3(omega) Damage: Growth Mitigation
The design of high power UV laser systems is limited to a large extent by the laser-initiated damage performance of transmissive fused silica optical components. The 3{omega} (i.e., the third harmonic of the primary laser frequency) damage growth mitigation LDRD effort focused on understanding and reducing the rapid growth of laser-initiated surface damage on fused silica optics. Laser-initiated damage can be discussed in terms of two key issues: damage initiated at some type of precursor and rapid damage growth of the damage due to subsequent laser pulses. The objective of the LDRD effort has been the elucidation of laser-induced damage processes in order to quantify and potentially reduce the risk of damage to fused silica surfaces. The emphasis of the first two years of this effort was the characterization and reduction of damage initiation. In spite of significant reductions in the density of damage sites on polished surfaces, statistically some amount of damage initiation should always be expected. The early effort therefore emphasized the development of testing techniques that quantified the statistical nature of damage initiation on optical surfaces. This work led to the development of an optics lifetime modeling strategy that has been adopted by the NIF project to address damage-risk issues. During FY99 interest shifted to the damage growth issue which was the focus of the final year of this project. The impact of the remaining damage sites on laser performance can be minimized if the damage sites did not continue to grow following subsequent illumination. The objectives of the final year of the LDRD effort were to apply a suite of state-of-the-art characterization tools to elucidate the nature of the initiated damage sites, and to identify a method that effectively mitigates further damage growth. Our specific goal is to understand the cause for the rapid growth of damage sites so that we can develop and apply an effective means to mitigate it. The prevailing hypothesis for the growth mechanism of laser-initiated damage involves a synergism of some means for absorption of 3{omega} light at the damage site and local field enhancement due to cracks. A proposed mechanism for damage growth involves an initial damage at a precursor resulting in the transformation of basically non-absorbing SiO{sub 2} to form an absorbing layer of d-SiOx. In this context d-SiOx implies SiO{sub 2} modified in terms of either the formation of other stoichiometries (eg., SiO, Si, or more generally SiOx with 0<x<2) or significant concentrations of defects (broken bonds, vacancies, etc.). Earlier efforts focused on the characterization of the absorption mechanisms and measurement of the field enhancement due to cracks. The FY00 effort continued the identification of the absorbing species and the characterization of damage morphology while emphasizing the development of growth mitigation techniques directed at removing both the absorbing species and the cracks. We applied a variety of analytical tools to characterize the damage morphology, including; photoluminescence (PL) spectroscopy, optical and photothermal microscopies, high resolution transmission electron microscopy (TEM) and electron-spin-resonance (ESR) spectroscopy, x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), x-ray micro-tomography (XMT) and cathodo-luminescence (CL). The objective of the surface damage mitigation effort is to experimentally validate methods that could effectively stop the growth of 3{omega} laser-initiated damage. A specific goal is to obtain data and information on successful methods for fused silica optics, which would be sufficient to enable the down-selection to a single approach. Future effort could then be focused on developing a primary method for actual implementation on NIF. It is also the intent of this study to prioritize the remaining successful methods, so that there will be a back-up selection if the primary method fails to meet requirements. The mitigation methods selected for the study are chemical etching, plasma-etching, CO{sub 2} laser processing, and micro-flame torch processing. The number of experiments differed for the various methods and therefore the data for some techniques is limited. Nevertheless, there is sufficient data and information to choose a primary mitigation method. The CO{sub 2} laser processing shows the most significant effect to halt damage growth, and there is already enough data to have confidence in the approach for mitigating damage on NIF relevant silica optics.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15013546
- Report Number(s):
- UCRL-ID-142773; TRN: US0600830
- Country of Publication:
- United States
- Language:
- English
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