Scanning electron microscopy images of raster-scan initiated laser damage sites on MLD diffraction grating samples. Different damage morphologies are observed when using 30-ps pulse duration (left), and 1-ps pulse duration (right). In each case the damage appears to be caused by a buried nodular inclusion which has ejected through the multilayer coating, leaving a crater and damaging nearby grating structure.
(Download Image)Assessing the Damage Resistance of ARC Gratings
When completed, NIF’s Advanced Radiographic Capability (ARC) will be the world’s largest and highest-energy short-pulse laser, capable of delivering peak power exceeding one quadrillion (1015) watts. ARC is designed to produce brighter, more penetrating, higher-energy x-rays than can be obtained with conventional radiographic techniques. ARC’s ultimate performance, however, will depend in part on the ability of its multilayer dielectric (MLD) diffraction gratings and other downstream optics to resist damage during the few picoseconds (trillionths of a second) they are exposed to ARC’s intense, petawatt-class laser beams.
In a paper in the June 15 issue of Optics Express, LLNL researchers reported the results of measurements of the picosecond laser damage behavior of gratings used in the compression of chirped pulses, such as those used on ARC, in order to determine ARC’s safe operational limits. Previous laser damage assessments of MLD diffraction gratings have been based on catastrophic laser damage tests of witness samples performed on small areas (~10−3 cm2) in air. These measurements do not necessarily predict operational performance limits of ARC’s one-meter-wide gratings located in a vacuum chamber. To investigate these issues, the researchers developed a short-pulse in-vacuum laser damage test station (VLDTS) that enables testing of large areas (~20 cm2) under expected use conditions.
The team measured the cumulative optical damage density, ρ(Φ), of MLD gratings by raster scanning 1-cm2 sample areas with picosecond duration laser pulses in a clean vacuum environment. Measurements were also made using the commonly-used “R-on-1” test method where ramp-to-failure tests are performed on several sample locations to estimate the damage probability. The ρ(Φ) data provide a “significant improvement of the damage performance estimate over the commonly used small-area R-on-1 test,” the researchers said. While ρ(Φ) measurements are used to predict performance of NIF optics subjected to nanosecond duration pulses, these measurements show that it should also be applied for short-pulse damage characterization. Large-area testing is more relevant to performance in a laser system because it scans for regions that initiate damage at low fluence and lower the overall optic performance.
The team also looked into what caused the damage observed in the raster-scan measurements. A close-up view of these damage sites using scanning electron microscopy (see graphic) shows that a ~10-micron-diameter pit exists in the MLD stack which is characteristic of a buried nodular inclusion which has ejected from the coating. This also causes damage to nearby grating structure which depends on the incident laser pulse duration. These particular defects (nodules) limit the overall performance and are also sparsely located so they require advanced methods (like the raster scan) to characterize.
The maximum laser energy that ARC can deliver on target will also depend on the compressed pulse duration. ARC will have the ability to vary the pulse duration from 1 to 50 ps which is a range, the researchers said, “where the (laser damage) scaling is not well understood due to the many material-dependent condensed matter processes which are relevant.” To estimate the scaling of the operating limit with pulse duration (τ) the authors have performed R-on-1 measurements at 1-30 ps, resulting in a scaling of τ0.22 for both air and vacuum environments. Understanding what physical processes are responsible for causing damage from picosecond pulses (the focus of a current Laboratory Directed Research and Development effort) also has potential to lead to performance improvements.
To simulate the environment in which the gratings are used, the researchers also conducted the first study of the effect of controlled exposure in a vacuum to vapor-phase organic contaminants (VOCs) on picosecond laser damage of diffraction gratings. Operating MLD coated optics in vacuum chambers can expose them to VOCs, chemically remove oxygen from the MLD coating stack via laser-assisted reduction reactions, and also deform the MLD coating stack from outgassing of trapped water vapor. Each of these processes depends on the environmental conditions as well as the characteristics of the MLD coating stack and has the potential to alter its performance.
“No change in the R-on-1 damage probability was observed over a VOC exposure range exceeding that expected for the ARC compressor vessel,” the researchers said. “It is important to note that these initial results do not rule out effects of these VOC exposures on a single-shot laser system (such as ARC) as the fluence ramping effect of the R-on-1 test may laser clean the surface hydrocarbon contamination. Further testing using different damage test methods such as 1-on-1 are needed to address this issue.”
Lead author David Alessi was joined by LLNL colleagues Wren Carr, Dick Hackel, Raluca Negres, Ken Stanion, Jim Fair, David Cross, James Nissen, Ron Luthi, Gabe Guss, Jerry Britten, Bill Gourdin, and Constantin Haefner.
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