High Temporal Resolution Framing Imaging Technology Based On Time Domain Modulation

Inertial confinement fusion(ICF)is one of the most promising technologies to drive the revolution of energy source through controlled nuclear reaction process.Owing to ultra-short duration(~200 ps)and ultra-small volume(~200 um)of the pellet implosion process in ICF,framing cameras with high time resolution and two-dimensional image-detection ability have been critical diagnostic devices in high energy physics,especially in measurement of the pellet implosion symmetry,hydrodynamic instability,and plasma uniformity in ICF researches.For the current framing camera based on gated MCP technique,the imaging is limited by the transit time of photoelectron in MCP,so that the time resolution can't exceed 60 ps.Due to this shortage,only 3 images can be obtained during the key process of pellet implosion,which is far more unsatisfied in ICF research.Aiming to meet ICF demands for ultrafast two-dimensional diagnostic technology,a new picoseconds framing imaging system,which can achieve picoseconds time resolution(3.3 ps),based on temporal modulation technique has been designed and developed to solve the diagnosis problem during the key process of the pellet implosion.By theoretical researching its working mechanism and by fabricating and testing the core photoelectric device,the effects of tube structure,high voltage parameter,and laser characteristics on the detector performance are elucidated,the feasibility of using temporal modulation technique to obtain ultra high time resolution is verified.The main contents of this thesis are as follows:1.The principle and process of the temporal modulation in the proposed framing technique are analyzed and researched.According to the deduction and transition process of photoelectron,the theoretical model of the temporal and spatial characteristics of temporal modulation electron optics system is established,the equation of temporal and spatial resolution control is proposed,and the factors that estrict the temporal and spatial resolution are determined.Based on the analysis and calculation of this theoretical model,the system time resolution is mainly determined by the temporal broadening of the electron pulse when it is much larger than the physical time resolution(0.85 ps)of the system,and determined by physical time resolution itself when the two values are similar.2.The relationship between the parameters of the system are analyzed,and the values of the expansion factor,drift region length,scanning pulse slope,and scanning pulse amplitude are determined.A model based on the electron optics simulation software is established,and the electron optics design is completed.This design is analyzed and evaluated using a program based on temporal and spatial modulation transfer function analysis.Calculation results show that the theoretical time resolution of the temporal modulation framing technique can reach 1 ps.3.Based on the design,analysis and simulation,the image converter tube(including cathode,grid and focusing system),and gated MCP camera are developed.High voltage drive pulse of several kilovoltage and 100 picoseconds are realized based on the avalanche transistor and tapered transmission line technique.The mechanical structure design,machining,assembly and leak detection of the system are completed.4.Experiment schemes are designed and installed to test the static and dynamic time resolution of the imaging system.Using the spatial resolution test board and ultraviolet lamp,the spatial resolution of the imaging system is tested.According to the Rayleigh criterion,the static spatial resolution exceeds 5lp/mm.Using 8ps ultraviolet laser and Mach-Zehnder interferometer,the dynamic temporal resolutionis tested.Based on the analysis of the experimental results,the dynamic temporal resolution of the imaging system achieves better than 3.3ps,which meets the design requirements.The innovation of this thesis is shown as follows:1.Electron optics theory is proposed for the first time to be applied in framing echnique.A novel imaging method based on temporal modulation technique is presented,which enlarges the duration of the electron pulse and therefore improve the time resolution(from tens ps to 0.85 ps).2.A theoretical model of temporal and spatial characteristics of the temporal modulation electron optics system is established according to the deduction and transition process of photoelectron.The principle and process of the image converter tube in the proposed framing technique are analyzed and researched.Results show that the optimization of the amplitude and slope of the high voltage drive pulse and the flight distance of the photoelectron is crucial for the improvement of the system time resolution.3.A high-voltage(amplitude about 3.2kV)? low jitter(about 20ps)? high speed(width about 200ps)pulse generation circuit has been designed.A "constant current conduction" method is proposed.Tapered transmission line was designed so that the avalanche transistor current is consistent.Dynamic inductor of the pulse generation circuit was effectively reduced.Technical support was provided for time domain modulation framing camera.4.Long magnetic lens has been used in time domain modulation framing camera.Through the optimization design of the magnetic field distribution,photoelectron emission energy distribution and space charge effect caused by the electronic space dispersion were overcome.Spatial resolution of the imaging system was improved.