Bragg reflector has been used downstream of the cathode in an X-band relativistic backward wave oscillator (RBWO) to radiate a TE_n mode at the output. Simple analytical formulas were used to design the basic parameters of the Bragg reflector, which were later optimized numerically since there is no exact closed form solution for the electromagnetic fields within a periodic waveguide structure or cavity. The fully electromagnetic, fully relativistic particle-in-cell (PIC) code MAGIC was used to simulate the problem. The RBWO was driven by a voltage pulse that has a half sine wave-like shape, 460 kV amplitude, and FWHM duration of 12 ns. A uniform static magnetic field of 1.9 T was applied throughout the simulation volume. With these parameters a microwave power of 330 MW at a frequency of 9.9 GHz in a clean TE_n mode pattern was detected at the output.