The velocity interferometer system for any reflector (VISAR) coupled with a streaked optical pyrometer (SOP) has been used as a diagnostic tool in inertial confinement fusion (ICF). The VISAR diagnoses the evolution of moving surface velocity by measuring the shifted interference fringes, while the SOP analyzes the radiance temperature of a shocked thin layer by comparing the measured self-emission intensity with the brightness temperature from a standard Plankian blackbody radiator.

To validate the process of adiabatically compressing the fuel shell through precise tuning of shocks in experimental campaigns for the double-cone ignition (DCI) scheme of ICF, a compact line-imaging VISAR with a SOP system is designed and implemented at the Shenguang-II upgrade laser facility, as shown in Fig.1. The temporal and spatial resolutions of the system are better than 30 ps and 7 μm, respectively. An illumination lens is used to adjust the lighting spot size matching with the target size. A polarization beam splitter and λ/4 waveplate are used to increase the transmission efficiency of our system. This system has been used in different DCI campaigns, where the generation and propagation processes of multi-shock are carefully diagnosed.

This work has been published in High Power Laser Science and Engineering, vol. 12, Issue 1 (Dawei Yuan, Shaojun Wang, Huigang Wei, et al., Design, performance and application of a line-imaging velocity interferometer system for any reflector coupled with a streaked optical pyrometer system at the Shenguang-II upgrade laser facility[J]. High Power Laser Science and Engineering, 2024, 12(1): 010000e6).

Figure 1: (a) The optical ray tracing of VISAR and SOP system, (b) Schematic layout of the system including five components: VISAR laser, illumination sub0system, imaging sub-system, Mach-Zehender interferometers, SOP sub-system and streaked cameras.

Figure 2: Typical parameters of the VISAR in the DCI campaign. (a) A 500-μm-diameter light-spot with imposed interferometer fringes is obtained by using an illumination lens with F = 2200 mm. (b) The spatial resolution of the whole system is better than 7 μm using the tested reticle placing at the TCC. (c) The obtained relationship between the emission temperature and the measured count value. (d) The static fringes in the field of view are swept by the Streaked camera.

In direct-driven ICF experiments, for example DCI, inhomogeneous compression can cause hydrodynamic instability, which greatly limits the success of ignition. How to detect and suppress hydrodynamic instability in experiments is a challenge. In general, inhomogeneous compression will inevitably induce the changes in the velocity and temperature of the shocked material. Therefore, VISAR and SOP system with high spatiotemporal resolution, velocity resolution and temperature resolution can directly reflect the uniformity of the compression process. In the future, the line-imaging VISAR and SOP system will be further upgraded into two-dimensional imaging one, which can give two-dimensional velocity field distribution.