Guangqiang Wang

Numerical studies of powerful terahertz pulse generation from a super-radiant surface wave oscillator

The results of theoretical and numerical studies of coherent stimulated terahertz radiation from intense, subnanosecond electron beam are presented. The mechanism of terahertz pulse generation is associated with self-bunching of the beam and slippage of the wave over the whole electron flow. This so called Cherenkov super-radiance (SR) is used to propose a compact terahertz generator with high peak power. A large cross-section (overmoded), slow wave structure is designed to support the high power handling capability, and the mode competition is avoided by operating the device in the surface wave status. With 2.5 D particle-in-cell simulation, the “hot” characteristics of the proposed super-radiant terahertz generator are investigated, and the numerical results show that the SR peak power could be further increased by optimizing the amplitude profile of electron pulse. Under the condition of 0.5 ns pulsewidth, 500 kV voltage, and 1.5 kA current, the 110 ps, 680 MW, and 0.14 THz SR pulse is achieved with a ...

A repetitive 0.14 THz relativistic surface wave oscillator

Preliminary experimental results of a repetitive 0.14 THz overmoded relativistic surface wave oscillator (RSWO) are presented in this paper. The repetitive RSWO is developed by using a rectangularly corrugated slow-wave structure with overmoded ratio of 3 and a foilless diode emitting annular electron beam with thickness of 0.5 mm. The high quality electron beams at the repetition rate of 10 are obtained over a wide range of diode voltage (180 kV < U < 240 kV) and current (700 A < I < 1.2 kA). The generation experiments of RSWO are conducted at an axial pulsed magnetic field whose maximum strength and duration can reach about 2.7 T and 1 s, respectively. The experimental results show that the RSWO successfully produces reasonable uniform terahertz pulses at repetition rate of 10, and the pulse duration, frequency, and power of a single pulse are about 1.5 ns, 0.154 THz, and 2.6 MW, respectively, whereas the dominated radiation mode of the RSWO is TM02.

Analysis of electromagnetic modes excited in overmoded structure terahertz source

A method on electromagnetic mode analysis in overmoded structure terahertz source is studied in this paper. This mode analysis method is based on Fourier-Bessel series and the particle-in-cell simulation. The result of mode analysis shows that there are TM02 and TM03 modes present in the waveguide motivated by mode change in the discontinuity. The mode content keeps stable when the device starts to work. The magnetic field and the voltage of beam do not change the mode content to some extent. The parameters of diode affect the mode content significantly.

Small-signal theory of subterahertz overmoded surface wave oscillator with distributed wall loss

A small-signal theory of the overmoded surface wave oscillator (SWO) with distributed wall loss is presented in this letter. The wall loss considered here includes the surface resistance and surface roughness. The cold and hot characteristics of 0.14 THz SWO are studied by the small-signal theory. Numerical results show that as the increase of wall loss, the working frequency decreases slightly, the rise time and startup time of oscillation increase significantly, and the output power decreases dramatically. Particle-in-cell (PIC) simulation confirms the prediction by the small-signal theory.

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.

Particle simulation on high power terahertz wave generated by backward wave oscillator

Microwave vacuum electron devices (MVED) are capable of handling more power in a smaller interaction volume than the solid-state devices. An over-mode high power terahertz backward wave oscillator (BWO) is proposed. The slow wave structure (SWS) is constructed as follow: a series of equidistant annular slots are cut in the inner wall of a cylindrical waveguide. The dispersion relationship of the SWS is theoretically analyzed. The parameters of this structure are optimized by using 2.5D UNIPIC code. The influences of structure parameters and working parameters on the performance of the device are investigated, such as the period number of the SWS, the beam voltage, and the strength of external guiding magnetic field. The numerical results indicate that the frequency of the device is not sensitive to the beam voltage. This is the typical characteristic when the device works at π-mode. Under the condition of the electron beam with the voltage of 200 kV, the current of 1100 A, the inner radius of 2 mm , the outer radius of 2.5 mm, and magnetic field of 5 Tesla, the TM01 mode wave begins to oscillate at 3.5 ns, whose average power is about 30 MW, frequency is 0.147 THz. And an efficiency of 14% is also obtained with a fine spectrum characteristic. The numerical results demonstrate that the over-mode electrodynamic structure can be used to decrease internal electric field strength while avoiding multimode generation and maintaining good spectral purity.