Research Of Fast Neutron Penumbral Imaging In Inertial Confinement Fusion

Due to its great significance in overcoming the crisis of energy and in national defence, research of Inertial Confinement Fusion (ICF) is given enormous supports by many countries. In experiments, the hydrodynamics instabilities and the mixture of the fusion materials, which occur in the implosion process of the fusion targets, may lead to the failure of the implosion, and they may be caused by the driving dissymmetry of the driving source. For the compressing dissymmetry of the target by the driving source will finally affect the shape of the fusion core, it is very important to develop diagnosis techniques to observe this effect. Because fusion neutrons are charge free, they can escape easily from the dense plasma. So the shape of the fusion area can be observed by the fusion neutrons with some neutron imaging techniques.There are some neutron imaging techniques, while neutron penumbral imaging is a suitable one at low neutron yield level. Considering the neutron yield of Shenguang-III which is to be established in the near future, neutron penumbral imaging will be an important diagnosis technique on it. Some preparations of the technique need to be done while Shenguang-III is in building. The paper presents some of these preparations we have done. The neutron penumbral imaging technique includes two main parts: the construction of the equipments and the study of the reconstruction methods. In reconstruction of the penumbral images, linear reconstruction methods, for example, Wiener Filter method, are often used, but the linear approximations in these methods always cause serious distortion in the reconstruction results. Recently, by introducing the conception of virtual atom and virtual cluster, Molecular Dynamics (MD) was used to solve the optimization problem in Intensity Modulated Radiation Therapy Plan, and good results were obtained. In the same way, the MD method can be applied to reconstruct the neutron penumbral images, since the reconstruction of the neutron penumbral images is actually a nonlinear optimization process. By far, the reconstruction program based on the MD method has been completed, and some good reconstruction results of simulated penumbral images on different noise level were also obtained. Using the method, the resolution of 20μm of neutron source image is achieved, and some discussions of the equipments were also given with the help of the Monte Carlo method which can be employed to simulate the neutron penumbral imaging processes.The paper mainly introduces the application of the MD method in the reconstruction of the neutron penumbral images. In Chapter 1 the research background is introduced. Then Chapter 2 presents the principle of penumbral imaging technique. In the Chapters 3-4, by comparing the MD method with the Wiener Filter method, some advantages of the MD method in reconstructing the neutron penumbral images are given in detail. And by the MD method and imaging simulations, the paper gives some discussions on equipments, and some demands on point error of aperture and neutron yields are also put forwards.