Cover interpretation
The cover image shows a laser-driven deuterium fusion process, in which a femtosecond pulsed laser with a high repetition rate is focused on a thin layer of D2O. Under laser irradiation, two deuterium nuclei in D2O fuse to form a pair of relativistic neutrons and helium nuclei. The cover showcases the dynamic interaction between light and matter, highlighting the fusion reactions that are taking place.
The ideal "probe" of the microscopic world - neutrons
Aero engine is known as the heart of the aircraft and the symbol of a strong country. The key to ensuring that the "heart" beats strongly is to address the problem of metal fatigue in engine blades. At tens of thousands of revolutions per minute, the blades are highly susceptible to metal fatigue, increasing the risk of breakage. The neutron source can be used to non-destructively detect blade stress and prevent metal fatigue, ensuring safety in the aviation industry. This is due to the uncharged nature of neutrons, which cannot interfere with their forward movement due to Coulomb interactions, giving them extremely penetrating power and can be used to probe microscopic material structures. At the same time, neutrons have magnetic moments, which can be used for high-resolution magnetic structure studies. In addition, neutrons are also widely used in cancer treatment, light element detection and other fields. Therefore, neutrons are regarded by scientists as an ideal "probe" to explore the microscopic world.
However, traditional mobile neutron sources often do not provide high spatial resolution, and neutron sources with high resolution, such as nuclear reactors, are not. This has led to an urgent need for mobility, high-resolution neutron sources. Over the past two decades, compact, high-resolution neutron sources based on ultra-intense laser technology have been extensively studied and experimentally verified. However, most experiments still use a single-shot protocol, which is far from the minimum standard for seed yield (~109/s) in practical applications. Recently, the use of high repetition rate laser technology has provided a new solution to this hot problem.
High-repetition rate laser-driven deuterium-deuterium fusion
Recently, a research team from the Department of Engineering Physics at the Ohio Air Force Institute of Technology in the United States used 8 mJ, 40 fs laser pulses to drive deuterium-deuterium (D-D) fusion on thin (<1 μm) D2O liquid sheets to produce neutrons (∼105/s). Under the conditions of relativistic intensity (∼5×1018 W/cm2) and high repetition rate (1 kHz) laser, the system achieves stable neutron generation for up to 1 hour. To accurately measure neutron doses, they employ three separate detection systems, including an organic scintillator detector, a 3He proportional counter, and 36 bubble detectors. The time delay analysis of the photon and neutron signals in the scintillator detector data shows that the scheme produces a single neutron of 2.45 MeV.
成果发表在High Power Laser Science and Engineering 2024年12卷第1期的封面文章(Benjamin M. Knight, Connor M. Gautam, Colton R. Stoner, Bryan V. Egner, Joseph R. Smith, Chris M. Orban, Juan J. Manfredi, Kyle D. Frische, Michael L. Dexter, Enam A. Chowdhury, Anil K. Patnaik. Detailed characterization of kHz-rate laser-driven fusion at a thin liquid sheet with a neutron detection suite[J]. High Power Laser Science and Engineering, 2024, 12(1): 010000e2)。
The distribution of the device is shown in Figure 1, with the main beam generated by a high repetition rate (1 kHz) titanium-sapphire laser with a center wavelength of 780 nm, an energy of 8 mJ, a focal spot of 1.65 μm, a pulse width of 40 fs, and an oblique incidence of 45° on the liquid target (s-polarization). The liquid target is a submicron-thick flowing sheet formed by two intersecting D2O cylindrical jets with a diameter of 25 μm, which can be stabilized at frequencies of kHz or higher.
Fig.1 Simplified top view of the target chamber and surrounding detectors
Detection and verification of neutron sources at kHz rate
When a super-intense laser pulse interacts with a deuterium-rich target, there are two processes that can produce neutrons. First, when the laser energy absorbed by the hot electrons is transferred to the ions, the local temperature in the focal area becomes very high, which can lead to D-D fusion. Second, most of the hot target explodes after a period of time, and the high-energy deuterium nuclei collide with each other, causing D-D fusion to occur (a few nanoseconds after the pulse leaves the target). At a focused light intensity of up to 1018 W/cm2, the high-energy deuterium nucleus is mainly accelerated outward from the surface by the target normal sheath acceleration (TNSA) by the target normal sheath acceleration (TNSA) method, and hits nearby deuterium-rich secondary targets. At an energy of about keV, the colliding deuterium nuclei can undergo D-D fusion, with half of the fusion reaction yielding 3He and a neutron, and the other half producing tritium and a proton:
In a neutron-producing reaction, the reactive release energy of 3.27 MeV will be converted into kinetic energy of the product. The mass of the free neutrons accounts for about 1/4 of the total mass of the product, and about 3/4 of the energy is obtained, i.e., 2.45 MeV. As shown in Figure 2, there is a peak structure around 2.45 MeV in the discrete energy data of the scintillator detector, confirming the occurrence of D-D fusion.
Figure 2 Energy histogram of emitted neutrons, measured by scintillator detectors with the time delay of photon and neutron signals
At the same time, the team also used the particle grid (PIC) simulation code WarpX to validate the experiment. The code was recently updated with a fusion model algorithm developed by Higginson et al. The results of the 2D simulation are shown in Figure 3, where neutrons are generated from the central region of the focal spot, and the energy spectrum has a single-energy structure of 2.45 MeV, which is the expected energy output of the neutrons in D-D fusion. The simulation results show that the neutron yield is increased by 2.6 times compared with the S-polarized laser.
Fig.3 2D3v PIC simulation results. (a) Simulated image at s polarization at 600 fs (red is the neutron distribution, black is the tritium distribution). (b) Evolution of neutron counts over time. (c) Energy spectra of deuterium (dashed line) and neutron (solid line) at 500 fs
Summary and outlook
This research is expected to achieve high-repetition rate hybrid radiation in laser-driven systems. This includes the proven MeV ions, electrons, X-rays, and neutron radiation in this paper. A continuous mixed radiation environment can be used for nuclear radiation hardening or space meteorological testing. In addition, due to the small size of the neutron-generating target, this neutron source is well suited for neutron photography techniques. A neutron source driven by a high repetition rate laser provides a low-cost, on-demand test bench for these applications. This technology will bring more opportunities in the fields of materials science, medicine, and energy and environment, and provide important support for future scientific research and engineering applications.
From: Laser Review
Welcome everyone to participate in the activities held by the Yangtze River Delta G60 Laser Alliance:
Application of laser intelligent manufacturing in new energy vehicles:
July 29-31, 2024, Hefei Fengda International Hotel, Anhui
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