Experimental setup

The experimental setup is shown in Fig. 1. Eight arms of main laser pulses delivered a total energy of nearly 2 kJ to the K-shaped D-Li or D-D target. The laser beams were divided into two groups of four laser beams each, which were simultaneously focused onto the facing surfaces of the target with a focal spot diameter of 150 μm. Another laser pulse with a wavelength of 526 nm and a 70 ps duration was used as a probe beam. By changing the delay time between the main laser pulses and the probe beam, the process of plasma expansion and interaction was recorded at different delay times using a Nomarski interferometer and shadowgraph instruments. Four neutron detectors were employed to measure the neutron spectrum via the time-of-flight (TOF) method. The detectors were numbered from 1 to 4 and located at 5.1 m, 4.4 m, 5.5 m, 5.8 m, respectively, from the fusion plasma. All of the TOF signals were recorded by oscilloscopes with a bandwidth of 1 GHz.

Figure 1: Experimental setup. Eight laser beams of laser were divided into two groups of four beams each, which were simultaneously focused onto the facing surfaces of the targets. Two types of K-shaped targets (D-Li and D-D) were employed. Two plastic and two EJ-301 liquid scintillator detectors were employed to measure the neutron yields via the TOF method. The process of plasma expansion and interaction was recorded by shadowgraph instruments and a Nomarski interferometer. Full size image

Experimental results

A high density plasma with numerous energetic deuterium ions and lithium ions was generated by the laser beams focused on the two-sided target. The plasma streams from both sides expanded toward the opposite target face and collided with each other. During this process, the deuterium ions completely blended together with the lithium ions and collision events occurred, resulting in great potential for the occurrence of 7Li (d, n) nuclear reactions. Figure 2 shows sample TOF signals obtained from detector No. 4. The neutron detection threshold was calculated to be 3 × 104 by considering the detection length, active detection area, and detection efficiency of the detector. Figure 2(a) shows the typical neutron results from the D-Li target. The initial X-ray peak shows a falling edge at 98.8 ns. Considering the distance between detector No. 4 and the fusion plasma as well as the speed of X-rays in the atmosphere, the initial instant of laser-plasma interaction was accurately calculated to be 79.7 ns. The narrow neutron peak not far behind the large X-ray peak has a falling edge at 194.4 ns which corresponds to the energy of 13.32 MeV. There is a tiny deviation between the measured energy and the intrinsic energy (13.36 MeV), which may originate from the measurement error in the neutron flight distance. When the resulting 8Be nuclei were in their first or second excited states, a small amount of 10.7 MeV or 3.27 MeV neutrons could have been generated in the experiment. However, no such neutrons were detected in this shot because of the low yield. The pulse duration of the neutron peak in Fig.2(a) is 9.6 ns (full width half maximum, or FWHM). Figure 2(b) presents the TOF signal from D-D target as a reference. The falling edge that neutron peak starts at a time point which is corrosponding to the energy of 2.44 MeV. This is a direct proof that d (d, n) nuclear reactions happened. There is also a tiny energy deviation read from the intrinsic energy (2.45 MeV) which is coming from the measurement error as mentioned above. By calculating the area of the neutron signals and considering the same detector parameters used in ref. 25, we estimated the neutron yields from the d (d, n) nuclear reactions to be ~106. Again considering the neutron signal from the 7Li (d, n) nuclear reactions, the calculated area of the D-Li neutron signal is nearly 6 times lower than in the D-D case. Thus, the neutron yields from the 7Li (d, n) nuclear reactions were greater than 105. Figure 2(c) shows a typical cosmic ray signal recorded by liquid scintillator detector No. 4 under natural conditions. An individual cosmic ray particle was collected and left a sharp peak on the oscilloscope. The acquired pulse duration of the cosmic ray particle was 9.4 ns (FWHM), which indicates the inherent time resolution of the detector. In other words, the neutrons produced in the 7Li (d, n) nuclear reactions and the cosmic ray particle elicited detector responses of nearly the same time duration. This is strong evidence that the neutrons we obtained were quasi-monoenergetic.

Figure 2: Experimental results. TOF results for 13.36 MeV and 2.45 MeV neutrons from liquid scintillator detector No. 4. (a) K-shaped D-Li target irradiated with eight laser beams. (b) K-shaped D-D target irradiated with eight laser beams. (c) Typical signal induced by a cosmic ray particle under natural conditions. Full size image

To further illustrate the neutron energy acquired in the experiments, the neutron signals from several laser shots and different detectors were analyzed. The black curve in Fig. 3 gives a plot of calculated flight time versus flight distance in relation to the 13.36 MeV monoenergetic neutrons corresponding to Fig. 2(a). Similarly, the red curve shows the case of 2.45 MeV neutrons. From the absolute zero time, the 13.36 MeV neutrons started from the plasma interaction region with speed of 5.06 cm/ns (2.17 cm/ns for 2.45 MeV neutrons). At a special time, the neutrons arrived at the detector positions. The circles in Fig. 3 represent the mean value of flight time acquired from the detectors in several laser shots, and the error bar gives the variation range of these TOF data. The results implied that the experimental flight time points were highly matched with the calculated flight time curves and the measured neutron energy was confirmed by the nearly consistent flight times of different neutron detectors. The No. 1 and No. 2 detectors were not available during the D-Li experiments.

Figure 3: Experimental results. Flight time versus flight distance in relation to the 13.36 MeV and 2.45 MeV neutrons. Full size image

The Nomarski interferometer and shadowgraph instruments were used to obtain a visual representation of the plasma expansion and interaction at a specific point in time after the laser-plasma interaction. Figure 4(a) shows the interferogram for the original D-Li target, which provides a reference for density analysis. The black area without fringes corresponds to the target body. The deuterium target is on the left side, and the LiF target is on the right side. Figure 4(b) shows a shadowgraph of the D-Li target at 3 ns after irradiation. The extension of the black area is caused by the expansion of the high-density target plasma. When the plasma density is higher than a critical density of ~4 × 1021 cm−3, the probe beam cannot penetrate the plasma. Therefore, the color depth represents the plasma density. There is a distinct opaque area at the center of Fig. 4(b), where the plasma density is higher than the density of the surrounding area. In this region, the plasmas were strongly fused and 7Li (d, n) nuclear reactions occurred. Figure 4(c) shows an interferogram of the D-Li target at 3 ns. The displacement of the interferometric fringes in Fig. 4(c) indicates density fluctuations in that area. The intense displacement of the fringes was analyzed via Abel inversion to determine the plasma density. Figure 4(d) shows the spatial distribution of the plasma density at 3 ns, corresponding to Fig. 4(c). The crimson area corresponds to the no-fringe area in Fig. 4(c), where the plasma density is higher than the critical density. This area was manually cropped, and was not considered in the Abel inversion. The figure shows two plasma streams with a plasma density higher than 1020 meeting at the center. In this region, the deuterium ions completely blended together with the lithium ions and collision events occurred, resulting in a great potential for the occurrence of 7Li (d, n) nuclear reactions.

Figure 4: Optical imaging results. Optical imaging of the K-shaped D-Li target. (a) Interferogram of the original D-Li target. (b) Shadowgraph of the D-Li target at 3 ns after laser irradiation. (c) Interferogram of the D-Li target at 3 ns. (d) Spatial distribution of the plasma density at 3 ns, corresponding to (c). Full size image

Simulation results

The velocity and density results calculated from a simulation were used to verify the production of monoenergetic neutrons using the laser-collider method. Figure 5 shows the results of a MULTI 2D hydro-dynamics simulation of the K-shaped LiF target. The two-dimensional process of plasma expansion after laser irradiation was simulated. The K-shaped target appears on the right-hand side of the plots, as marked by a black dashed line. Two laser beams were focused on the K-shaped target, one on the upper part and one on the lower part. The plasma streams thus were produced and moved in the negative direction on the X axis. In Fig. 5, the X and Y axes represent the two spatial dimensions. The coordinates (X,Y) represent the position of the plasma. Figure 5(a) shows the spatial distribution of the plasma velocity at 3 ns. The color scale represents the velocity of the plasma stream. The highest velocity is 6 × 107 cm/s, which corresponds to the lithium ion kinetic energy of 13 keV. According equation (5), the calculated energy spread is less than 0.1%. The head-to-head collision significantly enhanced the cross section for 7Li (d, n) nuclear reactions. This provided sufficient conditions for monoenergetic neutron production. Figure 5(b) shows the spatial distribution of the plasma mass density at 3 ns. The color scale in this figure represents the plasma density. High-density plasmas containing energetic lithium ions were produced in the simulation. The two figures together show that the K-shaped target generated an overdense region along the X axis in the middle of the figure because of the overlap of the two plasma streams from the upper and lower parts of the K-shaped target, which had an opening angle of 120°. With this mechanism, a K-shaped target is distinctly advantageous for promoting a high plasma density compared with other alternatives25. This enhancement of the plasma density significantly increases the collision strength of the plasma streams, thereby also providing increased opportunities for the collision of deuterium and lithium ions and giving rise to essential conditions for monoenergetic neutron production.