Modeling Of Laser Induced Damage And Optic Usage At The National Ignition Facility PDF Download

Controlling damage to nominally transparent optical elements such as lenses, windows and frequency conversion crystals on high power lasers is a continuing technical problem. Scientific understanding of the underlying mechanisms of laser energy absorption, material heating and vaporization and resultant mechanical damage is especially important for UV lasers with large apertures such as NIF. This LDRD project was a single year effort, in coordination with associated experimental projects, to initiate theoretical descriptions of several of the relevant processes. In understanding laser damage, we distinguish between damage initiation and the growth of existent damage upon subsequent laser irradiation. In general, the effect of damage could be ameliorated by either preventing its initiation or by mitigating its growth. The distinction comes about because initiation is generally due to extrinsic factors such as contaminants, which provide a means of local laser energy absorption. Thus, initiation tends to be local and stochastic in nature. On the other hand, the initial damaging event appears to modify the surrounding material in such a way that multiple pulse damage grows more or less regularly. More exactly, three ingredients are necessary for visible laser induced damage. These are adequate laser energy, a mechanism of laser energy absorption and mechanical weakness. For damage growth, the material surrounding a damage site is already mechanically weakened by cracks and probably chemically modified as well. The mechanical damage can also lead to electric field intensification due to interference effects, thus increasing the available laser energy density. In this project, we successfully accounted for the pulselength dependence of damage threshold in bulk DKDP crystals with the hypothesis of small absorbers with a distribution of sizes. We theoretically investigated expected scaling of damage initiation craters both to baseline detailed numerical simulations presently underway and to aid identification of damage initiators. Ancillary experimental techniques intended to yield information on laser energy absorption and shockwave generation were investigated. We also determined the role of material evaporation and fluid motion accompanying low-level CO2 laser energy absorption, which can potentially ''heal'' surface mechanical damage. Section 2 of this report describes accomplishments of the project. Work reported elsewhere is mentioned briefly and cited. Section 3 describes the two proof of principle experiments carried out by UC collaborators. Section 4 has conclusions and recommendations for future work. Section 5 is a listing of reports and presentations arising from this project.