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Isaac Bass, Eyal Feigenbaum, Darin Anderson, Gabe Guss, Wren Carr
High energy and power laser systems such as NIF operate near the damage threshold of its optical components. Stable operation requires control of the initiation and growth of the inevitable damage that will occur on its high-value optics due to factors such as beam contrast, non-linear effects, and contamination. This is accomplished through an optics recycling loop.[1] A key element in this loop is the Final Optics Damage Inspection (FODI) [2] system that monitors damage on the final optics preceding the target chamber. When the damage size reaches an upper limit, the optic is removed from the system and transported to the Optics Mitigation Facility (OMF). For fused silica optics, the damage is removed by a machining process using a CO2 laser.[3] This leaves a conical shaped depression in the surface of the optic.
The core of the FODI system is an articulated telescopic imager at the center of the target chamber that can interrogate the final optics in any of the 192 beam lines entering the chamber. The detector in the imager is a two-dimensional charge coupled device (CCD) array. Individual optics in any beam line can be separately interrogated using an illuminator that injects light into the optic through one of its edges. This light then scatters or reflects from damage, defects, and cones to the FODI imager.
The walls of the cones on the input surface of the optic, because of their orientations, are particularly efficient reflectors/scatterers of light. In many instances, the magnitude is sufficient to highly saturate the CCD pixels. This produces the well know blooming effect in CCDs when the charge exceeds the storage capacity of a pixel and spills into adjacent pixels. This result is a long line of saturated pixels when the charge is read out. The area of this line can be sufficiently large to obscure the growth of damage to sizes exceeding the upper allowable limit for removal of the optic. This can lead to either loss of the optic for recycling, or loss of the optic for any further use. The issue may become exacerbated by the prospect of increased use of even larger input cones to control the growth of exit surface damage by shadowing.[4]
We will report on our study of light scattering by input surface cones and techniques to suppress the blooming effect it produces on the FODI imager. It includes high resolution imaging of the scattered light, its dependence on cone size and shape, modeling of the scattering process, and diversion of the scattered light by opposing exit surface cones.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-??????.
References:
[1] Optics Recycle Loop Strategy for NIF Operations above UV Laser-Induced Damage Threshold, M. L. Spaeth, P. J. Wegner, T. I. Suratwala, M. C. Nostrand, J. D. Bude, A.D. Conder, J. A. Folta, J. E. Heebner, L. M. Kegelmeyer, B. J. MacGowan, D. C. Mason, M. J. Matthews & P. K. Whitman, Fusion Science and Technology Volume 69, 2016 - Issue 1
[2] Kegelmeyer LM, Clark RD, Leach RR, McGuigan DL, Miller Kamm V, Potter D, Salmon JT, Senecal JD, Conder AD, Nostrand M, Whitman PK. Automated optics inspection analysis for NIF. "Fusion Engineering and Design", Volume 87, Issue 12, December 2012, Pages 2120-2124
[3] I. L. Bass, G. M. Guss, M. J. Nostrand, and P. L. Wegner, “An improved method of mitigating laser-induced surface damage growth in fused Silica using a rastered, pulsed CO2 laser,” Proc. SPIE 7842, 7842202010.
[4] R. N. Raman, R. Garcha, M. C. Rushford, G. Guss, and C.W. Carr, “A shadowing technique to arrest laser-induced damage growth on exit surface silica,” Proc SPIE 11173, 1117303 (2019)
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