Juntao He

A low-impedance transit-time oscillator without foils

A low-impedance transit-time oscillator (LITTO) without foils is proposed and studied by simulation. By using a coaxial structure, the space-charge limiting current can be improved significantly, thus allowing higher input and output powers. With no foils to erode, the only factor limiting the repetition rate is the ability to maintain an adequate vacuum. By contrast with conventional transit-time oscillators, the proposed LITTO has the advantages of low diode impedance, rapid saturation time, and a possibility of repetitive operation. As indicated in PIC simulation, the average microwave output power is over 5.0 GW at the main frequency of 1.6 GHz, with an input electron beam of 36.0 kA current and 600 kV voltage, and an external magnetic field of 0.45 Tesla.

An oversized X-band transit radiation oscillator

An oversized transit radiation oscillator is designed to generate high power microwave at X-band. By using a coaxial structure, the power capacity of the device is improved significantly. In order to realize long-pulse operation, the cathode-anode gap, the collector location, and the surface field strength have been emphatically considered in our design. With a 710 kV, 14.5 kA beam guided by a 0.8 T magnetic field, a 2.7 GW microwave at 9.38 GHz has been obtained in the simulation. The power conversion efficiency is 26.2%. The simulation also indicates that the highest axial electric field strength on the surface of electrodynamic structure is only 590 kV/cm, which could be further decreased by increasing the radial dimension of the X-band device.

Effects of Intense Relativistic Electron Beam on the Microwave Generation in a Foilless Low-Impedance Transit-Time Oscillator

A low-impedance transit-time oscillator without foils has been proposed in our previous work. Recently, the correlated experiments have been carried out in our laboratory. In the experiments, two kinds of explosive-emission cathodes made of copper dielectric and polymer velvet were employed, and the electron collectors mainly consist of stainless steel and graphite. The experiments show that the cathode and collector materials have great influence on microwave generation in the device. This paper presents the experimental results, along with comparisons with the simulation results. It was found that the combination of the polymer-velvet cathode and the graphite collector is much more advantageous to the generation of high-power microwaves.

Focusing electrode and coaxial reflector used for reducing the guiding magnetic field of the Ku-band foilless transit-time oscillator

Based on the theoretical analysis of the intense relativistic electron beam propagation in the coaxial drift-tube, a focusing electrode and a coaxial reflector is proposed to lessen the demand of the coaxial Ku-band foilless transit-time oscillator (TTO) for the guiding magnetic field. Moreover, a Ku-band TTO with the focusing electrode and the coaxial reflector is designed and studied by particle in cell simulation. When the diode voltage is 390 kV, the beam current 7.8 kA, and the guiding magnetic field is only 0.3 T, the device can output 820 MW microwave pulse at 14.25 GHz by means of the simulation. However, for the device without them, the output power is only 320 MW. The primary experiments are also carried out. When the guiding magnetic field is 0.3 T, the output power of the device with the focusing electrode and the coaxial reflector is double that of the one without them. The simulation and experimental results prove that the focusing electrode and the coaxial reflector are effective on reducing the guiding magnetic field of the device.

A novel coaxial Ku-band transit radiation oscillator without external guiding magnetic field

A novel coaxial transit radiation oscillator without external guiding magnetic field is designed to generate high power microwave at Ku-band. By using a coaxial structure, the space-charge potential energy is suppressed significantly, that is good for enhancing efficient beam-wave interaction. In order to improve the transmission stability of the unmagnetized intense relativistic electron beam, a Pierce-like cathode is employed in the novel device. By contrast with conventional relativistic microwave generators, this kind of device has the advantages of high stability, non-guiding magnetic field, and high efficiency. Moreover, with the coaxial design, it is possible to improve the power-handing capacity by increasing the radial dimension of the Ku-band device. With a 550 keV and 7.5 kA electron beam, a 1.25 GW microwave pulse at 12.08 GHz has been obtained in the simulation. The power conversion efficiency is about 30%.

Suppression of the asymmetric competition mode in the relativistic Ku-band coaxial transit-time oscillator

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.

High power microwave generation from the low-impedance transit-time oscillator without foils

A low-impedance transit-time oscillator without foils (LITTO) has been proposed in our previous work. Recently, the experiment is carried out on an intense relativistic electron beam (IREB) generator, which is capable of producing a 50 ns duration electron beam in the voltage range of 0.4-1 MV. With a 600 kV, 24 kA electron beam guided by an external magnetic field of 0.5 T, a radiation power of 2.7 GW at 1.64 GHz has been achieved and the corresponding power conversion efficiency is 18.75%. With the similar voltage and current parameters, the experimental results are reexamined and confirmed by the particle-in-cell(PIC) simulation.

Experimental research on Ku-band magnetically insulated transmission line oscillator

An improved Ku-band magnetically insulated transmission line oscillator is proposed and investigated experimentally. In the particle-in-cell simulation, the Ku-band MILO generates the microwave with a power of 1.62 GW and a frequency of 13 GHz at the input voltage of 474 kV. The device is fabricated based on the simulation results, and an experiment system is designed. In the preliminary experiments, output microwave with frequency of 13.02 GHz, power of 150 MW, and pulse width of 17 ns is generated, under the diode voltage of 450 kV. Analysis on the experiment results shows that plasma produced due to the large current hitting to the outside of the collection tank is the essential cause for the low amplitude of the microwave power and short pulse width.

A novel Ka-band coaxial transit-time oscillator with a four-gap buncher

A novel Ka-band coaxial transit-time oscillator (TTO) with a four-gap buncher is proposed and investigated. Simulation results show that an output power of 1.27 GW and a frequency of 26.18 GHz can be achieved with a diode voltage of 447 kV and a beam current of 7.4 kA. The corresponding power efficiency is 38.5%, and the guiding magnetic field is 0.6 T. Studies and analysis indicate that a buncher with four gaps can modulate the electron beam better than the three-gap buncher in such a Ka-band TTO. Moreover, power efficiency increases with the coupling coefficient between the buncher and the extractor. Further simulation demonstrates that power efficiency can reach higher than 30% with a guiding magnetic field of above 0.5 T. Besides, the power efficiency exceeds 30% in a relatively large range of diode voltage from 375 kV to 495 kV.

An Improved

An improved