Experimental Study Of Hohlraum Energetics

The laser-driven hohlraum can produce radiation temperature of 3 million degrees,electron temperature of tens of millions degrees,and pressure of 1 billion atmospheres.It is one of the major ways to drive nuclear fusion and extreme astronomical phenomena in laboratory.The primary goal of hohlraum energetics was to provide suitable x-ray sources for the capsule implosion,and understands the energy partition in the hohlraum,including the propagation and absorption of the laser,the hohlraum radiation peoperties,and the wall and spot motion in the hohlraum.In this study,a suit of experimental methods are established according to the whole process of hohlraum energetics,especially the multiple line-of-sight LEH flux method.Hohlraum energetics are studied through experiments combined with simulation.The radiation properties of different regions inside the hohlruam were studied from multiple line-of-sight,and several technical problems restricting the application of this method were solved:1)The continuous phase plates(CPP)was proposed and designed to suppress LPI.The fluctuation of radiation temperature was reduced from 10～15 eV to 5 eV,and the x-ray conversion efficiency was increased from～70%to～90%.It established the principle that the Shenguang-Ⅲ prototype and subsequent laser facilities must be equipped with CPP to improve the repeatbility of experiments.The focal spot and LEH of this size are still the standard configuration for the Shenguang-Ⅲ prototype.2)The low power prepulse was designed to ablate the LEH membrane,which significantly improved the laser absorption efficiency,and made the x-ray flux of the gas-filled and vacuum hohlraum can be compared.The quantitative effect of the sealing membrane on the measurement of the x-ray radiation was found in the cryogenic gas-filled hohlraum.3)It is found that the x-ray from laser ejection hole(LEH)clipping,unconverted light are the main source of unwanted x-ray background,and a variety of techniques are developed to reduce the noise signal to～3.5%.4)The factors leading to the abnormal repeatability of data between different lines of sight were found.The evaluation method combining experiment with three-dimensional view factor code was established,and the effects of the beam-to-beam energy balance and the laser pointing accuracy on the x-ray radiation were obtained.A variety of information,such as the total amount of x-ray radiation,spot non-spot contrast,corona plasma motion and so on,were obtained based on the multiple lines of sight method:1)the influence of two-dimensional effects,such as laser cone and blow-off bubble stagnates around the hohlraum axis,are evaluated.It is found that the x-ray emission near the hohlraum axis may be the dominant factor leading to the difference between the simulation and experiment.2)The influence of electron conduction on the energy distribution in the hohlraum is analyzed.The simulation results using the flux limiter fe=0.1 and removing x-ray emission around the hohlraum axis are in good agreement with the experiments.3)The temporal behavior and fraction of the hohlraum x-ray radiation were obtained under various conditions.The fraction of the M-band radiation in the vacuum hohlraum basically showed a linear increase relationship with the peak radiation temperature.4)By opening double diagnostic holes in the hohlraum wall,the difference of x-ray radiation between the laser spot and the re-emitted wall was studied,and the temporal evolution of the contrast between the spot and non-spot x-ray radiation was obtained.Aiming at the two most important regions inside the hohlraum,the motion of laser spot and the laser-heated coronal plasma are studied by multi-view and multi-energy x-ray imaging observation:1)By observing thin-walled hohlraum at perpendicular to the axis,laser spot movement along the axial depends on competition between the radiation ablation pressure and laser ablation pressure.Therefore there is little difference between different laser energy.However the effect of the flux limiter is large,a flux limiter of fe=0.07 matches the experiment well;2)By observing hohlraum along the hohlraum axis,the corona motion along the radial direction was studied.The difference of corona movement was very small under different flux limiter,and the difference was large under different hohlraum filling gas density.