Study On Small-Scale Self-Focusing Effects In ICF Divers

Nowadays, more and more attention has been paid to the Inerlial Confined Fusion (ICF) solid drivers because of its tremendous functions in energy sources, scientific research, military affairs and so on. In this high power laser system, nonlinear self-focusing effect especially the small-scale self-focusing is the main limiting factor. The system's output power has been limited and its cost of operation has been enhanced because of filament destroy caused by small-scale self-focusing. So it is vital important to study the small-scale self-focusing problems. In order to fully understand the physical mechanism governing the small-scale self-focusing and extract a guide line to the optimization of drivers, in this work, we discuss some problems about it.Based on the nonlinear paraxial equation and B-T theory, a differential equation, which describes the small-scale self-focusing in gain (loss) media has been derived. The change rule of the maximal gain frequency, cutoff frequency and integral exponential gain have been analyzed approximately. The results indicate that after the beam has propagated for a minimal distance in gain media the modulation begin to increase and after the beam has propagated for a maximal distance in loss media the modulation stop increasing. We have calculated the effects which gain and loss impose on the formation distances of filaments of small-scale modulations, the results indicate that properly increasing the gain coefficient can counteract the small-scale self-focusing in certain degree.In addition, the formation of the so called "hot image" has been studied experimentally and theoretically. We have calculated the position and intensity of the "hot image". Studies indicated that image distance is independent of input intensity above a certain threshold and distance between modulate plane and input plane of media, but dependent on the gain coefficient and modulation size; The intensity of the "hot image" is independent of modulation size and distance between modulate plane and input plane of media, but dependent on the intensity of input beam and the gain coefficient.