Fig. 1. Schematic of neutron source from an optical trap (OT). (a) Two lasers with the same frequency, sub-picosecond duration, and moderate intensity first overlap inside the near-critical density plasma. These are termed OT pulses. Then, a laser pulse with relativistic high intensity (boost pulse) is injected after certain time delay for ion energy boosting. (b) Closeup of adjacent plasma gratings outlined by the black rectangle in (a), corresponding to pre-acceleration of ions in the OT. As the density grating grows, the thermal pressure expels electrons, which results in the generation of an electric field that reflects the ions. (c) Neutron generation triggered by colliding ions after energy boosting.

Fig. 2. Evolution of an OT and self-generated electric fields. (a) Time–space evolution of electrostatic fields in an OT, where a closeup of only three periodic layers is shown. T₀ is the laser cycle and λ₀ is the laser wavelength. (b)–(d) Closeups of density distributions of electrons (red dotted lines) and ions (blue dashed lines), and longitudinal distributions of self-generated fields E_x (black lines) formed near a density peak at t = 140T₀, t = 178T₀, and t = 212T₀, respectively. (e) Phase space of ions near a density peak at t = 140T₀ (top) and t = 220T₀ (bottom).

Fig. 3. Ion phase space and longitudinal electrostatic fields along the laser axis (white dashed line) from 2D simulation. The top plot is at 640 fs, before the two boost lasers arrive. The middle plot is at 1.06 ps, without boost laser injection. The bottom plot is at 780 fs, with the boost lasers.

Fig. 4. Boosted ion energy features according to 2D PIC simulations. (a) Ion energy angular distribution. (b) Ion energy spectrum at two emission angles: 0° (red full line) and 90° (blue dashed line).

Fig. 5. (a) and (b) 1D simulation results for deuteron phase space and density distributions at 820 and 860 fs, respectively, with energy boosted by boost lasers.

Fig. 6. Boosted ion velocity as a function of four different parameters, namely, the time delay of the boost lasers, the initial plasma density, the boost laser intensity, and the OT laser wavelength, obtained from the simulation and the toy model. (a) Dependence of the velocity (blue line with solid circles) and OT density (red dashed line with triangles) on the time difference between OT and boost lasers from the simulation, together with the velocity as estimated by the toy model (black open squares). The blue open circle is the simulation result without the boost laser. (b) Dependence of the velocity (blue line with solid circles) and layer distance (red dashed line with triangles) on the initial plasma density, together with the velocity as estimated by the toy model (black open squares). (c) Dependence of the velocity on the boost laser strength (upper plot) and on the OT laser wavelength (bottom plot), with fixed a_OT and a_boost and the same normalized plasma density n₀/n_c(λ_OT) = 0.8. The dashed line in the lower plot is the average value of the simulation results.

Fig. 7. Neutron production results in OT via bulk acceleration, given by 1D3V simulations. (a) Temporal evolution of the neutron production rate P (black full line) and neutron yields N_n (red dashed line). The inset is the spatial distribution of the volumetric neutron production rates R at 0.5 ps. (b) Neutron energy spectra at 0° and 90° emission angles. The insert shows the total spectrum of the neutron source. (c) Angular distribution of total neutron yield dN/dΩ.

Fig. 8. The upper plot shows the central energy value of neutron spectra at different emission angles, and the lower plot the spectral ranges (blue) with the corresponding peak neutron production values (red).

Fig. 9. Neutron production results from 2D PIC simulations. (a) Spatial distribution of volumetric neutron production rate R at t₀ = 0.5 ps, corresponding to the boosted ion collision time. (b) Longitudinal distribution of R at 0.5 ps and averaged in the y direction. (c) Angular distribution of neutron emission at 0.5 ps.

Fig. 10. Results for the proposed scheme with different OT laser wavelength λ_OT, obtained with 1D PIC simulations. The red open circles are the maximum neutron production rate P_max in simulation, the blue one corresponds to the duration of neutron pulse δT_n for each group. An estimated model is provided here that the P_max is reversely scale by λ_OT according to X⁴ with shrink rate X = 1.057 µm/λ_OT.