The Stochastic Nature of Growth of Laser-Induced Damage

Laser fluence and operational tempo of ICF systems operating in the UV are typically limited by the growth of laser-induced damage on their final optics (primarily silica optics). In the early 2000 time frame, studies of laser damage growth with relevant large area beams revealed that for some laser conditions damage sites located on the exit surface of a fused silica optic grew following an exponential growth rule: D(n) = D-0 exp (n alpha(phi)), where D is final site diameter, D-0 is the initial diameter of the site, phi is the laser fluence, alpha(phi) is the growth coefficient, and n is the number of exposures. In general alpha is a linear function of phi, with a threshold of phi(TH). In recent years, it has been found that that growth behavior is actually considerably more complex. For example, it was found that alpha is not a constant for a given fluence but follows a probability distribution with a mean equal to alpha(phi). This is complicated by observations that these distributions are actually functions of the pulse shape, damage site size, and initial morphology of damage initiation. In addition, there is not a fixed fluence threshold for damage sites growth, which is better described by a probability of growth which depends on site size, morphology and laser fluence. Here will review these findings and discuss implications for the operation of large laser systems.