In the inertial confinement nuclear fusion (ICF) facility, the non-uniformity of the target plane irradiation aggravates the instability of various laser plasmas such as magnification into filaments, stimulated Raman scattering, and stimulated Brillouin scattering, and destroys the target pellet symmetry of compression. In order to improve the irradiation uniformity of the target plane, researchers have successively developed a variety of beam smoothing schemes, such as radial smoothing (RS), azimuthal smoothing (AS), dynamic interference smoothing, and the combination of induced spatial incoherence(ISI) and lens array(LA) based on broadband light proposed by Shanghai Institute of Laser Plasma. However, these schemes have some shortcomings. Therefore, there is an urgent need to further develop a more feasible ultrafast beam smoothing scheme with a picosecond time scale.

Taking the 2×2 beam as an example, by establishing a physical model of the ultrafast beam smoothing scheme based on the rotation arrangement of phase plates, the effects of parameters of phase plates, beam arrangement, spatial wavefront distortion, and spatial deviation of laser beams on beam smoothing are analyzed.

Through calculation and simulation, the effects of parameters of phase plates, beam arrangement, spatial wavefront distortion, and spatial deviation of laser beams on beam smoothing are first investigated, the smoothing effect when the scheme is combined with the traditional beam smoothing scheme is then analyzed, and finally the influence of spatial deviation in the actual system on beam smoothing is studied. The simulation results show that when the peak valley (PV) value of the phase plate is 2λ3λ, it can not only ensure the beam smoothing effect of the scheme, but also avoid the excessive energy loss caused by wavefront distortion (Fig. 4). In addition, changing the arrangement of phase plates has little effect on the smoothing effect of the scheme (Fig. 5). When the PV value of phase distortion does not exceed 3λ, the scheme can show a better smoothing effect, and at the same time, the energy utilization rate of continuous phase plate (CPP) is less reduced (Fig. 6). Moreover, the combination of beam smoothing schemes based on the rotation arrangement of phase plates and smoothing by two-dimension spectral dispersion(2D-SSD) can further improve the smoothing effect of the target plane (Fig. 7). Finally, this scheme also has a large tolerance for spatial deviation (Fig. 8).

This paper proposes an ultrafast beam smoothing scheme based on the rotation arrangement of phase plates. By rotationally arranging the phase plates with a rotational asymmetric distribution in the laser quad, different spatial phase modulations are first provided for each sub-beam in the laser quad. Then the dynamic interference of the sub-beams with a certain wavelength difference on the target plane makes the speckles within the focal spot sweep rapidly in multiple directions and multiple dimensions, so as to achieve the goal of improving the uniformity of the focal spot in the picosecond time scale. On this basis, the effects of parameters of phase plates, beam arrangement, spatial wavefront distortion, and spatial deviation of laser beams on beam smoothing are analyzed. The results show that in the ultrafast beam smoothing scheme based on the rotation arrangement of phase plates, only the same phase plates with a rotational asymmetric distribution are required to be processed at the same time, which can lower the design and processing difficulty of phase plates. In addition, this scheme is little affected by spatial wavefront distortion, beam arrangement, and spatial deviation of laser beams. Combining the beam smoothing scheme based on the rotation arrangement of phase plates proposed in this paper with the traditional beam smoothing schemes can significantly improve the uniformity of focal spots within a few picoseconds, which can be an effective supplement to the traditional beam smoothing schemes.