Department of Physics, University of Nevada, Reno, Nevada 89557, USA.
Japan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan.
Rev Sci Instrum. 2023 Mar 1;94(3):033511. doi: 10.1063/5.0130953.
High-power, short-pulse laser-driven fast electrons can rapidly heat and ionize a high-density target before it hydrodynamically expands. The transport of such electrons within a solid target has been studied using two-dimensional (2D) imaging of electron-induced Kα radiation. However, it is currently limited to no or picosecond scale temporal resolutions. Here, we demonstrate femtosecond time-resolved 2D imaging of fast electron transport in a solid copper foil using the SACLA x-ray free electron laser (XFEL). An unfocused collimated x-ray beam produced transmission images with sub-micron and ∼10 fs resolutions. The XFEL beam, tuned to its photon energy slightly above the Cu K-edge, enabled 2D imaging of transmission changes induced by electron isochoric heating. Time-resolved measurements obtained by varying the time delay between the x-ray probe and the optical laser show that the signature of the electron-heated region expands at ∼25% of the speed of light in a picosecond duration. Time-integrated Cu Kα images support the electron energy and propagation distance observed with the transmission imaging. The x-ray near-edge transmission imaging with a tunable XFEL beam could be broadly applicable for imaging isochorically heated targets by laser-driven relativistic electrons, energetic protons, or an intense x-ray beam.
高功率、短脉冲激光驱动的快电子可以在高密度靶体流体动力学膨胀之前迅速加热和离化它。使用电子诱导 Kα 辐射的二维（2D）成像研究了这种电子在固体靶体中的输运。然而，目前的研究仅限于无时间分辨率或皮秒时间分辨率。在这里，我们使用 SACLA 自由电子激光（XFEL）演示了固体铜箔中快电子输运的飞秒时间分辨 2D 成像。聚焦的准直 X 射线束产生了具有亚微米和 ∼10fs 分辨率的透射图像。XFEL 光束调谐到其光子能量略高于 Cu K 边缘，能够对电子等容加热引起的透射变化进行 2D 成像。通过改变 X 射线探针和光学激光之间的时间延迟来进行时间分辨测量，结果表明，在皮秒时间内，电子加热区域的特征以约 25%的光速扩展。时间积分的 Cu Kα 图像支持透射成像中观察到的电子能量和传播距离。可调谐 XFEL 光束的近边透射成像可以广泛应用于激光驱动相对论电子、高能质子或强 X 射线束等对等容加热靶体的成像。
