We report the first experimental demonstration of coherent combining of phase-steerable high power microwaves (HPMs) generated by X-band relativistic triaxial klystron amplifier modules under the guidance of pulsed magnetic fields. Electronically agile manipulation of the HPM phase is achieved with a mean discrepancy of 4° at the gain level of 110 dB, and the coherent combining efficiency has reached as high as 98.4%, leading to combined radiations with equivalent peak power of 4.3 GW and average pulse duration of 112 ns. The underlying phase-steering mechanism during the nonlinear beam-wave interaction process is furthermore explored by particle-in-cell simulation and theoretical analysis. This Letter paves the way for high power phase array in large scale and may stimulate new interest in research of phase-steerable high power masers.

In this paper, the simulation and experiment research on a Ka-band coaxial transit-time oscillator is conducted. The asymmetric-mode-competition phenomenon was observed in the experiment, and TE_1 1 and TE_29 1 modes exist in the output microwaves. Based on the analysis of the mechanism of the asymmetric modes, support posts need to be improved to suppress the asymmetric modes. By using the improved support posts, the effective TEM mode transmission, the small reflection of the TE_1 1 mode, and the small excitation of the high order asymmetric modes are expected. In the improved experiment, the output microwaves with the power of 650 MW at the frequency of 26.5 GHz can be obtained with the diode voltage of 450 kV and the beam current of 9.1 kA. The asymmetric modes are effectively suppressed. It should be noted that the power loss caused by the material cannot be ignored in the Ka-band. The corresponding power loss of stainless steel is above 30% compared with the perfect conductor.

A relativistic Ku-band coaxial transit-time oscillator has been proposed in our previous work. In the experiments, we find that the asymmetric competition mode in the device limits the microwave power with the increase of the input electric power. For solving such a problem, the methods for analysis and suppression of the asymmetric competition mode in the device are investigated theoretically and experimentally. It is shown that the structure and the material of the collector, the concentricity, and the electron emission uniformity play an important part in the suppression of the asymmetric competition mode in the relativistic Ku-band transit-time oscillator. In the subsequent experiments, the asymmetric mode was suppressed effectively. At a low guiding magnetic field of 0.7 T, a microwave pulse with power of 1 GW, frequency of 14.3 GHz close to the simulation one, and efficiency of 20% was generated.

An improved foilless Ku-band transit-time oscillator with low guiding magnetic field is proposed and investigated in this paper. With a non-uniform buncher and a coaxial TM₀₂ mode dual-resonant reflector, this improved device can output gigawatt level Ku-band microwave with relatively compact radial dimensions. Besides the above virtue, this novel reflector also has the merits of high TEM reflectance, being more suitable for pre-modulating the electron beam and enhancing the conversion efficiency. Moreover, in order to further increase the conversion efficiency and lower the power saturation time, a depth-tunable coaxial collector and a resonant cavity located before the extractor are employed in our device. Main structure parameters of the device are optimized by particle in cell simulations. The typical simulation result is that, with a 380 kV, 8.2 kA beam guided by a magnetic field of about 0.6 T, 1.15 GW microwave pulse at 14.25 GHz is generated, yielding a conversion efficiency of about 37%.

A novel L-band slow wave structure (SWS) for compact and high-efficiency relativistic Cerenkov oscillator is proposed. The SWS is composed of orthogonal array of two all-metal metamaterial units. Since the metamaterial used for constructing the SWS has a negative equivalent permittivity and permeability, the fundamental mode of the SWS has a negative dispersion and can work below cutoff frequency of circular waveguide with the same size, indicating that the structure has advantage of miniaturization. Analysis of high-frequency characteristics shows that interaction impedance of the fundamental mode (quasi TM₀₁ mode) is greater than 70 Ω across operating frequency band, which indicates that this structure is expected to achieve the high efficiency of high power microwave sources. By adopting an orthogonal arrangement between metamaterial resonant units, the angular uniformity of electric field in SWS space is significantly improved, and the non-uniformity of electric field due to asymmetry of the metamaterial units is reduced. In addition, an L-band relativistic Cerenkov oscillator is constructed using this metamaterial SWS. In the acceleration voltage U = 550 kV, beam current I = 1.5 kA, and axial uniform magnetic field B = 1.5 T conditions, preliminary particle simulations show that the Cerenkov oscillator obtains a 240 MW average power at 1.405 GHz, with an efficiency of 29.1%, and axial length around one $\mathrm{\lambda }$ . This Cerenkov oscillator obtains a high power and efficiency in a miniature and compact microwave device.

A novel all-metal metamaterial slow wave structure (MSWS) which suits high-power microwave sources was proposed in our previous work. Recently, an experiment on the transmission characteristics of the MSWS was carried out. The experiment results show that the MSWS can transmit electromagnetic waves in the 1.40–1.60 GHz frequency band, while the same size circular waveguide TM₀₁ mode has a cutoff frequency of 2.4 GHz. The experiment proved that the MSWS has the advantage of miniaturization. Then, based on the transmission characteristic experiment, a novel L-band relativistic Cherenkov oscillator with miniaturization and high conversion efficiency is proposed. By introducing a new coupled output structure and adjusting the MSWS period at both ends of the output structure, the electromagnetic field is better matched between the MSWS and the output coupler, and the conversion efficiency is increased from the previous 29.2% to 55.8%, with an average output power of 460 MW. Through the analysis of the electric field distribution at the main frequency of the generated microwave, it is determined that the working mechanism of the device is the Cherenkov mechanism.