| Wire arrays are exploded by pulsed power facility and a burst of x rays are produced during the stagnation of wire-array plasma.A capsule is driven by the z-pinch hohlraum and fusion reaction takes place by thermonuclear fuels with huge energy releasing.This is called Z-pinch inertial confinement fusion(ICF).Z-pinch ICF has potential to provide people clean,safe,and sustained energy.As a result scientists worldwide are strongly motivated for the pursuit of z-pinch technologies.China has actively participated in z-pinch ICF,and has made great progress in accelerator development,fusion target fabrication,z-pinch physics study,z-pinch diagnostics,and so on.One of the basic requirements for realizing z-pinch ICF is that radiation asymmetry smaller than a few percent.X-ray backlighting technology is able to directly show capsule images in the process of implosion,according to which shell departure from sphericity can be calculated and radiation asymmetry can be extrapolated.Z-beamlet backlighting system in Sandia National Laboratory is built for this purpose.There are no ready-made methods and systems to study radiation asymmetry in China.Scientists in Institute of Nulcear Physics and Chemistry are preparing to build x-ray backlighting system based on Thomson scattering method.However,this technology is touphly difficult,and the whole system needs quite a long time to build,more than ten years.Therefore it is urgent to develop a small-size backlighting system as a technology reserve to study radiation asymmetry in z-pinch ICF.We have developed an x-ray backlighting system to diagnose z-pinch capsule compression asymmetry in the thesis.The system is built based on JULONG-1 facility and is able to take two images of the capsule in two different compression moments.When the x-ray backlighting system is successfully built,tungsten wire backlighting experiment is carried out in the laboratory to test properties of the system.Dynamic imaging capability of the backlighting system has also been estimated.The backlighting system is composed of four parts,backlighting source module,scintillator module,optic module,and recording module.The four modules are connected by physical properties of the whole system,sensitivity,spatial resolution,and time resolution.The theory of backlighting technology is scintillator converts x rays passing through the capsule to fluorescence,which is collected and transmitted by optic module and imaged eventually on ICCD.X rays are produced by a pulsed X-ray tube.Compression time of the z-pinch capsule is only a few nanoseconds and x-ray pulse of the backlighting source is about 20ns,which provide beneficial conditions to take two images of the capsule.Focus point of the pulsed x-ray tube is 1 mm in diameter and fluence of x rays is about 108 cm-2.While the capsule is several millimeters in diameter and too much high-energy photons are produced in z-pinch experiments on JULONG-1 facility.Thus there is too much challenge in designing the backlighting system.A program base on MCNP and MATLAB softwares is developed to simulate the backlighting system.Capsule images are obtained by simulation.Factors such as focus point of the pulsed x-ray tube and geometric layout(distances among source,capsule and scintillator)are studied by simulation.Theoretical analysis and simulation calculations show that sensitivity of the system is determined by geometric layout,types of scintillator,and light connection efficiency of optical lens.Spatial resolution of the system is determined by focus point of the pulsed x-ray tube,thickness of the scintillator,resolution of the optic module,and resolution of ICCD.Temporal resolution of the system is determined by decay time of scintillator,and gate time of ICCD.By theoretical analysis,experimental study,and simulation calculation,the backlighting system is successfully developed through optimizing of the geometrical layout,choosing suitable scintillator,and designing appropriate light path.Theoretical analysis shows that spatial resolution of the system is 240?m and temporal resolution is 2ns.The Light beam is divided into two beams by transmission line and reflection line of the optic module,and each beam is recorded by a corresponding ICCD.By setting two different gate times of the two ICCDs,we acquire two images of the capsule in one z-pinch shot experiment.High-energy photons are measured in z-pinch experiments on JULONG-1 facility.Fluence,pulsed waveform and other parameters of high-energy photons are obtained.High-energy photons mainly come from pinch region.Fluence of high-energy photons is tens of times higher than that of x rays of the backlighting source.Pulsed wavefonn of high-energy photons is several nanoseconds later than z-pinch soft x rays.Experimental results show that high-energy photons cause serious background to the backlighting system.High-energy photons mainly influence scintillator module and recording module of the backlighting system.High-energy photons outside of the field of view of the backlighting system can be shielded by 3cm-thick lead.High-energy photons inside of the view of the backlighting system should be reduced according to time difference between high-energy photons and z-pinch soft x rays.Single tungsten wire backlighting experiment is carried out in the laboratory after the backlighting system is manufactured.By using EJ256 scintillator,the backlighting system can take images of W wire 250?m-in-diameter.Experimenatal results show that spatial resolution of the backlighting system is 235?m by processing the W-wire image using MATLAB procedure.Signal-to-noise ratio is estimated according to gray value of W-wire image and measurement results of high-energy photons.The approach to set times of two images of the backlighting system is studied,in which high-energy photon background can be avoided to a maximum extent.Calculation results show the signal-to-noise ratio is around 7.4.Dynamic imaging capability is estimated for the backlighting system.Capsule images of three different moments in the process of implosion are shown by simulation.The theorem to distinguish boundary of the capsule image is studied,and the method to calculate shell departure from sphericity is provided.Backlighting images of the multi-shell target is simulated,and the ability of the backlighting system to distinguish shells of the target is analyzed according to results of W-wire backlighting experiments.We have successfully developed a backlighting system able to take two capsule images in one z-pinch shot.Single tungsten wire backlighting experiment is carried out in the laboratory,and some valuable results are obtained.Research achievements in the thesis provide technology reserve for diagnosing radiation assymmetry,and make progress in z-pinch diagnostic technologies. |
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