Cover caption: Laser-irradiated gold cones emit intense hard X-rays (>2 keV), inducing pre-ablation on inner walls to form Au pre-plasma. This causes implosion asymmetry and high-Z mixing (local Au mass fraction: 5%). Substituting tungsten cones reduces radiation intensity by 70% and pre-plasma generation by 50%, increasing peak areal density by 15% and reducing high-Z mixing significantly. (Left/right halves contrast plasma dynamics under Au/W cone configurations.)

Abstract

The radiative properties of gold cones in double-cone ignition (DCI) systems have been identified as a critical factor affecting fusion efficiency. Under laser irradiation, the gold emits intense hard X-rays (>2 keV), which trigger pre-ablation of the cone's inner walls, forming low-density gold pre-plasma. This pre-plasma interacts with the fuel, destabilizing the implosion process and reducing peak temperatures by 25%. The research team led by Prof. Xiaohu Yang at the National University of Defense Technology (NUDT) has uncovered these mechanisms through advanced radiation hydrodynamics simulations. Their groundbreaking solution is that replacing gold cones with tungsten to reduce harmful X-ray emissions by 70% and boost fuel compression performance.

The Double-Cone Ignition Scheme: Promises and Challenges

Inertial confinement fusion (ICF), a leading approach to achieving controlled nuclear fusion, uses high-energy lasers to compress millimeter-scale fuel capsules (deuterium-tritium mixtures) to extreme densities and temperatures. However, traditional ICF faces three major hurdles: enormous energy demands (e.g., the U.S. National Ignition Facility requires lasers exceeding 2 megajoule), hydrodynamic instabilities that disrupt compression symmetry, and inefficient laser-plasma coupling.

The DCI scheme, a novel alternative, positions a spherical fuel capsule between two open-tipped gold cones. Lasers drive the fuel to collide at high speeds (>200km/s) along the cone walls, enabling more efficient energy transfer. While DCI reduces energy requirements and improves tolerance to asymmetries, experiments revealed an unexpected issue: gold cones emit intense hard X-rays during laser irradiation. These high-energy photons penetrate the fuel, ablating the inner cone surfaces to form gold pre-plasma. This pre-plasma mixes with the fuel, altering implosion dynamics and increasing energy losses—critical factors that risk ignition failure.

Figure 1. Schematic diagram of the double-cone ignition structure.

X-ray Pre-Ablation: A Hidden Threat to Implosion Efficiency

NUDT's simulations quantified the impact of gold-generated X-rays. Key findings include:

Radiation Intensity: Gold cones emit hard X-rays at 81 times the intensity of pure CH (carbon-hydrogen) materials, accounting for 15% of total radiative energy flux (Figure 2a).

Pre-Plasma Formation: These X-rays induce subsonic ablation on the inner walls of the cone, creating low-density gold pre-plasma. The plasma expands and collides with the fuel pre-plasma (generated by rarefaction waves), raising cavity pressures to several megabars (Figure 2b).

Figure 2. (a) Radiation spectra across different regions. Dashed lines represent blackbody radiation spectra at equivalent temperatures. (b) Pressure distribution during implosion. Dashed lines mark the boundaries of the gold pre-plasma region.

Implosion Disruption: The pre-plasma slows shell velocity near the cones, distorting compression symmetry (Figure 3a). Fuel areal density drops to 0.26 g/cm²—far below the 0.6 g/cm² ignition threshold.

Fuel Contamination: Gold ions infiltrate the fuel at a 5% mass fraction locally (Figure 3b), increasing bremsstrahlung radiation losses by 46% and cooling the plasma (the peak temperature falls to ~300 eV).

Figure 3. (a) Evolution of peak temperature and areal density in the collision zone

Directional variations (radial vs. axial) highlight symmetry distortion. (b) Density distribution of colliding plasma and localized gold ion mixing. Gold ion mass fraction reaches up to 5% (red regions).

Tungsten Cones: A Game-Changing Solution

To mitigate these effects, the team proposed replacing gold cones with tungsten. Simulations demonstrated:

Reduced X-ray Emissions: Tungsten's M-band radiation is 70% weaker than that of the gold, lowering cavity radiation temperatures by ~20 eV (Figure 4a).

Limited Pre-Plasma Expansion: Tungsten pre-plasma forms at half the rate of gold and expands more slowly, avoiding fuel core contamination.

Improved Performance: Implosion symmetry and compression efficiency increased, which increased the fuel areal density by 15% (to 0.3 g/cm²) and peak temperatures to 500 eV (Figure 4b).

Figure 4. Performance of tungsten cones. (a) Radiation spectra. (b) Evolution of peak density and areal density.

Future Directions

This work highlights the importance of cone material in DCI and provides a practical pathway to optimize the fusion performance. Next steps include exploring advanced radiation-shielded target designs and experimental validation using next-generation high-energy lasers. These findings not only advance DCI's feasibility but also offer new insights into radiation-matter interactions in high-energy-density physics.

This research was published in High Power Laser Science and Engineering (Bihao Xu, Xiaohu Yang, Ze Li, Bo Zeng, Zehao Chen, Lingrui Li, Ye Cui, Guobo Zhang, Yanyun Ma, Jie Zhang, "Effects of X-ray pre-ablation on the implosion process for double-cone ignition," High Power Laser Sci. Eng. 13, 02000e24).