Research on target uniform irradiation method using linearly modulated light and special grating dispersion

In indirect-drive laser fusion research, filamentation instability (FI) severely influences laser injection efficiency and beam pointing accuracy in hohlraum. Experiments indicate that high frequency modulations on the far field of the laser driver are responsible for the filamentation. To solve this problem, a new smoothing technique is proposed using linear modulation and special grating dispersion. The linear modulation was realized by stacking a series of 100 ps chirped pulses in fiber delay lines. The temporal smoothing characteristics of the focal spot adopting star and circular grating dispersion were studied. Simulation results indicate that both kinds of grating investigated restrained the high frequency modulations. The star grating has a better smoothing effect when compared to circular grating. The insertion of the star grating could realize “color” variation across the beam in the azimuthal direction and make the far-field pattern rotate with time. Therefore, the time-integrated focal spot would be smoothed in a more comprehensive way. This smoothing method bears the advantage of simple configuration, two-dimensional angular spectral dispersion and not enlarging the far field too much.