基于神光Ⅱ升级装置,研究了纳秒/皮秒双束激光联合驱动双层靶的伽马(γ)辐射特征。利用ns束激光与CH薄膜靶相互作用,产生大尺度近临界密度等离子体,然后将ps束激光作用在该等离子体上,产生高能电子,高能电子穿过2 mm厚的Au靶,通过轫致辐射产生γ射线。对不同方向的γ辐射能谱和靶室外的γ辐射剂量分布进行实验测量,发现γ辐射集中在激光前冲方向,具有较小的发散角,而且在该方向上高能段的γ辐射较强。这说明双层靶的设计可以提高ps束激光与等离子体的能量耦合效率,提高高能电子温度,增加高能电子数目,有利于高能段γ辐射在ps束激光的前冲方向集中。另外,在靶室外距离靶点1.25 m处测到的50 keV以上γ辐射的单发次最大剂量为277 μGy。本研究结果对γ辐射的防护和应用具有参考价值。

数千电子伏特(Multi-keV)量级的X射线背光成像是高能密度等离子体物理实验中常用的一种诊断技术。在神光II激光装置上, 研究了纳秒激光驱动钛4.7 keV波段及氯2.7 keV波段背光面源的性能。研究结果表明, 氯背光能谱以类He线及类H线为主, 其中2.7 keV波段的类He-α线最强; 在当前神光II激光加载能力下, 氯背光面源相对强度超过钛背光面源一个量级, 因此, 可选用氯He-α线的X光探针进行背光诊断。

To probe the electron density distribution of the plasma,double-frequency grating shearing interferometry based on the soft X-ray laser technology is developed.The 13.9 nm Ni-like Ag soft X-ray laser is used as a probe to make attempt on the diagnosis of laser produced plasma from a planar gold wafer target.Clear interference fringes image is achieved,which fully demonstrates the practicability of this technology.However,there are some problems such as the unclear target surface in the image.Analysis indicates the main reasons of these problems and a reasonable solution is proposed to improve this technology.

针对高温稠密激光等离子体临界密度附近电子密度的诊断需求,发展了基于软X射线激光的软X射线双频光栅干涉诊断技术.利用波长13.9 nm的类镍银软X射线激光作为探针,掠入射式的双频光栅作为分光干涉元件,对激光辐照金平面靶产生的等离子体进行诊断尝试.实验获得了清晰的干涉条纹图像,充分表明该技术的实用性.但结果中存在靶面不清晰等问题,为此,提出了初步的解决方案.

It is important to diagnose electron density of a plasma irradiated by lasers for inertial confinement fusion, in high energy density physics and related fields, especially for measuring high-Z plasma near the interface. Use of soft X-ray laser as a probe is an important method in diagnosis of plasma electron density distribution. However, it is difficult to carry out the research in high-Z laser plasma, because of the problem of excessive plasma spontaneous radiation. In view of the characteristics of soft X-ray laser, several specific experimental techniques have been developed. By using these techniques, which can greatly suppress effects of spontaneous radiation, diagnosis of high-Z plasma with soft X-ray laser probe method becomes possible. As a typical example, an experiment of diagnosing gold plasma is performed and clear images are obtained, indicating that the techniques are effective and feasible.

The diagnoses of laser-produced plasma electron density have important significance for inertial confinement fusion, plasma physics, high energy density physics and other relevant fields, especially for measuring electron density distribution information of medium and high-Z material plasma near the critical density surface. With 13.9 nm Ni-like Ag X-ray laser serving as a probe, using double frequency grating shearing interference technique, the electron density distribution of plasma produced by laser irradiating a gold planar target is measured. Clear interference fringe image is obtained. Preliminary deduction of the fringe shows that the maximum density measured is about 1.4 times the critical density. It is found that there are some discrepancies between experimental results and simulation results, which provides a useful reference to the further optimization of the simulation program. The experimental results fully demonstrate that the soft X-ray double frequency grating shearing interference technique is practical to diagnose near-critical-density plasma of medium and high-Z materials, which will have a good application value.

在神光Ⅱ激光装置上，针对外径260 μm的柱形靶，采用大焦斑拼接的办法（焦斑直径约200 μm），开展了八路激光直接驱动压缩实验。利用第九路激光驱动钼X射线背光，使用KirkpatrickBaez显微镜成像以及条纹相机记录的方法，获得了柱形靶内爆流线图，据此给出的压缩后密度约为初始密度的120倍。该密度处于快点火电子产生区和能量沉积区密度之间，正是电子束需要传输的密度区域。神光Ⅱ皮秒激光运行后，可以利用这种压缩的柱形靶开展电子束在稠密等离子体中传输的实验研究。