On The Beam Intensity Irregularities Induced By The 3～(rd) Order Nonlinear Effect In ICF Drivers

In an Inertial Confinement Fusion (ICF) laser driver, the 3rd order nonlinear effect can make beam irregularity grow, even damage the expensive optics. With the development of the research and fabrication of ICF drivers in our country, more and more attention has been paid to this issue. So, the two main effects (small-scale self-focusing and "hot image") relate to the 3rd order nonlinear effect have been studied in this work.At first, we describe the optical Keer effect induced by the 3rd order nonlinear effect, then we introduce the nonlinear paraxial equation and B-T theory. After describing the approximations adopted to arrive at the paraxial equation, the limitations of B-T theory have been identified. Then the influence of medium's gain and boundary conditions on ripple's gain has been discussed. It is pointed out that "the most quickly growing spatial frequency" and "the maximum growth spatial frequency" are different. The results also indicate that, in a piece of gain medium, the cutoff frequency,"the most quickly growing frequency" and the integral exponential gain are variables of the beam intensity, medium gain coefficient and propagation distance. Taking into account the boundary conditions, two results were gotten: 1) The nonlinear medium provides gain to small ripples which is beyond the Bespalov-talanov cutoff spatial frequency; 2) Large ripples can also get large gain.Besides the theoretical analysis, we also numerically studied the relation between the beam irregularity and B integral for a special four-pass amplifying system. Based on these calculations, a "red line" has been set up for the safety of the system. In addition, the variation of beam irregularity with the size of filter's hole has been studied.At last, we illustrate the imaging process of nonlinear medium by numerical simulation, and the image position and intensity are studied. At the same time, it is pointed out that a piece of nonlinear medium and high power laser beam can form a hologram system. For the first time, we found that a "dark image"(or "cold image") may emerge along with a "hot image".