Boron-Proton Nuclear-Fusion Enhancement Induced in Boron-Doped Silicon Targets by Low-Contrast Pulsed Laser

We show that a spatially well-defined layer of boron dopants in a hydrogen-enriched silicon target allows the production of a high yield of alpha particles of around ${10}^9$ per steradian using a nanosecond, low-contrast laser pulse with a nominal intensity of approximately $3\times {10}^{16}\mathrm{W}{\mathrm{cm}}^{-2}$. This result can be ascribed to the nature of the long laser-pulse interaction with the target and with the expanding plasma, as well as to the optimal target geometry and composition. The possibility of an impact on future applications such as nuclear fusion without production of neutron-induced radioactivity and compact ion accelerators is anticipated.

Popular Summary

Nuclear fusion involving boron has garnered interest since the 1930s because of the process’s ability to produce copious numbers of alpha particles, which can in turn be used for generating fusion energy without producing neutron-induced radioactivity. We build on these previous experiments using boron dopants in hydrogen-enriched silicon and recover significantly higher alpha-particle counts than previous studies using only moderate-power lasers that are readily reproducible in industrial settings.

We assemble samples of boron-doped silicon and carefully control the placement and concentration of the dopants. The boron atoms are placed with a density of ${10}^{22}{\mathrm{cm}}^{-3}$ in a layer 100 nm thick, and a 500-J laser at the Prague Asterix Laser System is fired at the sample. The laser pulses are characterized by a relatively low intensity, low power, and a low-contrast intensity ratio. We measure the craters produced by alpha-particle nuclear-track detectors. We verify that undoped silicon samples produce no alpha particles. We use a silicon carbide detector and a Thomson parabola spectrometer to recover the ion-energy distributions. An alpha-particle distribution of 4–8 MeV with a peak around 4.5 MeV is measured. Our experimental setup yields an alpha-particle flux of over ${10}^9$ particles per steradian, a 100-fold increase over previous studies. We attribute this increase to our use of both longer laser pulses (nanoseconds) than previous experiments (picoseconds) and the boron-doped silicon targets.

We have verified that it is possible to create a high-luminosity source with potential strategic applications in the field of laser nuclear fusion and the development of next-generation compact particle accelerators.

Figure 1

(a) Experimental setup. $T$ is the target, $p$ is the proton beam, $\alpha$ is the alpha-particle beam, PM-355 is the nuclear-track detector, TP is the Thomson-parabola spectrometer, MCP is the microchannel plate, and SiC is the silicon-carbide detector. The angles are measured with respect to the target normal. (b) Laser intensity ($I_L$) against time (${\tau }_L$) in arbitrary units. The laser-energy content in different time windows is reported in the legend.

Figure 2

2D pale hydrodynamic simulations of the boron-plasma expansion after the action of the laser pulse (in the range 1–1.85 ns) for (a) the B layer implanted in the SiH substrate, (b) the B layer deposited on the SiH substrate, and (c) the B layer diffused in the SiH substrate. The gradient scale on the right-hand side shows the B-plasma density.

Figure 3

PM-355 film snapshot showing craters produced by incident protons (small light craters) and alpha particles (large black craters) for (a) the Si-H-B target and (b) the corresponding histograms of particle crater diameters. (c) The PM-355 snapshot shows craters produced by incident protons (small light craters) for the Si target and (d) corresponding histograms of particle crater diameters. (e) The PM-355 calibration curve for alpha particles and protons after 2-h etching time. An Al foil of $15\mu \mathrm{m}$ in thickness is placed in front of the PM-355 films.

Figure 4

(a) Alpha-particle energy distribution obtained from PM-355 analysis using detectors with different Al filter thicknesses ($6–20\mu \mathrm{m}$) and placed at an angle of about 45° with respect to the laser-incident direction in the same laser shot. (b) Alpha-particle angular distribution obtained from PM-355 analysis using detectors placed at different angles with respect to the laser-incident direction in the same laser shot.

Figure 5

Laser-target, laser-plasma, and proton-boron interactions in the 2-ns range before the maximum laser intensity [0-III in Fig. 1]: $n_s$ is the solid density, 0 is the target surface position, $x_0$ is the depth of the boron atoms, $x_1$ is the substrate extension, $x_2$, $x_3$, and $x_4$ are the ablation depths at three given times, and $P1$ and $P2$ are plasmas generated by the laser pulse in the 0-I and I-II regions, respectively.

Figure 6

Ion TOF distribution for the Si-H-B target (orange curve) with the alpha-particle signal (${\alpha }_1$ and ${\alpha }_2$) and plasma ions measured by the SiC detector covered by a $8\text{−}\mu \mathrm{m}$ aluminum foil and placed at 65° with respect to the target normal. The ion TOF distribution for the Si target (blue curve) showing the presence of only plasma ions (no fusion event has occurred) is also reported for comparison.

Figure 7

(a) Typical TP spectrometer snapshot showing the presence of backward-accelerated protons and heavier plasma ions (a). The corresponding proton-energy distribution and the comparison with the alpha-particle one (obtained by fitting the TOF spectrum in Fig. 6) are shown in (b).