Research On Electronic Optics System And Backward Wave Oscillator Based On Carbon Nanotube Cold Cathode

Vacuum electronic radiation sources?VERS?have been applied widely in national economy and national military fields.The traditional VERS usually adopt the existing thermionic cathode technology.After decades of application and development,it has been found that the thermionic cathode has its inherent disadvantages,such as the need for filament heating,large working volume,long preheating time and the slow response.Therefore,the development of vacuum electronic devices towards high frequency and high power has been somewhat restricted.Moreover,it is difficult to realize the miniaturization,integration and convenience of the devices.The field emission?FE?cold cathode has been proposed to solve the problems.It has many advantages,such as low operating temperature,fast switch-on time,long life,facile and inexpensive integration,coupled to the potential for miniaturization.As a new type of cold cathode material,carbon nanotubes?CNT?are the excellent candidate for field emission due to its unique physical and chemical characteristics:large aspect ratio,good mechanical properties,both hardness and toughness,and good electrical conductivity.The particular advantages of CNT cold cathode are its simplicity,ease of fabricate and use,and low cost.Compared with the metal tip,it is not easy to be damaged.Above all,it has considerable field emission current density.Therefore,CNT has been selected as the emission electron source to design and study the cold cathode high-power vacuum electronic devices.Electron optics system is one of the most important components of VERS.Therefore,the development of a CNT cold cathode electron optics system with high quality electron beam and large emission current density is a focal point research in this dissertation.Combining with the theoretical calculation and experimental exploration in the early stage of this work,three kinds of electronic optical systems are proposed.First,A gridded CNT cold cathode electron optics system with array cathode substrate is proposed.The structure adopts grid-controlled field emission,which can reduce the regulating voltage.The cold cathode substrate is processed into an array of square columns,and every square column is coaxial with the grid mesh.It can effectively solve the problem of the grid intercept.The cold cathode substrate of array of square columns effectively not only reduces electrostatic shielding effects but also improves the field emission enhancement factor,thereby improving the field emission current and beam transparency.The simulation results show that the electron beam transparency of 100%is obtained,and the total emission current is 7 mA in the effective emission area of 0.18 mm² when the electric field intensity on the cathode surface is4.8kV/mm.Second,a dual-gridded CNT cold cathode electronic optics system with planar cathode substrate is proposed.The cathode surface of this structure is arrayed by the cathode grid.The separating control grid is adopted to apply the field emission electric field on the cold cathode surface.The cathode grid is coaxial with the separating control grid,which can effectively solve the problem of grid intercept and electrostatic shielding effects.Compared with the first structure,its outstanding advantage is that the cold cathode substrate is planar,which greatly lowers processing difficulties of cathode substrate and decreases the technology requirements of CNTs growth.The simulation results show that under the electric field intensity of 7.1kV/mm,the total emission current is 76.4 mA,and the final current is 76.3 mA after electrons moving through the grid and anode,the beam transparency of¹00%is obtained.The electron beam area is compressed from the original 42.25 mm² to 4 mm² with an electron beam compression ratio of 1/10.6.The experimental results of the triode show that under the electric field intensity of 7.154 kV/mm,the total emission current is 77.1mA,the grid interception current is 12.1 mA,the finally operating current of 65 mA is obtained,and the beam transparency is 84.31%.Third,a dual-anode CNT cold cathode electronic optics system with curved cathode substrate is proposed.In this structure,a cone wall is used as cold cathode surface.Due to the particularity of the structure,emission cathode surface is greatly improved,thereby enhancing the emission current.Compared with the previous grid-controlled structure,the structure adopts a control anode instead of control grid,which effectively solves the problem of grid intercept and greatly improves the beam transparency.The simulation results show that under the electric field intensity of6.9kV/mm,the total emission current is 230 mA and the beam transparency is up to¹00%.The electron beam area is compressed from the original 46.34 mm² to 0.9 mm²,with an electron beam compression ratio of 1/51.5.The experimental results of the diode show that the maximum emission current is 260 mA at the electric field intensity of 7 kV/mm.The second and third electronic optics system are proved by the experimental results,so the 8 mm and 0.22 THz backward wave oscillator?BWO?are designed by using these two electronic optical systems.The simulation results of the 8 mm BWO based on the third structure show that the operating current of 220 mA is obtained,and the average output power of the BWO is 180⁴95 W.The simulation results of the0.22 THz BWO based on the second structure show that the operating current is 50 mA,the operating voltage is 21 kV and the average output power is 32 W.The experimental test results of the 8 mm disk-loaded waveguide BWO based on the third electronic optical system show that when the operating voltage tuning range is36.88³7.8 kV,there are two high-frequency output signals of 33.412 GHz and 33.645GHz,corresponding operating current is 285 mA and 248 mA respectively,and the maximum output power is 240 W and 230 W respectively.A ka-band circular waveguide TM₀₁-rectangular waveguide TE₁₀ mode convertor is adopted in the output circuit.The frequency range in this mode converter is 32.3³4.7 GHz with the transmission coefficient of-1dB,and the mode purity is greater than 99.5%.Here we demonstrate that it is possible to realize,for the first time,a hundred watt-level ka-band BWO using a CNT-based field electron emitter.Our results indicate that the output power of the CNT-based cold-cathode VERS has been improved from mW level by a factor of 10³,which represents a significant breakthrough in the application of CNT cold-cathode in VERS and provides a new implementation method of miniature,compact high-power VERS for the future.Furthermore,it has initiated a pioneering exploration for the development of high-power,high-frequency,ultra-wideband micro-nano cold cathode radiation sources in the 5G era.