| X-ray framing cameras have the characteristics of three-dimensional resolution of time,two-dimensional space and energy spectrum.It has become a very important diagnostic device for X-ray measurement in ICF experiments.This equipment is an important technical support for research on high-energy density physics,inertial confinement fusion,photo-physics,and photochemistry.The data obtained is an important basis for analyzing ultrafast processes.Frame imaging diagnostic technology has important supporting significance for further research in related fields in China.The post-combustion stage of ICF implosion is about 100 ps,and the target core is compressed to tens of micrometers.It is extremely challenging to effectively diagnose the detailed changes of the plasma conditions in such short combustion duration.This makes it more important to improve the time resolution of the detector to fully characterize the ICF combustion process.There are two main ways to improve the time resolution capability of the detection signal: one is to improve the time resolution of the detection equipment itself;the other is when the time resolution capability of the detector cannot be improved,the measured signal will be widened in time,and then the outputted signal by detector will be time-compressed and pulse-reconstructed.The second approach is to load an excitation pulse that changes with time to a photocathode-mesh structure,thereby introducing a time-dependent photoelectron acceleration potential.The electric field imparts axial velocity dispersion to photoelectron beam.Due to the different velocities of the front and back electrons in the electron beam,the electron beam is stretched when it reaches the input surface of MCP through the drift region.In this way,the time resolution of the splitting camera and the photodiode can be improved.This thesis focuses on the theoretical model,specific influencing factors and experimental verification in the process of using pulse dilation technology to widen the detected signal to improve the resolution of the device.In this dissertation,the main completed research work and innovations has been shown below.1、The model of the time dilation framing camera is established with Matlab by using the Monte Carlo method,the finite element method and the finite difference method.Based on the model,the temporal resolution and spatial resolution of the time dilation framing camera are simulated.When three magnetic lens are used at an imaging ratio of 1:1,the photocathode(PC)is applied with-3.0 kV DC,and the anode mesh is at ground potential,the spatial resolutions of the centre,6mm,12 mm 18mm and 24 mm off-axis points are 20.8 lp/mm,11.4 lp/mm,5.4 lp/mm,2.7 lp/mm,and 1.7 lp/mm,respectively,which are higher than the spatial resolution of the same off-axis position when using single lens and double lens.The farther the electrons are emitted from the cathode surface from the axis,the smaller the distance between the imaging position and the photocathode,and the worse the spatial resolution is.The spatial resolution can be improved by combined imaging.The transient performance of the image tube is numerically simulated.By using the electron beam pulse dilation technology,when the PC is applied with about-1.4 kV DC,and the pulse slope is 10 V/ps,the time resolution of the image tube can be improved to about 2ps.The relationship between the temporal resolution and the PC bias voltage,the gradient of the PC excitation pulse,the drift length is studied.The temporal resolution is improved with the decreasing PC bias voltage,increasing PC pulse gradient and increasing drift length.2、The spatial performance of the experimental platform of the pulse dilation framing camera is measured by using resolution plates photoetched on the PC.When the PC is applied with-3kV DC and the imaging ratio is 1:1,the static resolutions of single-lens system are measured to be 9.4 lp/mm,4.7 lp/mm and 4.2 lp/mm for 6mm,12 mm and 15 mm off-axis points,respectively.The larger the PC bias voltage,the better the spatial resolution of the imaging;the farther the electron emission point is from the axis,the worse the spatial resolution of the imaging.When the spatial resolution is better than 10 lp/mm,the single lens,double lens,and triple lens are in the range of 6 mm,9 mm,and 18 mm off-axis,respectively.The temporal resolution is measured by a fiber bunch.When the PC bias is applied with a DC bias voltage of-3.0 kV and an excitation pulse with a slope of 3.0 V/ps,the time resolution of the device was 9 ps,which was much higher than 78 ps without using pulse dilation technology.The relationship between the temporal resolution and the PC bias voltage is obtained.The variation of the temporal resolution with the gradient of the PC excitation pulse is also provided.It shows that the temporal resolution is improved while the bias voltage between the PC and the mesh is decreased.The temporal resolution is improved while the gradient of the PC excitation pulse is increased.The dynamic paraxial spatial resolution is measured to be better than 10 lp/mm.3 、 According to the Time-Domain Reflectometry(TDR)method,the transmission speed of electrical pulses on PC and MCP microstrip lines is basically the same.In the case of an imaging ratio of 2,the transmission shutter effect of a pulse dilation frame camera was tested using X-rays.The FWHM of the image signal obtained by the framing camera is about 9.8 ps.Through analysis,it is proposed by limiting the imaging ratio,thereby eliminating the adverse effect of the transmission shutter effect on the imaging of the pulse dilation framing camera.4、A photodiodes detector with a transmission photocathode is provided to measure the ultrashort electron beams among 1.8-3.0 ke V.Both of the photocathode and the collector are formed with a planar transmission line structures to achieve fast response time.The induced current in the collector produced by the transited electron beam is simulated by using the Ramo theorem.The FWHM of induced pulses on the cathode and collector are measured.They are approximately 190 ps and 600 ps respectively and the electron beam TOF from cathode to collector is about 15.46 ns,while the photocathode is applied with a-3.0 kV bias voltage.The time of flights of the electron pulses from photocathode to collector are also measured corresponding with different bias voltages applied on photocathode.The experimental results show that the intensity of the output signal of the collector exhibits high-order nonlinearity with the length of the electron beam from the collector;the FWHM of the output signal increases with the decrease of the cathode bias voltage.5、The pulse dilation technology was used to experimentally test the dynamic characteristics of the photodiode,and a time recovery algorithm for the pulse dilation was established.When the PC bias is applied with a DC bias voltage of-3.0 kV and an excitation pulse with a slope of 3.0 V/ps,experimental results show that the average broadening ratio of the pulse of the electron beam is 10.6 times,and the output signal width of the detector is 255ps;Using this algorithm,the time resolution of the pulse dilation photodiode is improved to be 25 ps,which is about 10 times higher than the time resolution of 261 ps without the pulse-dilation technology. |
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