On Issues Of Laser Indirect-drive Inertial Confinement Fusion Implosion

For laser driving inertial confinement fusion (ICF), It is a key point to assemble the required plasmas for ignition by the method of implosion. The implosion physics is an important part of ICF research. The achievement of ICF ignition requires to understand the implosion physics and to obtain physical laws of implosion. This thesis is dedicated to the physics and the applied technology of the ICF implosion.In chapter one, we review the development and present status of ICF research, introduce the fundamental processes of ICF and the main physical issues involved.In chapter two, the difference of the two phases of shock compression and inertial compression in the implosion is firstly discussion. Then we propose that a two-peak structure is formed in the fusion rate by adjusting the shell thickness of a capsule to vary the relative strength of shock compression and inertial compression. According to this proposal and the given conditions of Shengguang ? Prototype laser facility, we design a type of targets with low pressure (10atm) gas fill and a type of targets with high pressure (100atm) gas fill. In the experiments of the low pressure gas targets, the theoretically predicted two-peak fusion rate is obtained, but the experimental magnitude of the two peaks is still much lower than the predictions of one-dimensional simulations. In the experiments of the high pressure gas targets, the neutron yields of the shock compression reach 74% of the predictions of one-dimensional simulations, while the neutron yields of the inertial compression reach only 5%?30% of the predictions of one-dimensional simulations. It is shown in the experiments of both types of targets that the inertial compression is much more sensitive than the shock compression to the affection of multi-dimensional imperfect factors.In chapter three, the physical laws of the deceleration phase are investigated. A "uniform density thick shell model" is firstly developed from R. Betti's thick shell model by assuming that the shell's density is uniform. The scaling relations between the stagnation quantities and the variables at the beginning of deceleration are derived from the uniform density thick shell model. The scaling laws are in reasonable agreement with the numerical simulations of implosion. The uniform density thick shell model and simulations indicate that the compression could not be adequately fulfilled unless the shell mass reaches a critical mass. When the shell mass reaches the critical mass, the stagnation quantities depend on the ratio of the shell ?u2 (p is density, u is velocity) to the driving pressure. The improvement of ?u2 enhances the inertial compression and improves the stagnation hot spot pressure. Using the scaling laws and the general Lawson Criterion, the requirements for ignition are discussed.In chapter four, we investigate the control of fuel adiabat by shaping the driving pulse. Starting from the adiabat level required for low-adiabat implosion, we analyze the changes in fuel entropy coming from the heating of shocks generated by the driving pulse, discuss the principle for lowering the fuel adiabat by controlling the number, strength and launching time of shocks, and present the driving pulse shape fulfilling the demand of fuel adiabat control.In chapter five, we propose that a denser layer is used as a pusher to enhance the implosion and release the requirement of fuel entropy control. The pusher is insert between the fuel and the ablator. The fuel in the capsule should be reduced to preserve the shell velocity. The pusher enhances the shell average ?u2 to improve the hot-spot pressure. Two conceptual designs of ignition target are presented. The designs both select Aluminum (Al) as the material of the pusher. One of the designs use CH plastic as the ablator and the other use high density carbon (HDC). Simulations of the implosions indicate that the hot-spot pressure of the two pusher target designs are both close to the low-adiabat (adiabat factor?1.5) traditional ignition target designs, although the adiabat factors of the CH and HDC pusher targets are as high as 2.92 and 2.38 respectively. The pusher target is theoretically more ablation stable as the implosion is driven by a high-adiabat pulse. The high-adiabat pulse is shorter because it launches a stronger first shock. The driving pulses of the conceptual pusher target designs here are short enough that the implosion may be driven by a "low gas fill density" hohlraum in which the unfavorable affect of laser plasmas interaction is mitigated.In chapter six, the dynamics of the deceleration phase and physical mechanism of the hot-spot pressure improvement are investigated in the implosion of shock ignition by the method of numerical simulation. It is indicated that the reason of hot-spot pressure improvement in shock ignition lies in that the shell density is enhanced by the collision of the return shock and the ignitor shock. Comparing to the traditional central ignition, the coupling efficiency of hot-spot energy is not increased in shock ignition. The ignition energy of shock ignition is reduced at the price of the smaller hot-spot volume.