The Precise Measurement And Modeling Of M-band Radiation Flux From Void Hohlraums

In indirect-drive scheme of inertial confinement fusion, the interior wall of high-Z hohlraum is heated with intense laser pulses to generate soft x-rays, which ablate and compress the fusion capsule and finally ignite it. The M-band x-rays in the range of1.6keV～4.4keV are also generated. These x-rays can penetrate beyond the capsule ablation surface, preheat the fusion fuel prior to the arrival of shock front, and thus seriously degrade the implosion efficiency. In order to minimize the preheating effect, we need accurate information of the time-resolved absolute intensity and the angular distribution of M-band x-rays to design doped ignition capsules.The work described in this thesis is to create a diagnostic system for M-band X-rays, and explore the physics of the M-band X-ray emission from void hohlraums. The main work consists of three parts:1.the development of a flat-response x-ray detector and the establishment of an accurate measurement system for diagnosing M-band x-rays;2. the establishment of a3D simple model to calculate the M-band radiation flux from empty hohlraums;3. the research of M-band x-rays emission principles based on the comparison of the measured data and the calculated results.The most significant improvement of the flat-response detectors is the design and production of a novel compound filter consists of three layers of thin films, i.e. a layer of720nm boron (B), a layer of3.9μm scandium (Sc), and a layer of34μm Sc with a void ratio of18%.The compound filter enables a aluminum (Al) cathode XRD to have a flat spectral response in the range from1.6to4.4keV. A pinhole array is used to realize the void ratio. And this way reduces the difficulty of the absolute calibration. An accurate diagnostic system is established based on filtered XRD, including the association with the absolute time, the absolute calibrations of the spectral response and the time response, and the correction of the measurement results. The time resolution of corrected radiation flux reaches20ps and its uncertainty is just about5%. The simple model describes that M-band x-rays is emitted from the volume in the field of view of the detector. This volume is the overlap of the laser channel and radial plasma expansion from the laser spot. In this volume the isothermal expansion of the electron density can be analyzed by a self-similar rarefaction wave.The evolution of the electron temperature is simplified that it increases linearly with time and decreases linearly with density in the area with the density above the laser reflection ones. The emission rate of M-band x-rays is calculated by the electron density and temperature using an average atom model such as FLYCHK code.The comparison of experimental data and model results of M-band x-rays shows that the simple model is relatively accurate when the hohlraum is larger and the number of lasers injecting into one LEH is four. As hohlraums become smaller and the number of lasers becomes more, the simple model will fail due to the following reasons.The horizontal and axial expansion near the laser spot increases the emission volume. The non-uniform distribution of laser power in the spot region enables a significant emission increase from its center area. The distribution of the electron density and temperature is changed by the concentration of plasma in the center of the hohlraums and other lasers in the vicinity. The energy difference between the lasers exacerbates the fluctuation of the measurement results.