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Laser-plasma interaction experiments in the context of inertial fusion

机译：惯性融合背景下的激光-等离子体相互作用实验

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In laser fusion, the coupling and the propagation of the laser beams in the plasma surrounding the pellet must be well controlled for to succeed in producing a high energy level. To achieve thermonuclear ignition and high gain, the coupling efficiency must be as high as possible, the uniformity of the energy deposition must be very good and the fast electron generation must be minimized. This implies a deep understanding of the laser-plasma interaction mechanisms to keep the nonlinear processes at a low level. Important advances in laser-plasma interaction physics have been achieved thanks to the converging efforts of the experimental and theoretical approaches.Among the different studies of the last few years, we will report results on three themes which are important for future fusion experiments. The first concerns the ability of plasmas to induce temporal and spatial incoherence to the laser beams during their propagation. Beam smoothing, beam spraying and increased incoherence may in turn reduce the level of backscattering instabilities.In laser fusion, multiple beams are used to irradiate the target. The effect of the overlap of the laser beams on parametric instabilities may complicate the problem. Not only is there the interplay between instabilities driven by one beam, but also the interplay between instabilities driven by different beams. In the Laboratoire pour l'Utilisation des Lasers Intenses (LULI) experiment, although the overall stimulated Brillouin scattering (SBS) reflectivity was reduced, a well-defined resonance of the amplitude of ion acoustic waves (IAWs) associated with SBS has been observed for waves propagating along the bisecting direction between two laser beams. Energy transfer between two identical laser beams has been observed and correlated with plasma induced incoherence.The nonlinear saturation of stimulated scattering instabilities is a fundamental ingredient of the understanding of the observed and future reflectivity levels. Using Thomson scattering, the decay of the primary IAWs associated with SBS in secondary IAWs has been observed and correlated with the saturation of SBS.The experiments were performed with the six-beam laser facility of the LULI at Ecole Polytechnique, using one or two 1.053 mum interaction beams in the nanosecond regime and a well-characterized preformed plasma. Multiple diagnostics, including Thomson scattering of a probe beam with spatial, temporal and spectral resolution, were used. The comparison between the experimental results and numerical simulations is used to improve the physics included in the codes.
机译：在激光熔融中，必须很好地控制激光束在围绕小球的等离子体中的耦合和传播，以成功产生高能级。为了实现热核点火和高增益，耦合效率必须尽可能高，能量沉积的均匀性必须非常好，并且必须使快速电子生成最小化。这意味着对激光-等离子体相互作用机制的深刻理解，以将非线性过程保持在较低水平。在实验和理论方法的共同努力下，激光-等离子体相互作用物理学取得了重要进展。在过去几年的不同研究中，我们将报告三个主题的结果，这对未来的融合实验很重要。第一个问题涉及等离子体在其传播过程中引起激光束在时间和空间上不相干的能力。光束平滑，光束喷涂和增加的不相干性可能反过来会降低反向散射不稳定性的水平。在激光聚变中，多束光束用于照射目标。激光束重叠对参数不稳定性的影响可能使问题复杂化。不仅在一个光束驱动的不稳定性之间存在相互作用，而且在不同光束驱动的不稳定性之间存在相互作用。在激光强度实验室实验（LULI）中，虽然总体受激布里渊散射（SBS）反射率降低了，但已观察到与SBS相关的离子声波（IAW）振幅的明确共振。在两个激光束之间沿等分方向传播的电磁波。已经观察到两个相同激光束之间的能量转移，并且与等离子体诱导的不相干性相关。受激散射不稳定性的非线性饱和是理解观察到的和未来反射率水平的基本要素。使用Thomson散射，观察到了次级IAW中与SBS相关的初级IAW的衰减，并与SBS的饱和度相关联。该实验是在Ecole Polytechnique的LULI六束激光设备上进行的，使用一或两个1.053纳秒级的相互作用光束和特性良好的预成型等离子体。使用了多种诊断，包括具有空间，时间和光谱分辨率的探测光束的汤姆森散射。实验结果与数值模拟之间的比较被用来改善代码中包含的物理性质。

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