Neutron Generation by Laser-Driven Spherically Convergent Plasma Fusion

We investigate a new laser-driven spherically convergent plasma fusion scheme (SCPF) that can produce thermonuclear neutrons stably and efficiently. In the SCPF scheme, laser beams of nanosecond pulse duration and $10^{14}–10^{15}\mathrm{W}/{\mathrm{cm}}^2$ intensity uniformly irradiate the fuel layer lined inside a spherical hohlraum. The fuel layer is ablated and heated to expand inwards. Eventually, the hot fuel plasmas converge, collide, merge, and stagnate at the central region, converting most of their kinetic energy to internal energy, forming a thermonuclear fusion fireball. With the assumptions of steady ablation and adiabatic expansion, we theoretically predict the neutron yield $Y_n$ to be related to the laser energy $E_L$, the hohlraum radius $R_h$, and the pulse duration $\tau$ through a scaling law of $Y_n\propto (E_L/R_h^{1.2}{\tau }^{0.2}{)}^{2.5}$. We have done experiments at the ShengGuangIII-prototype facility to demonstrate the principle of the SCPF scheme. Some important implications are discussed.

Figure 1

Schematic plot of laser-driven spherically convergent plasma fusion processes.

Figure 2

Illustration of the experimental setup, and the to-scale side-on view of the two-LEH spherical hohlraum with laser beams.

Figure 3

In the panel, (a) is the GXI images of the convergent plasma emission above 2 keV with the time pinpointed, and the gate time is 70 ps; (b) is the temporal x-ray flux measured by FXRD with an energetic range from 100 eV to 4.4 keV from LEH with 30° to the axis; (c) is the nTOF line with a fitting curve.

Figure 4

The neutron yield data versus the scaling parameter. The error bars of the data are about 10%–15%, approximately equivalent to the vertical size of the varied scatters. $E_L$ in units of J, $R_h$ in units of mm, and $\tau$ in units of ns. Different scatters represent varied parameters; refer to the text for details.