Research Of Pulse-Driving Field-Emission Performance And Its Application

The field-emission electron source has the advantages of instantaneous start and strong pulse emission,and has good application prospects in the micro vacuum devices.In the field-emission devices such as X-ray tube and microwave tube,the cold cathode electron source is in the pulse mode,and only the vacuum convection current contributes to the radiation and high frequency power output of the device.Compared with the steady-state field-emission,the physical process of the pulse-driving field-emission is more complex.It involves not only the tunneling electron emission from field emitters,but also the displacement current between the electrodes in the vacuum environment,and the influence of the charge and discharge of the capacitance on the field emission performance.Therefore,it is very important to study the physical process and circuit model of pulse-driving field-emission and to explore the instantaneous state of the pulse-driving field-emission convection current.The study of field-emission performance of DC mode is relatively mature,while the research of field-emission performance of the pulse mode is not intensively and systematically.Aiming at the above problems and from the application of X-ray tube perspective,the pulse-driving field-emission performance of the diode and triode structure is studied.At last,the pulse-driving field-emission is applied to the X ray tube.The main research work and results are as follows:1.In order to study the performance of pulsed field emission,a high voltage pulse source is designed and fabricated.Due to the special requirements of pulsed field-emission,a high voltage pulse source is developed first in this work.The high voltage pulse source connects five IGBTs in series and uses FPGA to generate short pulse signal.The short pulse signal will be divided into five signals by the optical fiber synchronization circuit,to achieve synchronous control of the five IGBT switches.The source can produce 0-6kV amplitude,400W peak power,ms level pulse width and adjustable high voltage pulse.2.The pulsed field emission circuit model is established,and the cause of the transient current fluctuation is analyzed,and the physical process of the pulse-driving field-emission is studied experimentally.In the experiment of pulse-driving field-emission performance of diode structure,it is find that the transient current of pulse-driving field-emission is not stable,and the fluctuation of field-emission current increases with the increase of the field-emission current.In order to analyze the phenomenon,three sets of experiments are designed and a circuit model is established.The results of experiments and numerical simulations show that the non-linearity of the field-emission and the inter-electrode capacitance lead to the fluctuation.3.The transmittance of electron beam through the gate grids at high gate voltage in the pulse-driving field-emission is improved by depositing MgO film and covering the carbon film on the grid.In the experiment of pulse-driving field-emission performance of triode structure,it is shows that the transmittance of the grid decreases sharply with the increase of grid voltage.For example,when the field-emission current is 6.3mA,the anode current is only 1.25mA,the transmittance is 19.8%.In the experiment,in order to improve the performance of the grid,150nm thick MgO film and carbon film are coated on the grid.The final results show that the transmittance of the deposited MgO film is above 54%,and the transmittance of the coated carbon film is more than 43%,which is better than that of the original untreated metal grid.