Picosecond time-resolved X-ray holography

在进行X射线全息照相研究之初，我们就面临着激光等离子体X射线强度不足的问题。然后，我们改变了计划，瞄准了时间分辨率为10ps的激光烧蚀过程的时间分辨成像，并尝试拍摄金属材料和有机材料的X射线吸收图像。研究发现，对于金属样品和有机材料，可以在10次拍摄和100次拍摄中实现足够的图像对比度。因此，为了进行单次激发成像，需要进行10~100倍的X射线增强。激光等离子体X射线的产额强烈依赖于等离子体前的分布。我们通过数值模拟研究了预等离子体的条件，发现抑制预等离子体的膨胀将使X射线的量增加10倍。为了提高激光对比度，进行了几项实验。然而，这些方法目前还没有应用于实际的X射线产生实验。由激光等离子体产生的Cuka的光子数实验估计为4.4x10^9[光子数/射程/4πsr]。激光对比度提高了一个数量级(ASE强度~1012W/cm2)。改善激光对比度的努力仍在继续，例如半导体晶体中的相变、气体喷射中的相对论自聚焦以及金属涂层靶材。通过预脉冲控制，X射线产额将提高十倍以上。我们预计，由于激光升级，X射线的数量将在不久的将来进一步增加。

We faced a problem of insufficiency of laser plasma X-ray intensity at the start of this study for X-ray holography. Then, making a change in our plan, we aimed time-resolved imaging of a laser ablation process with 10 ps time resolution and tried to take X-ray absorption images of metallic materials and organic materials. It was found possible that sufficient image contrast would be achievable in 10 shots for metallic samples and 100 shots for organic materials. Thus, X-ray enhancement to the extent of 10 〜 100 times was necessary in order to make a single-shot imaging. The yield of the laser plasma X-ray is strongly dependent on the pre-plasma distribution. We investigated the condition of the pre-plasma with numerical simulations and found that the amount of X-rays would be increased 10 times by suppression of the pre-plasma expansion. Several experiments were carried out to improve the laser contrast. Those methods, however, were not applied to actual an X-ray generation experiment at this time. The number of photons of CuKa generated from a laser plasma was estimated experimentally as 4.4 x 10^9 [photons/shot/4 π sr]. Laser contrast was improved about one order ( ASE intensity 〜 10^12 W/cm^2). The efforts to improve the laser contrast such as phase transition in a semiconductor crystal, relativistic self-focus in a gas-jet, and metal-coated targets are continued. X-ray yield is going to increase more than ten times by pre-pulse control. We expect further increase of X-rays because of the laser upgrade to be done in the near future.