Investigation Of Related Characters Of Intense Current Diode Based On The Carbon Nanotubes Cathode

The intense electron beam sources based on the explosive electron emission(EEE)cold cathode have been investigated extensively in the field of High-Power Microwave(HPM).To enhance the performance of HPM generation,many aspacts of the intense electron beam sources should be studied deeply,such as emission material,fabrication technology,physics in diode and diagnostics of electron beams characteristics.The carbon nanotubes(CNTs)discovered in 1990 s might be a candidate for pursuing a novel intense electron beams source for its extraordinary perforamce in the domain of field emission(FE).By far,the properties of intense pulsed electron emission of CNTs cathode have not been discussed comprehensively.In this dissertation,the related aspects of carbon nanotubes are studied systematically and profoundly,in terms of the effects of microstucture and material peropety of CNTs cathode on its performance,fabrication of the CNTs cathode,the intense pulsed electron emission properties investigation of CNTs cathode,establishment of platform for diagnostics of electron emission uniformity and diode physics with anode effects.The detailed contents and conclusions include the followings:1.Screening effects of CNTs array and its FE optimal densityThe visualized screening effects of carbon nanotube array were obtained by using the finite-element method,which based on the COMSOL Multiphysics software.The generation criteria and the influences of parameters of the screening effects were discussed.The results indicate that when the value of normalized spacing s/l(l referred to as the length)is less than 2,the screening effects of CNTs array must be considered.For CNTs array with a fixed dimension,only to increase the density(or the number of emitter)could not achieve the purpose of obtaining the highest current.There definitely exits an optimal arrangement.Furthermore,the optimal value of normalized spacing s/l has been calculated to get the maximum emission current density for various geometrical aspect ratio l/r and electric field strength for a CNTs array with a fixed dimension.2.Initiation of vacuum breakdown and failure mechanism of the CNTs during thermal field emissionTo model thermal field(T-F)emission,Nottingham effect and heat radiation,a method based on electrostatic and transient thermal finite element analysis is developed.Considering the vacuum breakdown of CNTs,the Fowler-Nordheim(F-N)equation is not rational for the high temperature and electric field strength.The Murphy-Good equation is used to express the emission current density.By using the finite element methodology(COMSOL Multiphysics software),the influence of CNTs size,electric field strength,CNTs material parameters(such as resistivity and thermal conductivity), contact resistance,and the kind of substrate material on the initiation of vacuum breakdown and failure mechanism of the CNT are studied numerically.It was found that the thermal response of CNT during T-F emission presents three different states: heat equilibrium,vacuum breakdown,and substrate melting.There are two critical length(Lb and Lm)for the state change.The characteristics of the CNTs,electric field strength,contact resistance,and the kind of substrate material influence the value of critical length.3.Fabrication of the CNTs cathode and the relative intense electron emission propetiesThree different CNTs cathodes have been made by using electrophoresis deposition(EPD),Chemical Vapor Deposition(CVD)and also CNTs paper(or buckypaper)gluing.The intense emission properties of velvet and CNTs cathode have been studied in various aspects,such as emission capability,cathode plasma expansion,cathode initialization,emission uniformity,operation stability,outgassing property and so on.Results show that the emission capabilities of CNTs array and CNTs paper cathode are better than the velvet cathode.The orientation of CNTs does not affect the emission capability of CNTs cathodes.The small radius of the tube wall and the existence of defects are suggested as the reasons for the emission of electrons from the body of the tubes.The threshold electric field strength of intense electron emission of CNTs is about two-thirds of velvet cathode.The onset delay time of CNTs cathode is shorter than the velvet cathode about 12~17ns with the same electric field growth rate.The emission uniformity of cathode was studied by analyzing the distribution of cathode plasma spots and Cherenkov radiation light,which were captured by the high speed frame camera(HSFC).The emission uniformity of CNTs cathode is much better than the velvet cathode.Especially,the cathode plasma spots on the whole surface of CNTs array cathode are very dense and uniform.The peak outgassing pressure of the CNTs paper cathode is 0.3 Pa,which is fifth of the velvet cathode.The peak outgassing pressure of the CNTs array cathode is 0.042 Pa,which is the lowest.4.Establishment of platform for diagnostics of electron emission uniformityAn optical diagnostics based on the Cherenkov radiation is proposed to investigate the uniformity of the electron beams.Comparing to other optical diagnostics methods,it has many advantages,such as high temporal resolution,high light yield under low-energy electron beams injection.Paticle in cell(PIC)and Mento Carlo codes(CASINO and Fluka)are utilized to determine the structure parameters of the anode sleeve and quartz target and obtain the radiation characteristics.In experiment,the Cherenkov radiation spot and its time evolution processes were carried out on this platform.The temporal resolution was 10 ns.5.Characterization of a short-pulse high-power diode operated with anode effectsBased on the continuity equation,conservation of energy and the steady-state Posson equation,the laminar space-charge flow mode of the infinite relativistic planar diode and its critical value were deduced with considering the effects of anode ions flow.The PIC code was used to study the dynamics of anode plasma in high-power electron beam diode.The effect of gas type,dynamic characteristic of the ions on the diode operation with bipolar flow model were presented.The conclusions that we can draw are as follows.The type of gas desorbed from the anode material would affect the generation of anode plasma or ions flow.A higher saturation level of CO2 layer could be obtained at lower pressure than H2 O vapor.The transition time of ions could be used to estimate the onset of current flattop.The transition time is in direct proportion to the A-K gap,ion mass,saturation factor,and is inversely proportional to the diode voltage.When the ratio of ions density of anode to cathode is bigger than 25,the ions flow could be saturated to neutralize the space charge effect of electron flow.Experimental studies were also carried out.The results indicate that the time for generating the anode plasma tp decreases with increase of the deposit power on the anode.tp increases with the A-K gap decrease under the same deposit power.The effect of the fast gap closure is particularly detrimental to the long pulse diode(?s)or short A-K gap diode.The appearance of local cathode plasma flares(or nonuniform electron emission)is particularly critical for the diode closure.Once the temperature and pressure in the melted region increase up to their critical values,the materials are ejected from the surface layer.After multi-pulses,the original smooth fibers surface is covered with big sphere particulate matter(PM)and tine dust.Some fibers are gooey by each other by flake material.