Abstract
Max-of-N fluence is the maximum peak fluence at a given location over N number of shots and is important for calculating fluence dependence for intrinsic laser-induced optic damage. Previously, we observed the Max-of-N effect on the National Ignition Facility and developed an ad-hoc model to calculate its effect. In this work, we attempt to understand the fundamental mechanism that causes this Max-of-N effect. We conclude that the primary fundamental mechanism responsible for this effect is dominated by the combination of fluence variations and pointing jitter of the laser. This discovery both strengthens our model for predicting optics longevity and gives us insight into how to mitigate this effect.
Highlights
Control of the onset of damage on modern optical surfaces has improved so significantly[1,2,3] that the damage that occurs on the National Ignition Facility (NIF)’s fused silica optics surfaces is largely dominated by in-situ contaminations.[4,5]
For laser systems in which the laser varies significantly in terms of beam contrast, such as the 3-ω shot sequence where the range of beam contrast is on the order of 5% or more, we would either treat each single shot as different energy level shots or calculate the asymptotic contrast using the minimum of the beam contrast
We have explored the fundamental mechanism that is responsible for the Max-of-N effect on NIF and have found that it is driven by fixed spatial structure and the slight pointing jitter of the laser
Summary
Control of the onset of damage on modern optical surfaces has improved so significantly[1,2,3] that the damage that occurs on the National Ignition Facility (NIF)’s fused silica optics surfaces is largely dominated by in-situ contaminations.[4,5] The fluence and shot dependence of these damage mechanisms and efforts to eliminate them are discussed elsewhere.[4,5] As these sources of contamination are removed, operating limits are still be constrained by the limits of pristine surfaces. Max-of-N fluence distribution is the maximum peak fluence over N number of shots and was developed[7] to explicitly address the effect of repeated laser exposure to optics. The Max-of-N effect is the change of the fluence distribution as a function of repeated, constant-energy laser exposure. Our empirical formalism for calculating the Max-of-N fluence distribution and our approach to connecting the physical mechanism to the observed effect would provide a beneficiary foundation for all other high-energy lasers systems. Φ used for initiation calculations is the damage equivalent fluence (e.g., equivalent to a 3-ns Gaussian pulse shape) This is straightforward for a single shot, where we assume that the fluence distribution is Gaussian with mean μ and standard deviation σs as follows: ðφ−μÞ2. We investigated the possible mechanism behind the observed Max-of-N effect and addressed the various concerns described above that have limited the confidence in the current model
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