Study of High-Power Gridless Inductive Output Tube With L-Shaped Cavity

The inductive output tube (IOT) remains a popular microwave amplifier for sub-gigahertz (GHz) applications due to its cost, efficiency, and size. This article presents a study on a gridless IOT with an L-shaped, high- <tex-math notation="LaTeX">Q</tex-math> cavity, with an experimental cold test of the cavity. The L-shaped cavity is designed to maintain a compact IOT diameter. The cavity is coupled to a coaxial line to obtain output power in the TEM mode, at an operating frequency of 431 MHz. The tapered configuration of the coaxial line allows for power extraction in the TEM mode, without the use of conventional TEM coupling loops. Time-domain solver in CST Studio is used to obtain the impedance locus for <tex-math notation="LaTeX">Q</tex-math> -factor calculation and optimize the geometry of the output line. Particle-in-cell (PIC) results are presented to validate IOT design, show the gridless operation of the annular beam, and analyze the beam-cavity power transfer. The IOT PIC simulation is performed with a 70-kV beam voltage, controlled by applying a modulation signal to nonintercepting modulating disks, and an axial magnetic field of 0.1 T. IOT simulations show 2.24-MW average output power, a maximum gain of 16.5 dB, and an average efficiency of 53.9%. Cavity simulations show an unloaded <tex-math notation="LaTeX">Q</tex-math> -factor <tex-math notation="LaTeX">Q₀ = 2025</tex-math> and <tex-math notation="LaTeX">R/Q = 39.4~Ω</tex-math> . Experimental cold test shows <tex-math notation="LaTeX">Q₀ = 1812</tex-math> , <tex-math notation="LaTeX">R/Q = 32.7~Ω</tex-math> , and an acceptable 0.9% deviation in frequency from simulation.