The LMJ target chamber is designed to withstand high fusion energy produced by 20 MJ yield shots and 600 nm yield shots per year on a large variety of targets. We show how the target chamber and its equipment have to be protected from damaging by 14 MeV neutrons, high x-ray fluences, debris and shrapnel. A first wall made of louvers is required to prevent material ablated by x-rays form reaching the debris shields and reducing their live time. Constraints on the target design is also considered to limit the sources of vaporized or ionized mass and shrapnel coming directly from the target. The use of pre-shield for large mass targets is suggested to meet the requirement on the frequency cleaning of the debris shields.
Fused silica windows were artificially contaminated to estimate the resistance of target chamber debris shields against laser damage during NIF operation. Uniform contamination thin films were prepared by sputtering various materials. The loss of transmission of the samples was first measured. They were then tested at 355 nm in air with an 8- ns Nd:YAG laser. The damage morphologies were characterized by Nomarski optical microscopy and SEM. Both theory and experiments showed that metal contamination for films as thin as 1 nm leads to a substantial los of transmission. The laser damage resistance dropped very uniformly across the entire surface. The damage morphology characterization showed that contrary to clean silica, metal coated samples did not produce pits on the surface, B4C coated silica, on the other hand, led to a higher density of such damage pits. A model for light absorption in the thin film was coupled with a simple heat deposition and diffusion model to perform preliminary theoretical estimates of damage thresholds. The estimates of the loss due to light absorption and reflection pointed out significant differences between metals. The damage threshold predictions were in qualitative agreement with experimental measurements.
A major preoccupation for the design of the LMJ laser is the mirrors laser damage threshold. SAGEM SA, in collaboration with the CEA, has conducted a study in order to improve the laser induced damage threshold under operational conditions.
Large components are needed to build up the Laser Megajoule which belongs to the future generation of fusion laser. It is essential to determine the laser damage threshold of full-size components with a large-size laser beam, and to compare this value to the threshold obtained on small samples with a small gaussian laser beam. So, we have, in the first place, tested some small samples from the same coating run on REOSC, CEA and LLNL facilities and, in a second series of experiments, we have tested three full-size optical components on the LLNL beamlet laser. Two types of optical components have been tested on the beamlet: - one polarizer: 710 X 400 nm; - two HfO2/SiO2 mirrors, from the same coating run: 620 X 440 mm. Before beamlet irradiation these components have been continued at 10 ns on the LLNL Plato facility. Concerning the mirrors, two different conditioning procedures have been applied: - on the first mirror a 5-step conditioning procedure; - on the second mirror a 2-step conditioning procedure. Then, each mirror has undergone the same test on the beamlet. In this paper we report the result obtained on the polarizer and the mirrors after beamlet irradiation.
Inertial fusion targets are of two basics types: direct drive and indirect drive. The french Laser MegaJoule (LMJ) will preferentially use indirect drive experiments. However, to contribute to the determination of the best target for inertial fusion energy, both direct and indirect drive will be considered on LMJ-facility. That is why studies on materials for use as first-wall LMJ target chamber and for different applications are of significant interest. In direct drive, targets will be directly heated and imploded by intense 351-nm wavelength laser light and part of this UV energy will be back-scattered off the target surface and consequently deposited onto the target chamber first wall. Such a deposited fluence should average 0.3-J/cm2. The aim of this study was to determine both the fluence level at which physical damage occurs on some considered protective materials and the associated damage mechanisms. Investigations have been carried out on boron carbide and carbon-based materials. Basically, experiments consisted of illuminating samples with a 355-nm gaussian pulse of roughly 3-ns duration generated by frequency conversion through KDP- crystals of a YAG-laser light. In our investigations, characterization of ejected species and transient hemispherical directional reflectivity measurement of the illuminated materials were conducted. All these experiments have shown that boron carbide was a better candidate than carbon-based materials as first-wall LMJ target chamber use. Analyzing the defect type, size and density and the composition and physical-state of the considered materials has allowed predicting the impact of induced-ablation of the first-wall on LMJ target chamber use and lifetime. Composition analysis were addressed using micro-Raman spectroscopy. Preliminary experiments have revealed significant changes in damage mechanism versus laser fluence.
Large components typically 620 mm X 440 mm are needed to build up the LMJ (laser megajoule) which belongs to the future generation of the fusion laser. Today, the laboratory laser damage facilities allow to test small samples: generally witnesses with 50 mm in diameter. However, more of the test procedures are manually and small areas are examined. It means that usually just 20 and 50 sites are tested with a small Gaussian beam. Therefore, it is essential to verify if the large components needed for the fusion laser have the threshold above the requirements. It is also important to determine which law exists between the threshold of the witnesses samples and the threshold of the final components. In this paper, these problems are addressed in using the most powerful pulsed Nd:YAG laser in France. With this particular facility it becomes possible to test full scale optics on a few sites with a beam size diameter of 6 cm. Except the test on the real system, these kinds of experiments allow to observe the large beam damage morphologies. A study is at the beginning to try to find the scale law which linked the result obtained on the small witnesses with various methods. It also should lead to a procedure to qualify the full scale optics during the production time.
A set of fused silica windows were artificially contaminated by sputtering various thin film materials onto the surface to simulate target chamber contamination of high fluence optical components. The samples were then tested 1/1 and N/1 at 355 nm in air and vacuum with a 3- ns Nd:YAG laser to measure the damage threshold as a function of film thickness and film composition. The damage morphologies were characterized by Nomarski optical microscopy and SEM. The results show that thin film contamination leads to a decrease in damage threshold. The test also show a difference in damage behavior between air and vacuum. The results were compared to a model of the absorption of laser light in the film, shockwave generation and reflection, and heating dependence on film composition and thickness.
The manufacture of modern optical components requires the use of high performance
interferometers, usually based on phase-shifting techniques.
However, there is currently no commercial phase-shifting interferometer having the capacity to
measure large parts, such as those found in Inertial Confinement Fusion Lasers (NIF and
Méga-Joule), and other large systems.
Standard interferometers lack the simultaneous qualities of accuracy and spatial resolution
required for the measurement of such components. Indeed, it has been shown that surface
ripples with wavelengths of around 1 to 10 millimeters are extremely dangerous for large ICF
optics, even at low amplitude, because of the process of non-linear ripple amplification present
in high power laser systems.
In order to circumvent the restrictions on size and performance of standard interferometers, we
have designed and built a scanning interferometer, using a standard "small" diameter phase
shifting interferometer. A PC computer is used to control the measurement process, acquire
the interferograms and stitch the measurements together to produce the original large surface.
The measurement sequence is completely automated.
The advantages ofthis technique are low cost, small size, and no loss ofspatial resolution.
One system has been in actual use for the characterization of large size mirrors (approx. 400 x
600 mm) since december 1994.
In this presentation, we shall look over the design of the system, produce actual measurements,
and discuss the technical implications of the stitching process in relation to specifications such
as those currently being derived for ICF large components.
This work is supported by CEA/CEL-V, as part of the Laser MégaJoule Program.
It is now accepted that absorption phenomena are not directly responsible for the flux resistance of multilayer optical components exposed to pulsed lasers. However, such studies involved `overall' absorption by components, which is now known does not characterize an intrinsic property of thin film materials. If the absorption of thin film materials is mapped by exciting different zones less than 100 micrometers in diameter, a clear disparity in absorption thresholds is shown up which must be attributed to heterogeneity in the imaginary refractive index or the extinction coefficient. The heterogeneity of absorption may be related to the incorporation of foreign matter in the materials or to dislocations, nodules, etc... Under these circumstances a key issue is whether the damage thresholds can be correlated with these microscopic absorption sites. We present the equipment used for carrying out the different types of characterization in this study: absorption and diffusion mapping, damage threshold measurements, and analysis by atomic force microscopy. An attempt is made to correlate these different results.
For years, contamination has been known to degrade the performance of optics and to sometimes initiate laser-induced damage to initiate. This study has started to quantify these effects for fused silica windows used at 355 nm. Contamination particles (Al, Cu, TiO2 and ZrO2) were artificially deposited onto the surface and damage tests were conducted with a 3 ns Nd:YAG laser. The damage morphology was characterized by Nomarski optical microscopy. The results showed that the damage morphology for input and output surface contamination is different. For input surface contamination, both input and output surfaces can damage. In particular, the particle can induce pitting or drilling of the surface where the beam exists. Such damage usually grows catastrophically. Output surface contamination is usually ablated away on the first shot but can also induced catastrophic damage. Plasmas are observed during illumination and seem to play an important role in the damage mechanism. The relationship between fluence and contamination size for which catastrophic damage occurred was plotted for different contamination materials. The results show that particles even as small as 10 micrometers can substantially decrease the damage threshold of the window and that metallic particles on the input surface have a more negative effect than oxide particles.
Simultaneous absorption and scattering mappings are used to study the influence of local defects on the laser damage threshold of thin films. The same area is mapped for absorption an d scatter before and after irradiation at the threshold fluence. The study is performed for an uncoated fused silica substrate and two single-layer films deposited on fused silica substrates at a wavelength of (lambda) equals 1.06 micrometers . Initial results seem to indicate that the irradiation can create and enhance absorption and scattering defects.
One of the concerns with the megajoule laser design is the laser-induced damage threshold of the transport mirrors. Earlier studies have shown that the main constraint on the laser damage threshold comes from nodules at the mirror surface. It is therefore important to restrict the number of such nodules. SFIM-ODS, in close collaboration with CEL-V, has initiated a special study to characterize these nodules as precisely as possible. The objective of the study is twofold: (1) to determine the origin of the nodules and subsequently to adapt the mirror fabrication process in order to limit their formation, (2) to analyze their shapes and dimensions in order to ascertain which nodules are critical for laser-induced damage. To understand the origin of the nodules and their effect on the laser damage threshold, the mirrors are characterized using various methods: (1) absorption and scatter mapping: does the presence of nodules result in specific absorption patterns, (2) surface analysis by atomic force microscopy: to characterize nodule shape and dimensions, (3) focused ion beam cutting of nodules: to locate the seed initiating the nodule and to characterize the seed shape and composition, (4) laser damage threshold measurements: to determine the laser damage threshold of them mirror and study the behavior of nodules under laser irradiation depending on their dimensions and shape.
Laser conditioning is a process which consists in illuminating a coating by a laser below damage threshold in order to increase this threshold value. In a previous study we had reported that laser conditioning actually results in minimization or elimination of surface defects present after coating elaboration. The present paper reports a statistical study whose goal is to seek a correlation between defects size and shape, and defect evolution during laser conditioning. In order to achieve this, hundreds of defects are individually analyzed before and after laser conditioning using Atomic Force Microscopy. The observed samples are e-beam deposited highly reflecting multilayers Zr02/Si02 and Hf02/Si02 mirrors, and also single layers of Si02 and Zr02.
A focused ion-beam milling instrument, commonly utilized in the semiconductor industry for failure analysis and IC repair, is capable of cross-sectioning nodular defects. Utilizing the instrument's scanning on beam, high-resolution imaging of the seeds that initiate nodular defect growth is possible. In an attempt to understand the origins of these seeds, HfO2/SiO2 and Ta2O5/SiO2 coatings were prepared by a variety of coating vendors and different deposition processes including e-beam, magnetron sputtering, and ion beam sputtering. By studying the shape, depth, and composition of the seed, inferences of its origin can be drawn. The boundaries between the nodule and thin film provide insight into the mechanical stability of the nodule. Significant differences in the seed composition, geometry of nodular growth and mechanical stability of the defects for sputtered versus e-beam coatings are reported. Differences in seed shape were also observed from different coating vendors using e-beam deposition of HfO2/SiO2 coatings.
The CEA Limeil-Valenton Inertial Confinement Fusion (ICF) program is currently addressing, in close collaboration with LLNL, the critical physics and technology issues for demonstrating and exploiting high-gain ICF. We believe that a compelling strategy for our national ICF program is to demonstrate ignition and moderate gain at the beginning of the next century with an upgrade to the existing Phebus laser to provide 1.8 MJ of 0.351- micron light in a nanosecond regime. The proposed MJ-range laser system will consist of 288 optically independent and individually targetable beams each having a final optical aperture of approximately 35 centimeters. While Phebus used a multistage, single-pass amplifier design, the proposed MJ laser will use a single-stage multipass amplifier design so-called L-Turn. In the L-Turn architecture, a laser pulse enters the cavity and makes 4 passes in the resonator using different pathways before being directed out. The pulse reflects off a pick-off 6 x 6 cm square mirror that may withstand very high fluences. (Abstract truncated.)
We have studied the changes of dielectric optical coatings after laser conditioning using atomic force microscopy (AFM). Laser conditioning is the process which consists of illuminating a film by a laser below damage threshold in order to increase this threshold value. This method is implemented for optics used in high fluence laser beam such as Nd-glass lasers (1064 nm, 3 ns pulse length). We have studied e-beam evaporation deposited highly reflective multilayer ZrO2/SiO2 mirrors, and also single layers of SiO2, HfO2 and ZrO2. We have observed the modification of the surface after laser conditioning using the AFM. We find that this process results in localized minimization or elimination of defects (nodules, craters, etc...). Furthermore, the origin of the increase in the size of the hillocks observed after laser conditioning and previously reported by other groups, has been experimentally identified as an instrumental artifact.
We have studied the changes of dielectric optical coatings after laser conditioning using atomic force microscopy (AFM). Laser coatings is the process which consists of illuminating a film by a laser below damage threshold in order to increase this threshold value. This method is implemented for optics used in high fluence laser beam such as Nd-glass lasers (1064 nm, 3 ns pulse length). We have studied e-beam evaporation deposited highly reflective multilayer ZrO2/SiO2 mirrors, and also single layers of SiO2, HfO2 and ZrO2. We have observed the modification of the surface after laser conditioning using the AFM. We find that this process results in localized minimization or elimination of defects (nodules, craters etc...). Furthermore, the origin of the increase in the size of the hillocks observed after laser conditioning and previously reported by other groups, has been experimentally identified as an instrumental artifact.
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