Fusion energy is the most promising scheme to solve the future energy requirement for a sustainable development of mankind. This has also been the long-cherished dream for scientists to realize for decades. Inertial confinement fusion (ICF) is one of the potential candidates and has naturally become the main research subject for almost all fusion-class laser facilities in the world. In direct-driven ICF experiments, fuels are compressed and heated simultaneously to high density and high temperature to achieve ignition at the center of the targets. However, the entanglement of compression and heating processes has been a great challenge for a more reliable and more efficient operation of fusion power station. The double-cone ignition (DCI) scheme of ICF is proposed to address this challenge by separation of compression and heating processes, same as the fast ignition scheme. Fuel shells embedded in gold cones are firstly compressed to a high density along the isentropic path, and then accelerated to form a dense jets with high implosion velocity in the cones. Head-on collision of the plasma jets from the tips of the cones form an isochoric plasma with sharp ends in the heating direction. A relativistic electron beam guided by a B Field heats the end of the plasma to ignite the isochoric plasma.

A proof-of-principle experiment to confirm the feasibility of the first two processes, namely compression and acceleration, for the DCI scheme was published in High Power Laser Science and Engineering, vol. 12, Issue 4(Huigang Wei, Dawei Yuan, Shaojun Wang, Ye Cui, Xiaohu Yang, Yanyun Ma, Zhe Zhang, Xiaohui Yuan, Jiayong Zhong, Neng Hua, Yutong Li, Jianqiang Zhu, Gang Zhao, Jie Zhang; Compression and acceleration processes of spherical shells in gold cones[J]. High Power Laser Science and Engineering, 2024, 12(1): 04000e43).

Figure 1: Experimental setup and results. In (a): Sketch of the experimental setup with the two-ramp pulse profile. In (b): VISAR images for two-ramp pulse shots. In (c): SOP images for two-ramp pulse shots.

Figure 2: Temporal density distributions in the CHCl shell.

The paper presents the properties of the generated plasma jets from the gold cones. The experiments were performed at the Shenguang II upgrade (SGII-U) laser facility and the shells were driven by laser beams with two-ramp pulse, as shown in Fig.1. The compression and acceleration of the CHCl shell are diagnosed with VISAR (Velocity Interferometry System for Any Reflector) and SOP (Streak Optical Pyrometer). The experimental results demonstrate that the shell was compressed by the first ramp along an isentropic path and further accelerated by the second ramp. The experiments were simulated with the 2D cylindrical hydrodynamic code FLASH. A plasma jet with a density of up to 15 g/cm³ and a velocity of 126.8 ± 17.1 km/s has been generated according to the simulation and experimental measurements. The good agreements between experimental data and simulations are documented.

Present experiments have shown that compression and acceleration are feasible for DCI scheme. In future, the laser pulse shape will be further optimized to generate denser jets with higher temperatures, which are prepared for the colliding and heating processes.