In this letter, alow-profile and ultrawideband capacitive circuit absorber with enlarged unit periodicity is developed by using impedance sheet model for RC synthesis and grating lobes suppression. The developed absorber achieves a simulated (measured) 10 dB absorption bandwidth ratio of 12.4:1 (12.5:1) at 0.093 λ _L , and its unit size is 0.3 × 0.3 × 0.093 λ _L ³ , where λ _L corresponds to the lowest 10 dB absorption frequency. By taking advantage of the enlarged unit periodicity to directly improve realizable capacitance diversity, the developed absorber can be realized conveniently without high dielectric substrate and complex frequency selective surface patterns, which makes it lightweight, cheap, easy to design, and thus, suitable for many low-frequency applications.

To explore the approach for generating high-power frequency-tunable subterahertz (sub-THz) to THz radiation, a 220-GHz high-order axial modes (HOAMs) gyrotron is developed, and the proof-of-principle experiment is presented in this paper. Experimental results demonstrate that driven by an electron beam of high voltage and high current, HOAMs are successfully excited in a long cylindrical cavity. By tuning the operating magnetic field, a mode transition between the first four axial modes was experimentally observed with a stepwise frequency tuning range of 0.79 GHz around 219 GHz, while a 0.58-GHz frequency tuning range via beam voltage tuning by operating in the first three axial modes. During the process of frequency tuning, the output power was kept not less than 0.45 kW, while the maximum output power was 3.80 kW with an output efficiency of 6%. These experimental results of mode transition and mode competition between axial modes in high-power millimeter-wave gyrotron could be beneficial for the development of frequency-tunable gyrotron toward some up-and-coming novel THz applications.

The output/input circuit is a core component in all high-power millimeter-wave (MMW) radiation sources, and its performance specifications and reliability directly impact upon the performance of the radiation source device. Central to achieving high power is the development of efficient mode converters. Here, we report on the development of a compact Ka-band circular TM ₀₁ to rectangular TE ₁₀ mode converter. The present mode converter adopts an all-metal waveguide structure and facilitates notable improvement in the system power capacity and is capable of realizing high-power propagation. The mode converter realizes effective mode conversion between high-order and fundamental modes, as well as allowing longitudinal and transverse transmission. Our simulation and empirical findings have shown mode purity ≥99.5% in the frequency range of 32.7-34.6 GHz, and the mode conversion efficiency is more than 96% at a center frequency of 32.9 GHz with a return loss S ₁₁ of -19.3 dB. The bandwidth of the converter is 2.4 GHz with transmission coefficient S ₂₁ ≥ -1 dB.

Here, we investigate, through parametrically optimized macroscale simulations, the field electron emission from arrays of carbon nanotube (CNT)-coated Spindts towards the development of an emerging class of novel vacuum electron devices. The present study builds on empirical data gleaned from our recent experimental findings on the room temperature electron emission from large area CNT electron sources. We determine the field emission current of the present microstructures directly using particle in cell (PIC) software and present a new CNT cold cathode array variant which has been geometrically optimized to provide maximal emission current density, with current densities of up to 11.5 A/cm² at low operational electric fields of 5.0 V/μm.

The dual-frequency radiation scheme of a G-band extended interaction oscillator (EIO) has been proposed. The analytic estimates of structure, mode characteristics, and oscillation-starting condition of the overmoded double-sheet-beam oscillator have been presented. The counterstreaming beams and optimized parameters were employed to reduce the fundamental mode competition in a TM₂₁ mode EIO, allowing the high-order transverse mode (TM₂₁⁺⁻) and the fundamental mode (TM₂₁⁺⁺) to operate simultaneously. The analyses are verified by simulations using 3-D particle-in-cell codes. The newly designed EIO was simulated with copper as a background material, and the dual-frequency radiation power of ~517 W is achieved when a double electron beam with 0.4 A and 21.5 kV is injected.