Dongyang Wang

Overmoded subterahertz surface wave oscillator with pure TM01 mode output

Overmoded O-type Cerenkov generators using annular electron beams are facing the problem of multi-modes output due to the inevitable structural discontinuities. A simple but effective method to achieve the pure TM01 mode output is applied on the 0.14 THz overmoded surface wave oscillator (SWO) in this paper. In spite of still using an overmoded slow wave structure to ensure the easy fabrication, the followed smooth circular waveguide is shrinkingly tapered to the output waveguide with appropriate radius that it cuts off other higher modes except TM01 mode. Moreover, the modified device here has the same power capacity as the previous one according to the numerical analysis. By optimized lengths of the transition waveguide and tapered waveguide, particle-in-cell simulation results indicate that the subterahertz wave with output power increased 14.2% at the same frequency is obtained from the proposed SWO under the previous input conditions, and importantly, the output power is all carried by TM01 mode as e...

Mode analysis and design of 0.3-THz Clinotron*

To develop a high-power continuous-wave terahertz source, a Clinotron operating at 0.3 THz is investigated. Based on the analyses of field distribution and coupling impedance, the dispersion characteristic of a rectangular resonator is preliminarily studied. The effective way to select fundamental mode to interact with the electron beam is especially studied. Finally, the structure is optimized by particle-in-cell simulation, and the problems of manufacture tolerance, current density threshold, and heat dissipation during Clinotrons operation are also discussed. The optimum device can work with a good performance under the conditions of 8 kV and 60 mA. With the generation of signal frequency at 315.89 GHz and output power at 12 W on average, this device shows great prospects in the application of terahertz waves.

A high-order mode extended interaction klystron at 0.34 THz

We propose the concept of high-order mode extended interaction klystron (EIK) at the terahertz band. Compared to the conventional fundamental mode EIK, it operates at the TM31-2π mode, and its remarkable advantage is to obtain a large structure and good performance. The proposed EIK consists of five identical cavities with five gaps in each cavity. The method is discussed to suppress the mode competition and self-oscillation in the high-order mode cavity. Particle-in-cell simulation demonstrates that the EIK indeed operates at TM31-2π mode without self-oscillation while other modes are well suppressed. Driven by the electron beam with a voltage of 15 kV and a current of 0.3 A, the saturation gain of 43 dB and the output power of 60 W are achieved at the center frequency of 342.4 GHz. The EIK operating at high-order mode seems a promising approach to generate high power terahertz waves.

Optimization of the multi-slot cavity and drift in a 0.34 THz extended interaction klystron

The configurations of the cavity and drift tube used in a 0.34 THz Extended Interaction Klystron (EIK) are theoretically studied. The results from particle-in-cell (PIC) simulations are presented and discussed. Based on the small signal theory, the coupling coefficient and beam conductance at the gaps were studied, leading to optimization of the multi-slot cavity. The physical analysis of electron movement was carried out to study the influence of electron drifting on device performance. PIC simulations were conducted with the results compared to analytical theory. Good agreement was achieved between analytical predictions and simulations, demonstrating the feasibility of the theoretical approach. The performances of an EIK under different conditions such as mismatching and self-oscillation indicate that an optimized structure can produce an output power of 143 W and a gain of 38.1 dB, demonstrating its potential to be a highly stable and reliable source of coherent sub-terahertz radiation.

Accurate model of electron beam profiles with emittance effects for pierce guns

Accurate prediction of electron beam profile is one of the key objectives of electron optics, and the basis for design of the practical electron gun. In this paper, an improved model describing electron beam in Pierce gun with both space charge effects and emittance effects is proposed. The theory developed by Cutler and Hines is still applied for the accelerating region of the Pierce gun, while the motion equations of the electron beams in the anode aperture and drift tunnel are improved by modifying electron optics theory with emittance. As a result, a more universal and accurate formula of the focal length of the lens for the electron beam with both effects is derived for the anode aperture with finite dimension, and a modified universal spread curve considering beam emittance is introduced in drift tunnel region. Based on these improved motion equations of the electron beam, beam profiles with space charge effects and emittance effects can be theoretically predicted, which are subsequently approved to agree well with the experimentally measured ones. The developed model here is helpful to design more applicable Pierce guns at high frequencies.

A megawatt-level surface wave oscillator in Y-band with large oversized structure driven by annular relativistic electron beam

High power vacuum electronic devices of millimeter wave to terahertz regime are attracting extensive interests due to their potential applications in science and technologies. In this paper, the design and experimental results of a powerful compact oversized surface wave oscillator (SWO) in Y-band are presented. The cylindrical slow wave structure (SWS) with rectangular corrugations and large diameter about 6.8 times the radiation wavelength is proposed to support the surface wave interacting with annular relativistic electron beam. By choosing appropriate beam parameters, the beam-wave interaction takes place near the π-point of TM01 mode dispersion curve, giving high coupling impedance and temporal growth rate compared with higher TM0n modes. The fundamental mode operation of the device is verified by the particle-in-cell (PIC) simulation results, which also indicate its capability of tens of megawatts power output in the Y-band. Finally, a compact experimental setup is completed to validate our design. Measurement results show that a terahertz pulse with frequency in the range of 0.319–0.349 THz, duration of about 2 ns and radiation power of about 2.1 MW has been generated.

An effective method to increase bandwidth of EIK at 0.34 THz

ABSTRACT To increase the bandwidth of Extended Interaction Klystron (EIK) at 0.34 THz, the method of staggered tuning on cavities’ configurations is proposed. Based on the analysis of phase relationship between gap voltage and the bunched beam, the buncher cavities in EIK are reasonably staggered-tuned to achieve various resonance frequencies, which is helpful to flat the gain response of the whole device. The characteristics of output cavities with different numbers of gaps are then researched and the issue of start current for the self-oscillation mode is also involved, leading to the optimum number of gaps to enhance the interaction and avoid the instability. By comparing the performances of various typical stagger-tuned models, the final configuration is accordingly confirmed. Particle-in-cell simulation is eventually applied to study performance of the optimised structure, whose gain is 34.8 dB in peak and −3 dB bandwidth reaches about 500 MHz, which is double that of the synchronous-tuned structure.

Study on the stability and reliability of Clinotron at Y-band

To improve the stability and reliability of Clinotron at the Y-band, some key issues are researched, such as the synchronous operating mode, the heat accumulation on the slow-wave structure, and the errors in micro-fabrication. By analyzing the dispersion relationship, the working mode is determined as the TM10 mode. The problem of heat dissipation on a comb is researched to make a trade-off on the choice of suitable working conditions, making sure that the safety and efficiency of the device are guaranteed simultaneously. The study on the effect of tolerance on devices performance is also conducted to determine the acceptable error during micro-fabrication. The validity of the device and the cost for fabrication are both taken into consideration. At last, the performance of Clinotron under the optimized conditions demonstrates that it can work steadily at 315.89 GHz and the output power is about 12 W, showing advanced stability and reliability.

Mode competition and selection in overmoded surface wave oscillator

The overmoded surface wave oscillator (SWO) is one of the promising devices to generate high-power millimeter and subterahertz waves for its merits of high efficiency and easy fabrication. But the employed slow wave structure with large diameter may introduce mode competition as the adverse effects. Therefore, the mode competition and selection in the overmoded surface wave oscillator are investigated in detail in this paper. By using the theoretical analysis and particle-in-cell simulation, the potential transverse mode and axial mode competition is pointed out, and the physical mechanisms and methods for mode selection are investigated. At last, the results are verified in the design of a 0.14 THz overmoded SWO without mode competition, which can generate the output power up to 70 MW at the frequency of 146.3 GHz with conversion efficiency almost 20% when beam voltage and current are, respectively, about 313 kV and 1.13 kA.

Increasing the bandwidth of an extended interaction klystron at 0.34 THz by staggered tuning of the multi-gap cavities

To increase the bandwidth of EIKs at 0.34 THz, a synthetical method containing several steps is proposed. As the crucial unit determining the performance of EIKs, the output cavity is studied. Based on the theory of space charge wave, the number of gaps in the output cavity is optimized to increase both the interaction efficiency and the instantaneous bandwidth. The approaches of tuning the shape of a barbell gap and the number of gaps in idler cavities are studied, leading to the practicable method of staggered tuning on all the intermediate cavities. As a result, the bandwidth of an EIK is improved step by step. Another approach of adjusting the drift tubes is also discussed and the synthetically tuning strategy for terahertz EIKs is concluded. Finally, the optimized nonuniform structure is verified by PIC simulations and the results show that it can effectively broaden the −3 dB bandwidth up to 500 MHz, which is double that of the uniform structure. The synthetical method is proved to be practicable in tuning the bandwidth and capable to keep the device operating in stability.