The Influence Of Subsurface Microcracks In Fused Silica On Its Damage Growth

Damage growth has been an important problem, which is highly concerned inthe high-power laser systems such as NIF, SG-III, because it determines the lifetimeof the optical elements, and thus determines the running cost of the entire opticalsystem. The sub-surface defects especially the subsurface micro-cracks play animportant role in the whole damage growth process. To research on the influence ofmicrocracks that are distributed in the subsurface of fused silica glass on its damagegrowth, will help us to understand the internal mechanism of damage growth and tofurther develop the damage growth mitigation methods. In this way, we can prolongthe life of optical components and reduce the running costs of the optical system.Firstly, this paper outlines the basic theory of laser damage, including theintrinsic and extrinsic damage theory, linear absorption mechanism, electron impactionization mechanism and the mechanism of multi-photon ionization, the modelsdescribing the damage process as three stages (initial stage, the energy absorptionstage and the energy dissipation phase). We also summarize the basic theory ofdamage growth, including its concept and growth mechanism (heat, pressure andscattering). By introducing the glass manufacturing processes, the reasons for theformation of micro-cracks and its classification are also provided.Then, this paper establishes a numerical model for the sub-surface microcracks,using COMSOL Multiphysics to simulate how the three cracks (radial cracks,lateral cracks and conical cracks), which are in the front and rear surfaces under theirradiation of1553nm and355nm laser, to modulate and to enhance theelectromagnetic field. A preliminary analysis is given to determine the focus andmethod of numerical simulation. On this basis, by varying the parameters of thecracks (depth, opening width, and angle of inclination), we systematically simulatethe field enhancement factor of these three cracks, and a deep analysis is also made.Finally, combining with damage growth mechanism, we summarize the rolesplayed by subsurface micro-cracks in the three aspects of the damage growthprocess, including their contribution to damage initiation and damage growth factor.By varying the width and depth of the conical cracks, the development of the cracksand its mechanical effects on damage growth is analyzed by combining the PETeasy-tearing film. Some damage mitigation methods basing on healing themicrocracks are also provided.