In sub-THz communication systems, compact, high-power, tunable radiation sources are required. Backward wave oscillators (BWO) have been practical devices for THz communication, but the maximum output power can be several times more than the minimum in the operation frequency range. To obtain the low peak-to-average ratio (LPAR) of the maximum power to the average power, the local maximum output power in the operation frequency range is first found theoretically and verified by particle-in-cell simulation. The LPAR 850 GHz BWO is then proposed. It applies a corrugated rectangular waveguide slow-wave structure, whose geometrical parameters are optimized using a multi-constrained optimization method. The optimized BWO operating in the frequency range from 790 to 882 GHz is simulated by the CST Particle Studio. The device can deliver at least 0.58 W with a LPAR of 1.08 when the conductivity of the slow-wave structure is 5.96 × 10⁷ S/m. For a smaller conductivity of 3 × 10⁷ S/m, the BWO can deliver at least 0.4 W with a LPAR of 1.13.

Lossy dielectric bars have potential applications in high power microwave (HPM) radiation-effect experiments performed in wind tunnels. Under plane excitation, beat wave patterns are observed along lossy and lossless dielectric bars in 3-D electromagnetic simulation results. Mode analysis is performed to clarify the two main modes in the beat waves—the leaky and guided modes. Comparisons are made between the theoretical and simulated results and acceptable agreements are obtained. The absorbing array of lossy dielectric bars is further investigated. The array can attenuate 90% of the microwave power at 10 GHz when a ten-meter series of such arrays is arranged. It is expected that the proposed absorbing array can achieve the required attenuation, considering that the testing object under microwave radiation is also an absorbing structure itself.

Wide-band high-power relativistic traveling wave tubes (RTWTs) in X-band have the potential in phased-array antennas, microwave weapons, and so on. Thorough design and optimization are required to achieve wide-band amplification, especially with invariant beam voltage and interaction length. A corrugated circular waveguide is applied as the slow-wave structure for wide bandwidth. The geometric parameters of the structure are initially chosen by cold analysis and then verified by a nonlinear model and the 2D particle-in-cell simulation. The kinematic mismatch is proved to be vital to the wide-band amplification of the device. Optimization is performed so that high output powers in the bandwidth can be achieved with the same interaction length. With the beam voltage of 625 kV and the beam current of 1.92 kA, the optimized RTWT can deliver more than 100 MW from 8.43 to 9.32 GHz, and the corresponding bandwidth is ∼10%. In the frequency range from 8.65 to 9.32 GHz, the output power and efficiency are larger than 170 MW and 14%, respectively.

The current design period for various backward wave oscillators (BWOs) is still at least several months. How to find the best structure parameters with an efficient and stable optimization method is a problem facing researchers in both scientific research and engineering work. In this paper, a non-randomized iterative optimization method is proposed. It applies orthogonal design methods to find local solutions that can provide optimal ‘gradient direction’ for several successive next iteration steps. An evaluation function is designed to distinguish the better ones from the local solutions in the multi-constrained optimization of such BWOs. Optimizations from different starting points are performed separately for a global optimal solution. Two BWOs at different frequency ranges are optimized using the proposed method. The validity and stability of the method are verified. It is believed that the method can provide the global optimum and shorten the design period of THz BWOs.

This paper provides a detailed research on near-field-focused (NFF) wireless power transfer (WPT) application using a reflectarray antenna. We apply an analytical model to explore detailed performances on the efficiency of near-field-focused wireless power transfer assuming a Cosine distribution of the E-field amplitude on the aperture. The performance curves are obtained for planar square arrays as a function of the receiving area and the position of the geometrical focal point. A reflectarray with area of 1.3 m2 comprises 60 × 60 units is proposed and simulated at 5.8 GHz. The full-wave simulation results are in a good agreement with the design curves. Full-wave simulations demonstrate that the proposed reflectarray actually forms a focused spot at a distance of 6.3 m, and possesses a high transfer efficiency of 45% at 10 meters away from the array when the receiving area is 0.26 m2, which proves our model possesses significant guideline value for designing WPT system with high efficiency.

Two-dimensional periodic dielectric bars have potential applications in high-power microwave (HPM) radiation effect experiments performed in wind tunnels. Such a bar is designed to consist of two types of dielectric materials, and two lined-up blocks can be considered as a period along the bar. Under plane excitation, the theoretical period length of the beat wave pattern fits well with the simulation result, which requires modifying the previously presented field-matching method. The phase distribution on the cross-section can be non-uniform when two different guiding modes are excited independently and propagate along different materials. Directional reflection with a low reflection angle can be obtained by reasonably choosing the parameters of the dielectric array. The designed array can decrease the returned-back microwave power toward the microwave source by 6 dB according to the numerical simulation, which included the wind tunnel, the input antenna, the test target, and the reflect array in one model.