Simulation Research On High-order Harmonic Terahertz Gyrotron With Paraxial Small-orbit CNT Cold Cathode

Gyrotrons,fast-wave devices with some of the highest recorded output powers,have received widespread attention in space communication,thermonuclear fusion,and medical diagnosis.With the increase of operating frequency,gyrotron will greatly increase the demand for magnetic field.Although the required magnetic field,can be greatly decreased by engineering in high-order harmonic operation,mode competition aggravates as well at the same time.Compared with the traditional thermionic cathode,field emission cold cathode can almost support instantaneous turn-on,meet the requirements of ultracompact design and operate at near-room-temperature.The electron emission source based on carbon nanotubes(CNTs)cold cathode has been widely proved to provide high current density,excellent chemical and thermal stability,low opening voltage.It is a suitable alternative to thermionic cathode emission source and meet the demand of vacuum electron radiation source for excellent emission source.The major work of this paper is as follows:1.The parameters of gyrotron are designed and analyzed.The three-stage cavity structure is designed,and TE₃₇ mode third harmonic mode is selected.The starting current and mode competition in the guiding center radius of 0 mm,0.055 mm,0.305mm,and 0.56 mm have been analyzed,which support the result of aggravating mode competition as the increase of the guiding center radius.2.Based on the experimental results of CNTs,dual Ni80Cr20 alloy wires with CNTs were embedded in the groove to obtain the field emission parameters.The annular surface emitting band and dual Ni80Cr20 alloy wires embedded in the groove have been analyzed in detail in order to obtain the determination coefficient R² between the two structures.Simulation results show that dual Ni80Cr20 alloy wire embedded in the groove can be replaced by the annular surface emitting band.In order to reduce the mode competition of gyrotron caused by engineering in high-order harmonic,a paraxial small-orbit electron gun based on CNTs cold cathode has been designed with beam current of 160 m A at an accelerating voltage of 23.5 k V.The source supports the electron beam with a guiding center radius of 0.055 mm and a velocity ratio of 1.1.Simulation shows that the transverse velocity dispersion is 6.7%,the longitudinal velocity dispersion is 8.9%and the current density of electron beam at the exit port is420 A/cm².3.The nonlinear theory of gyrotron is adopted to calculate the beam wave interaction under different guiding center radius,which support that the beam wave interaction efficiency decreases with the increase of guiding center radius.At the same time,the magnetic field and voltage that affect the beam wave interaction efficiency are analyzed.The beam wave interaction efficiency of gyrotron is 5.28%,and the output power is 198 W.