This study is an experimental investigation aimed at addressing the limitation of pulsewidth in traditional Ku -band transit-time oscillator (TTO) devices within a long-pulse accelerator as highlighted in a previous study. By controlling the number of shots and analyzing the variations in breakdown phenomena within the device, we identified the junction between the extraction cavity and drift section as a susceptible area for breakdown discharges. Through an increase in the number of shots, we confirmed the validity of the “anode plasma impact” model. Building upon adjustments made to the internal structure of the drift section, we successfully optimized the output pulsewidth of the TTO device from 68 to 82 ns, effectively maximizing the energy efficiency of the pulse. This research holds significant implications for the design optimization of devices in long-pulse accelerators.

A novel method to realize modulated emission and transmission in the diode region is proposed to improve beam-wave interaction for high output power and conversion efficiency. Microwave energy leaked into the diode is utilized to form a modulated electromagnetic field to modulate the emission and transmission of the electron beam. This mechanism is conducive to achieving better modulation effects of electron beam and suppressing competitive modes through the resonant frequency component of initial current modulation. A low magnetic field (0.7 T) Q-band relativistic coaxial transit time oscillator (RCTTO) is designed with this mechanism to obtain a microwave power of 337 MW, under the diode voltage of 396 kV and current of 1.98 kA. Compared with an RCTTO with a normal emission mechanism, the conversion efficiency increases from 21.3% to 43%.

As the wavelength of an electromagnetic wave increases toward millimeters, the volume space of the resonant cavity drops rapidly, leading to insufficient electron beam modulation and small power handling capacity (PHC), eventually affecting the output power and pulse width. To solve the problems, this paper introduces a non-uniform 5-period modulation structure with mixed mode and output structure with TM03 mode at a W band. First, comparing the electron conductance, gap effective voltage, and fundamental modulating current of three different mode bunchers, it is demonstrated that the non-uniform 5-period mixed mode buncher achieves better modulation performance. Second, the TM03 mode is applied in the traveling wave output structure to reduce the surface field intensity and enhance the PHC output for pursuing highly efficient beam–wave interaction. Based on this analysis, a W band coaxial transit time oscillator (TTO) with a low surface field intensity utilizing the 5-period buncher with a non-uniform mixed mode and an 11-period output structure with TM03 mode is proposed in this paper. Particle-in-cell simulation results show that under the conditions of a 1.2 T magnetic field, a diode voltage of 343 kV, and a current of 4.06 kA, the TTO can generate an output power of 298 MW at 76.2 GHz with metal losses. The beam–wave conversion efficiency is 21.4%, and the maximum surface field intensity is 1.81 MV/cm.

Low guiding magnetic fields are favored for high-power microwave sources in practical applications for the sake of compactness and miniaturization, especially for permanent magnet packaging to replace the bulky and heavy solenoid system. A novel focusing cathode is proposed in a V-band transit-time oscillator to reduce the requirement for guiding magnetic fields. Particle-in-cell simulation results indicate that in the diode region, the radial electric field and electron beam current are reduced with the focusing cathode, further leading to improved transmission of the electron beam under a low guiding magnetic field. With an input of diode voltage of 393 kV, a beam current of 5.25 kA, and a low guiding magnetic field of 0.6 T, the transit-time oscillator outputs a microwave with an average power of 566 MW and a frequency of 58.6 GHz, resulting in a conversion efficiency of 27.3%. The proposed V-band transit-time oscillator with a focusing cathode can also stably work under a large range of input voltage and magnetic field, desired for further research with permanent magnet packaging.

Improving compactness is essential for high-power microwave (HPM) sources. In this paper, a novel reflective modulation cavity is proposed and investigated in a V-band relativistic coaxial transit-time oscillator (RCTTO). The cold cavity analyses and particle-in-cell simulations show that the reflective modulation cavity has larger reflection coefficients of TEM mode and stronger electron beam modulation capability when compared with a uniform modulation cavity. When the input diode voltage is 391 kV, the beam current is 4.91 kA, and when the guiding magnetic field is 0.6 T, the compact V-band RCTTO produces an output microwave power of 518 MW (conversion efficiency of 27.0%). Compared with the original RCTTO, the compact V-band RCTTO featuring a reflective modulation cavity exhibits a 24.8% increase in output power and a 5.4% improvement in efficiency, and the axial length of the magnetic field uniform region is reduced by 24.2%. The compact V-band RCTTO also demonstrates a broad operation voltage range, indicating potential for stable operation with voltage fluctuations in experiments. Furthermore, the reflective modulation cavity can be integrated into other high-frequency O-type HPM devices to enhance compactness, thereby diminishing the demands on the magnetic field region, which is advantageous for the future permanent packaging of HPM sources.

A novel V-band relativistic coaxial transit time oscillator (RCTTO) is proposed to operate with high efficiency under a low magnetic field. A two-stage cascade buncher, a TEM mode feedback mechanism, and a distributed reflector are proposed in the design of the V-band RCTTO to study electron beam modulation, microwave transmission, and microwave output. The two-stage cascade buncher enhances the velocity modulation of the electron beam. The TEM mode feedback mechanism improves the modulation effect of the second buncher by strengthening the gap voltage of the second buncher and deepening the fundamental harmonic current modulation depth. The distributed reflector can isolate the TEM mode leakage into the diode region and minimize the disturbance to the electron beam generation and transmission. With an input diode voltage of 391 kV, beam current of 4.91 kA, and low guiding magnetic field of 0.6 T, the RCTTO outputs a microwave with an average power of 658 MW and a frequency of 58.62 GHz, resulting in a high conversion efficiency of 34.3%.