Extensive efforts have been made to explore high-power vacuum electronic devices and super-resolution electromagnetic imaging technologies. Vacuum electronic devices use free-space electrons to excite electromagnetic-wave radiation between microwave to terahertz band, and play key roles in satellite, radar, wireless communication, and a series of other applications. High efficiency, high reliability and long life are the core scientific and technical problems that restrict theresearch of high power electric vacuum devices. High resolution and super resolution electromagnetic imaging are the important means to improve the information perception ability and the research hotspot. By combining with terahertz technology, a series of research works have been done. The main contributions are included as follows.

1) Broke through the key technologies of space traveling-wave tube.

Space traveling wave tube is the core component of all types of satellites and spacecraft, but the situation of long-term dependence on imports is a major national security risk. In order to solve the core scientific and technical problems of high efficiency, long life and practical application of the space traveling wave tube, the team established the mathematical model of the space traveling wave tube high-frequency slow wave system and the three-dimensional nonlinear field theory of electron beam-wave interaction. A high efficient multi-objective optimization algorithm was proposed, and an integrated design software platform was developed. Methods of suppressing secondary harmonic excitation, back-wave oscillation and improving phase distortion were proposed. A kind of straight-claw high-efficiency multistage depressed collector structure, which can suppress the secondary electron emission and improve the collection efficiency, was invented. The radiation radiator and the long-life cathode heat sub-components of the radiative cooling type space traveling wave tube were developed. All these systematic work laid a solid theoretical and technical basis for the development of high efficiency, high reliability, and long-life space traveling-wave tube. Software and related technology have been successfully applied on different satellites. Relevant achievements have been awarded the first prize of scientific and technological progress of Ministry of Education in 2013 and the second prize of national science and technology progress award in 2014.

2) Developed the terahertz gyrotron technologies.

The gyrotron is a new type of fast wave device based on the mechanism of relativistic electron cyclotron resonance stimulated radiation. In order to solve the problem of super strong magnetic field, mode competition, ohmic loss and broadband frequency modulation during the development and application of terahertz gyrotron, the team proposed a high-order harmonic large orbit electron gun based on new concept of constant-generalized-angular-momentum cathode. The current is about 1 order of magnitude higher than the existing international level, which solves the key problems in the development of high-order harmonic device. The high-Q cavity and the dynamic thermal start-up method were used to solve the efficient excitation problem of the third harmonic gyrotron. A scheme of wideband frequency modulation gyrotron using pre-bunched traveling wave resonant circuit and backward wave interaction was proposed. The nonlinear output waveguide is used to improve the Q value of the high-order axial mode, which breaks the limitation of the Q-value distribution law, reduces the ohmic loss by 10 times, and the power output curve of the axial mode transition is more flat. A concept of harmonic frequency modulation multi-frequency gyrotron was proposed, which suppresses the mode competition, the control mode and the frequency through the large cyclotron harmonic tuning, and adopts the long-cavity with high Q value to improve the efficiency of harmonic interaction. By breaking through the key technologies such as wideband mode exciter, quasi-optical mode converter, distributed loss circuit and terahertz energy window, a desktop type of broadband FM pulsed-magnet gyrotron was developed at 330GHz, which is of great significance to the biomedical applications. The main work has been published in the journals such as IEEE-EDL, IEEE-T-ED, IEEE-T-TST, IEEE-T-PS and POP, as well as the academic book "Millimeter Wave Gyrotron Traveling Wave Tube" published by the Springer.

3) Proposed new systems, models and algorithms for high-resolution electromagnetic imaging.

Electromagnetic imaging has important applications in biomedical, safety inspection, reconnaissance and resource exploration. The team proposed a variety of new systems for high performance electromagnetic imaging by breaking through the bottlenecks of existing phased array and synthetic aperture imaging system in real-time and system complexity. It provides a new way for high efficiency, high precision and low cost real-time electromagnetic imaging. The research has been published in Nat. Comm., Phys. Rev. E, IEEE-T-GRS, IEEE-GRSL and IEEE-T-AP, and so on. The electromagnetic data processing methods for object-oriented feature mining and imaging integration were also systematically proposed by breaking through the shortcomings of existing methods in imaging resolution, feature mining precision and efficiency, which has important applications in security and microwave remote sensing. The team was invited to write a long-article review for "Foundations and Trends ? in Signal Processing", and published an academic book "sparse perception" by Science Press.

4) Explored the terahertz-wave generation and super-resolution imaging techniques based on subwavelength artificial superstructures.

The interaction between electron beam and artificial surface plasmonic (SSP) or graphene surface plasmonic (GSP) is proposed to obtain enhanced and tunable terahertz radiation sources. Based on subwavelength artificial superstructure, SSP and Fabry-Perot oscillation effects, several terahertz super-resolution imaging schemes were proposed. The excitation and orientation excitation of SSP were realized, and the conversion of waveguide mode was realized by manipulating the SSP. Related results have been published in the journals such as Appl. Phys. Lett., J. App. Phys., Sci. Rep., Opt. Lett., Opt. Exp., J. Phys. D, IEEE-PJ, IEEE-T-PS, etc.