Small-scale Self-focusing Evaluation Of Beam Of Intense Laser Device Based On Near-field Entropy

As the tremendous uses in fusion energy, high energy density science research, military affairs and so on, the research on inertial confined fusion (ICF) has attracted the attention of many superpower countries. At the present stage, the facility output ability is relatively low to the requirement of some physical experiments. However, the strong nonlinear effects induced by this high intensity is very obvious, and the localized, extremely high-intensity modulation bring by small-scale self-focusing (SSFF) will lead to serious optics damage. This threatens the safety of the high-power laser facility. So assessing SSFF development and discriminating the start of nonlinear rapidly growth is therefore necessary for ensuring a "safe" high-power laser facility design. For classical methods using contrast, modulation, or power spectral density, it has some defects such as unclear on creation, relying on human experiences and so on. In terms of this issue, we carry out the research on finding some assessment methods, which could reflect the rapidly nonlinear growth in high-power laser facility more clearly.On the basis of theoretical analysis, we bring the Shannon entropy concept to assess the SSFF development in high-power leaser facility creatively, and propose the near-filed intensity entropy and LogPSD entropy methods. By making use of the property of Shannon entropy, we find that the intensity entropy will have a reverse change corresponding to the rapidly nonlinear growth in high-power laser facility after qualitative analysis. Analogously, the LogPSD entropy method is proposed by frequency domain analysis. LogPSD entropy decreases first and then increases with the SSFF development. Both of these two methods offer an obvious turning point while the rapidly nonlinear growth starts in the facility. Compare to the classical methods such as contrast and modulation, the near-filed Shannon entropy turns out a reverse change while the nonlinear effect increases obviously. Near-filed Shannon entropy methods could reflect the start of nonlinear rapidly growth in facility clearly, which show great superiority while compared to classical methods.Base on TIL facility, the proposed Shannon entropy methods are investigated by numerical simulation and experiment. We find that with the idle super-gaussian inject and pinholes opened up in the facility, the near-field intensity entropy increases first and decrease with the increasing ?B, and the LogPSD entropy decreases first and then increase at the same time. The intensity entropy and LogPSD entropy curves have an obvious point and two turning points coincide well. We analyze the high power experiment (with pinhole) data conducted in TIL, it turns that the intensity entropy increases first and then decrease, which has the same tendency as idle inject and without pinholes. But LogPSD entropy decreases all the time with increasing ?B, which due to the spatial filter pinhole.By use of the near-filed intensity entropy method, we analyze the safe output limit of ITB facility operated with different inject pulse width. Intensity entropy reflects the change of ITB facility output beam near-field well, and turns out a reverse change while the nonlinear effect increase obviously. This would offer a clear alarm of optics damage induced by strong nonlinear effect. According to this, we give the safe output limit graph of ITB facility operated with different inject pulse width. This work is significant for ITB facility.