The prediction that laser plasma heating distorts the electron distribution function away from Maxwellian and towards a super-Gaussian distribution dates back four decades¹. In conditions relevant to inertial confinement fusion, however, no direct evidence of this so-called ‘Langdon effect’ has previously been observed. Here we present measurements of the spatially and temporally resolved Thomson scattering spectrum that indicate the presence of super-Gaussian electron distribution functions consistent with existing theory². In such plasmas, ion acoustic wave frequencies increase monotonically with the super-Gaussian exponent³. Our results show that the measured power transfer between crossed laser beams mediated by ion acoustic waves requires a model that accounts for the non-Maxwellian electron distribution function, whereas the standard Maxwellian calculations overpredict power transfer over a wide region of parameter space. Including this effect is expected to improve the predictive capability of crossed-beam energy transfer modelling at the National Ignition Facility in California and may restore a larger operable design space for inertial confinement fusion experiments. This is also expected to motivate further inquiry in other areas affected by non-Maxwellian electron distribution functions, such as laser absorption, heat transport and X-ray spectroscopy.

激光等离子体加热会使电子分布函数从麦克斯韦方向偏离，并趋向于超高斯分布的预测可以追溯到四十年前¹。但是，在与惯性约束聚变有关的条件下，以前没有观察到这种所谓的“兰登效应”的直接证据。在这里，我们介绍在空间和时间上解析的汤姆森散射光谱的测量结果，这些测量表明存在与现有理论²一致的超高斯电子分布函数。在此类等离子体中，离子声波频率随超高斯指数³单调增加。。我们的结果表明，由离子声波介导的交叉激光束之间测得的功率传输需要一个考虑非麦克斯韦电子分布函数的模型，而标准麦克斯韦计算会在参数空间的宽泛区域内高估功率转移。预计包括该效应将改善加利福尼亚州国家点火设施的横梁能量传递建模的预测能力，并可能为惯性约束聚变实验恢复更大的可操作设计空间。在非麦克斯韦电子分布功能影响的其他领域，例如激光吸收，热传输和X射线光谱学，这也有望激发其他领域的研究。