Study On X-ray Preheating Phenomenon In The Void Hohlraum

Powerful lasers are injected into a cavity with high-Z wall to generate x-ray radiation field. These intense thermal x-rays are dedicated to drive high pressure shocks in the fusion ignition campaign and in high energy density physics studies. However, the x-ray radiation fields always deviate from ideal blackbody radiation. Some high energy photons are produced and cannot be fully equilibrated inside the hohlraum. These photons could preheat and change the hydrodynamic state of the target before shocks. The preheating effect is deleterious to the low entropy implosion process and could reduce the performance of capsule. Meanwhile, preheating would affect some of the most concerned issues in ICF, such as mixture and hydrodynamic instability. Thus, a detailed study on the preheating effect is an urgent demand. Nowadays, various preheating researches have been carried out worldwide, including both experiments and simulations. More precise diagnostics are established and reliable predictions are demonstrated. However, the preheating study of our own is still lacking.This thesis mainly concerns the multiple effects of preheating. We try to set up a self-consistent connection among different experimental observations through simulation. The work is mainly divided into two parts:1) the possitive diagnostic technique is applied to measure the preheating of the sample’s rear surface. We propose a new way to evaluate the evolution of the characteristics of the spectrum inside the hohlraum combined with numerical simulation;2) the pyrometer system is established to measure the brightness temperature of the sample. A new way is proposed to efficiently simplify the difficulties during calibration. This technique is capable of measuring temperature of preheating and shocks with adequately high spatial and temporal resolutions.1. Experimental observation of rear surface motion and relative simulationPreheating is the result of interaction between the driven source and the sample, which is largely affected by the evolution of the radiation source inside the hohlraum. Thus, the understanding of the characteristics of the raidiation source is of great importance. The common diagnostics for the radiation source is mostly from the laser entrance hole (LEH), which could be quite different from those observed by the sample for various reasons, such as cold plasma absorbtion, observation angles and shrinkage effect.We present a new approach to infer the evolution of radiation source by analyzing the rear surface motion of a sample due to preheating. For aluminum, x-ray around1.5keV and beyond3keV is the most efficient source for preheating. By properly adjusting the portions of the two parts, the simulated rear surface motion is matched with experimental results. Further more, with a suitable sample, we are proposed to infer the evolution of the x-rays within certain bands inside the hohlraum. Simutaneously, simulations provide the solution for the rear surface release state. Using the Drude model, we could calculate the reflectivity variations. The results also accord with VISAR results, which enable a crosscheck in one shot.2. Temperature measurement techniqueTemperature measurement is a direct way to observe the preheating effect and could associate with the simulation work. However, the uncertainty is relatively large and the usage of the technology is mostly limited by the complexity of calibration.In our work, we optimize the signal channel according to the experimental design and provide a suitable assessment on the performance. A relative simpler way for calibration is designed. We firstly measure the relative spectral response of the system. Then the standard material is used as a light souce for absolute calibration. This avoids the measurement error due to the differences in optical path between calibration and experiment. The platform is tested for the first time in laser drive shock experiment and resonable results are acquired. The uncertainty for the data is roughly estimated to be around9%to15%.Up till now, we have carried out a series of research on preheating effects. With the diagnostic technique demonstrated in experiment and simulation analysis, we are able to make a quantitative analysis of preheat phenomenon with internal coherence.