Study On Ablation Characteristics Of Si Doped Si / Ge Radiation

In indirect-drive inertial confinement fusion (ICF), laser light is absorbed by the interior gold walls of the hohlraum and converted to x-rays. The x-rays consist of soft x-rays(0-2keV) and M-band x-rays(2-4keV)。Because of the short mean free path, the soft x-rays can be stopped by the ablator before they get to the fuel. However, the M-band x-rays which are of long mean free path can transmit further, these x-rays can reach the fuel and preheat the fuel before the arrival of the shock wave. The preheating effect can significantly reduce the degree of compression of the fuel, these can be harmful to the ignition.There are two means to reduce the preheating effect. One is reducing the M-band fraction of the radiation source, the other is the use of a mid-Z dopant in the ablator by suppress the transport of the M-band x-rays. This thesis is mainly about the second mean. We can use a CH doped with Silicon(Si) or Germanium(Ge) target to reduce the preheat. However, the dopant can also be harmful to the velocity, ablation front instability and mix. In this thesis, we will discuss the preheat in three parts:First, we study the relationship between the transmission flux and the preheat temperature by using the radiation hydrodynamic code Multi-ID. The preheat can be divided into two processes:at first, the x-rays pass through the ablator and form the transmission flux, it’s a process that the ablator reforms the radiation source spectrum, and then the transmission flux preheat the fuel. We find the more transmission flux leads to higher preheat temperature by contrasting the transmission flux of the thin targets and the preheat temperature of the thick targets. This will be discussed in the third chapter.Second, we use the method mentioned before to evaluate the difference of the two dopants Si and Ge. We have performed the experiment in the ShenGuang-II high power laser facility. There were two kinds of targets for the experiment, one is thin target, the other is thick target. For the former, we studied the transmission flux of the soft x-rays. And for the latter, we measured the M-band transmission flux. In the experiment, we found that the Si coated target can not only allow more soft x-rays pass through, but also absorb more M-band x-rays. In simulation we got the same conclusion by using the radiation hydrodynamic code Multi-1D. This will be discussed in the fourth chapter.Third, we study the influence of the radiation source characteristic (consist of the radiation temperature and the M-band fraction) on the radiation ablation of the Si-doped CH target by simulation. We have computed the total M-band x-rays in the source with different temperatures and different M-band fractions. We find that the transmission flux of the same target in different radiation source is different. And the doped concentrations can also effect the ability of the target in restraining the transmission of the M-band x-rays. The preheat temperature is determined by the radiation source and the ablator. So it’s necessary to estimate the influence of the radiation source and the dopant on preheat when design the ignition target.In summary, we have studied the radiation ablation process of CH and CH with dopant, this can provide guidance for the design of the ignition target in the future.