Objective Inertial confinement fusion (ICF) is important to achieve controlled nuclear fusion. To solve the problem of low energy coupling efficiency in the traditional ignition scheme in a laser-driven ICF experiment, Zhang Jie and other researchers proposed double-cone ignition (DCI) as a new solution for ICF ignition. In DCI experiments, using laser irradiation, the surface area of the target material filled in the metal cone decreases sharply as the laser irradiates fusion targets; therefore, the focal spot size of the irradiated beam must be dynamically reduced accordingly. In this process, the control of shooting laser beamlines in the laser driver must be flexible. Thus, to meet the aforementioned demand, the dynamic focusing of shooting beams has been proposed. In 2013, to mitigate cross-beam energy transfer during low-adiabat cryogenic experiments on the OMEGA laser facility, researchers in the Laboratory for Laser Energetics of the University of Rochester in the United States proposed a technical solution of two-state focal zooming. The dynamic focusing process of the proposed technique is similar to that of the two-state focal zooming. Two combined spatiotemporal laser pulses are amplified by coaxial propagation. Then, a specially designed continuous phase plate is used to modulate the wavefronts of these two beams (denoted as beams 1 and 2) and smooth the focal spot separately. Next, the target material is irradiated by laser spots after focusing. In this study, to realize beam control using dynamic focusing technology, a new method for achieving dynamic focus using a combined spatiotemporal beam based on a preamplifier system is proposed to provide support for the subsequent research of DCI.
Methods Herein, near-field beam quality during the generation and propagation of the combined spatiotemporal beam is analyzed. First, based on the Fresnel diffraction propagation theory and the numerical simulation method of fast Fourier transform, a propagating model of the combined spatiotemporal beam is established. Then, the influences of the filter pinhole diameter, softening factors, extinction ratio, and phase difference on the propagation of the combined spatiotemporal beams are discussed. Finally, based on the overall plan of the spatiotemporal beam combination, an experimental laser system is constructed to verify the accuracy of the numerical simulation.
Results and Discussions In the analysis of the generation and propagation of the combined spatiotemporal beam, the outer size of beam 1 is 12 mm×12 mm, the outer edge softening factor is 0.1, the inner circle diameter is 6 mm, and the diameter of beam 2 is 6.6 mm. When the softening factor of the inner edge of beam 1 is Q3≥0.0625 and that of the outer edge of beam 2 is Q2≥0.055, and the pinhole of the spatial filter is greater than or equal to 32DL. After the propagation of laser in the 4f system, the near-field modulations of beams 1 and 2 are less than 5% at the positions of the image plane and 15 cm away from the image plane (Fig. 5). These results meet the demand for the near-field beam quality of the combined spatiotemporal beam. Furthermore, considering the perspective of risk prevention, when the time-domain extinction ratio is higher than 30 dB and the spatial extinction ratio is higher or equal to 20 dB, the influence of near-field intensity fluctuation on the system can be negligible [Fig. 6(b)]. Presently, the time-domain extinction ratio of the system can be greater than 40 dB and the spatial extinction ratio is ~20 dB, meeting the requirements of intensity stability of combined spatiotemporal beams in the propagation.
Conclusions Herein, to meet the physical experimental requirements of dynamic focus in the scheme of DCI using a high-power laser facility, a method based on the spatiotemporal beam combination laser preamplifier system in the laser driver is proposed. Based on Fresnel diffraction propagation theory, the laser propagation model of the combined spatiotemporal beam is established using the numerical simulation method of fast Fourier transform. The effects of the filter pinhole diameter, softening factor, extinction ratio, and phase difference on the propagation of the combined spatiotemporal beams are analyzed in the simulation, and reasonable softening factors and spatial filter pinhole diameters are obtained. The preliminary experimental results agree well with the simulation results. Moreover, the parameters of the facility can meet the intensity stability requirements of the propagation of the combined spatiotemporal beam from a risk prevention viewpoint. The numerical model can be used for optimizing other parameters, which can guide the propagation and amplification of the combined spatiotemporal beams in future research. The proposed method will be used in the spatiotemporal beam combination system of the preamplifier of the high-power laser facility. In the future, experimental research will be conducted on dynamic focus to meet target physics requirements.
