Research And Design On Electron Optical System In Terahertz Band

With the increasing insight into electromagnetic waves, scientists have found a ―blank‖ field of electromagnetism-the Terahertz wave(THz wave) which is between the millimeter wavelengths and infrared wavelengths and with great scientific research value.THz wave has a frequency range of 0.1T to 10T( wavelength range from 3000 to 30 micron). THz wave occupies a very special place within the scope of electromagnetic spectrum. It lies between millimeter wavelengths and infrared wavelengths. The main reasons why THz wave has not been fully developed: one is the technology, especially the processing technology, and the other is THz source has not been adequately addressed.This paper studied the electron optical system that THz wave source mainly concerns, and designed the system with low voltage and compact structure in THz band. The structure of electron gun is studied in this paper based on the analysis of force of electron beam in the system, electromagnetic field and motion trajectory equation of electron beam. The multidimensional and one-dimensional space charge limited current are compared in this paper. With the help of CST software many parameters are researched, including the voltage U between anode and cathode, the spacing d between anode and cathode, the radius R of cathode emission surface and the proportion of different coefficients K. The initial parameters of the electron are determined by the structural parameters of two-dimensional and one-dimensional space charge limited current. The main parameters of the electron gun are d=3mm, K=0.25, mr ≈0.05 mm.In order to obtain a good laminar flow and better transport of the electron beam, in this paper the magnetic structures of two different structures are explored, one is a solenoidal magnetic structure, and the other is a special permanent magnetic structures. The two structures are modeled and simulated to analyze the parameters how to impact on the magnetic field with FEMM and CST software. Through the simulation and optimization, optimal results of solenoidal magnetic structure and the permanent magnet structure are determined. With the above mentioned magnetic field, the voltage of 8.5kV and cathode emission current density J <5A/cm2, using CST software to optimize analysis of the transport of electron beam, the optimal results of the electron gun are gotten, for the solenoidal magnetic:beam transmissions of 93.7%,the emission current density J=3.47A/cm2,the circuit current density J1=138.8A/cm2,the compression ratio M2≈40, the perveance P =0.0281μP; for the permanent structure:beam transmissions of 97.23%, the emission current density J=2.1A/cm2, the circuit current density J1=111.3A/cm2,compression ratio M2≈53, the perveance P =0.0288μP.