Development of ultrahigh pressure apparatus based on high pressure produced by constraining thermal expansion

研制了一种基于约束热膨胀产生高压的超高压装置。在该装置中，压缩在两个金刚石砧之间的样品用高功率YAG激光脉冲通过金刚石窗照射。由于样品中心区域产生的超高温，可以通过限制热膨胀来产生超高压。然而，样品中超高温区域的存在给压力测量带来了困难。在该装置中，使用导热系数非常小的凝固气体作为样品，以便在样品上产生非常大的温度梯度。然后用有限差分法计算证明了在样品室的角落可以出现较低的温区。高压可以通过观察放置在较低温度区域的红宝石晶体的激光诱导荧光来测量。该仪器配备了温度和压力测量系统，其设计满足以下要求：(1)可以同时观察样品中任意三个位置的辐射光谱；(2)将激发激光束聚焦在预先选定的红宝石晶体上，观察到红宝石晶体的激光诱导荧光光谱；(3)在动力学模式下，采用冷却CCD可以同时实现这些光谱的时间分辨测量。本文还设计了一种金刚石砧槽内低温充注气体的技术，并将其应用于H_2充注。此外，实验还证实了高功率激光脉冲不会破坏光学系统和金刚石砧细胞。从而证明了用该装置可以进行以凝固的H_2为样品的超高压实验。

An ultrahigh pressure apparatus based on high pressure produced by constraining thermal expansion is developed. In the apparatus, a sample compressed between two diamond anvils is irradiated with a high-power YAG laser pulse through diamond window. Ultrahigh pressure can be produced by constraining thermal expansion due to ultrahigh temperature generated in the central region of the sample. However, the existence of ultrahigh temperature region in the sample gives rise to difficulties in the pressure measurements. In the apparatus, solidified gas having very small thermal conductivity is used as the sample in order to produce very large gradients in temperature across the sample. Then it is demonstrated by calculations using the finite difference method that the lower temperature region can appear in the corner of the sample chamber. The high pressure can be measured by observing laser-induced fluorescence of a ruby crystal placed in the lower temperature region. The apparatus is equipped with a temperature-and-pressure measuring system that is designed to satisfy the following requirements : (1) radiation spectra from three arbitrary positions in the sample can be simultaneously observed ; (2) excitation laser beam can be focused at point of preselected ruby crystal and the laser-induced fluorescence spectrum of the ruby crystal can be observed ; (3) simultaneous time-resolved measurements for these spectra can be achieved by using cooled CCD in kinetics mode. A cryogenic technique for loading gases in the diamond anvil cell is also devised, which is applied to the H_2 loading. Furthermore, it is confirmed experimentally that the optical system and the diamond anvil cell are not damaged by the high-power laser pulses. Thus it is demonstrated that the ultrahigh-pressure experiments using solidified H_2 as the sample can be executed by the present apparatus.