Laser Contouring Method for Wall Thickness Metrology on OMEGA Shock Breakout Targets

Transcription

1 Laser Contouring Method for Wall Thickness Metrology on OMEGA Shock Breakout Targets R. J. Sebring 1, J. J. Bartos 1, R. D. Day 1, J. M. Edwards 1, A. Nobile 1, G. Rivera 1 and R. E, Olson Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM Diagnostics and Target Physics, Sandia National Laboratory, Alb., NM Introduction The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory will attempt to achieve thermonuclear ignition of deuterium-tritium in approximately ten years. Ignition will be achieved by using the 1.8 MJ, 192 laser beam NIF to compress spherical targets containing deuterium-tritium fuel. An important physics issue that is being studied in experiments at the OMEGA laser facility at the University of Rochester is the manner in which laser-induced shocks propagate through target materials, and how target materials absorb and respond to x- ray radiation pulses. The Omega laser is presently the world most powerful laser with 30KJand 60 laser beams. At Los Alamos we are fabricating targets for these experiments. The metrology of these targets is a critical issue. In this paper, the unique advantages of measuring targets assembled in their ready-to fire physics targets, using dual, active confocal, lasers is described. Because suitable technology for measuring wall thickness to the accuracy and resolution required of these mesoscale target features does not exist, our lab developed a custom contour method. In the contour method, the target is mapped at extremely high spatial density by two opposing active confocal laser displacement probes, calibrated to each other for absolute wall thickness. The probes are scanned simultaneously over both surfaces of the target, resulting in the capture of 3D surface contour and wall thickness coordinate data maps. The motion, high speed data acquisition and analysis software system has been described elsewhere [1]. Targets The ablator targets were copper doped beryllium (0.9 atomic %), discs, 850 µm in diameter. Three styles of targets were fabricated with electric discharge machining and measured for surface contours and wall thickness variations using the contour method. The three styles of targets fabricated were 1) flats; 25 µm thick disc (Fig. 1A), 2) single steps; 40 µm at the thin end and 80 µm at the thick end (Fig. 1B) and 3) wedges; sloped region in the center with a small flat region at both ends (Fig. 1C). The target was attached to a cylindrical gold piece called a 'halfraum' and surrounded by a ultraviolet (UV) shine shield called a 'top hat.' The assembly was mounted to a long glass fiber called a 'stalk.' Due to this assembly, the target was inaccessible to conventional contact probe stylus or triangulation based laser probes (fig. 2). A B C 850 µm Fig. 1: Three styles of ablator targets were measured; A) a flat B) a step and C) a wedge 2003 Winter Topical Meeting - Volume

2 Fig. 2: A line diagram and two photographs showing the placement of the ablator target relative to the halfraum, UV shield and holding stalk. The metrology challenge was to measure the contours of both sides of the target after it was assembled into this assembly. Measuring Apparatus The targets were fixtured on our custom experimental measuring platform equipped with precision X and Y axes motions and high speed digital data acquisition. The motions are graphite air bearings, on round ways, mounted in a custom set of frames (Fig. 3A). Positioning is by linear motors and glass scales. The XY resolution is 100 nanometer, repeatability is ± 100 nanometers. The man/machine interface is LabView graphical programming software with Active X objects. Two opposing active laser confocal ranging probes were used to capture the part contours (Fig. 3B). These probes are commercially available and have a proven metrology record [2]. The principle of confocal is well described in the literature [3,4] and will not be described in this paper. Each probe (Keyence Corp. Model LT-8110). has a measuring range of 2 mm, a resolution of 0.2 µm, a spot size of 7 µm and a data rate of 1400 samples/sec. Data acquisition is via quadrature counters operating at 8 megahertz, and 16 bit A to D converters writing to a PC hard drive. CF A B CF Fig. 3: A) The LANL custom laser contour machine mounted on an anti-vibration table and its computer for motion control and data acquisition. B) close up of machine in white circled region of Fig. 3A showing two active confocal probes (CF) configured to scan ablator target assembled in halfraum (arrow). American Society for Precision Engineering 143

3 Scanning Laser active confocal probe calibration was verified for each probe using a National Institute of Standards and Technology (NIST) traceable quartz step height standard, 24.5 µm high. The two probes were carefully aligned to each other such that their optical axes were coaxial. A NIST traceable gauge block, 0.01 inch thickness, was inserted between the probes and the two opposing probes were set relative to this absolute thickness by zeroing them electronically. A motion control program was generated that created a XY raster motion over a 1.2 x 1.2 mm area. The first task the program does is to scan a 2mm length of the gauge block. This data is stored along with the target data and later used to correct the wall thickness data into an absolute thickness. The scan of the target immediately commenced following the gauge block scan. The gauge block and part were both attached to the Y axis, and moved relative to the probes at a rate of 0.2 mm/sec. while the probes remained stationary. Once a line of data 1.2 mm long is captured on the target, the probes, attached to the X axes, were stepped 0.01 mm. The whole scanning process, raster scanning and logging of data into a text file as XYZ triplets for top contour, bottom contour and wall thickness is totally automatic and takes about 15 minutes to complete. The raw coordinate data points, called clouds, were rendered and edited using reverse engineering software (Surfacer, Unigraphics Inc.) (Fig 4). A sub-section of the entire cloud, a cloud patch 150 µm wide x 860 µm long, was cut out and saved as a separate text file (Fig.5). The patch area corresponds to the exact region of the target as viewed by the diagnostic during the actual Omega shot. The 3D patch was plotted as a 2D wave, essentially collapsing the Y axis data into the X axis as a 2D plot, preserving the thickness variation data for the entire patch. A polynomial was fit to the wave and then sampled into 40 discrete, evenly spaced points. The data plot and the polynomial equation were provided to the experiment designer. The metrology was used to verify the wall thickness of the as-built target to choose the most appropriate target to shoot and assist in interpreting the post shot data. Fig. 4: 3D rendering of contour clouds of a wedge target. Top surface of wedge (blue colored points) and bottom surface of wedge (green colored points), lower flat region (yellow needles) and upper flat region (red needles).the needle colors and lengths represent deviations between the upper and lower surface points Winter Topical Meeting - Volume

4 Fig. 5: Wall thickness cloud of ablator step target. The whole target is in white and a sub section of this cloud, a patch 15 µm wide and 850 µm long that was extracted for analysis. Result Two of the three wedge shaped ablator targets, W2 and W3, varied in thickness relative to each other by about 10 microns (Fig. 6). Their wedge angles, about 4.1 degrees, were nearly identical. Their wall thickness varied considerably at the ends where the wedges were to transition into flat regions. This was more apparent for W2. Contour data from the top and bottom of the target W2 shows graphically that the majority of this unexpected wall thickness artifact is due to an uneven contour of the target's bottom surface The bottom surface is dish shaped instead of flat (Fig. 7) and would likely be caused by in the fabrication process. Even with these differences, the two targets were still within usable dimensional specifications for the experiment. But knowing the absolute wall thickness variations over the target, especially localized in the area of diagnostic interest, with this unprecedented level of detail was important to the precise interpretation of the shot breakout data. One of the three step shaped ablator targets, S1 (Fig.8 red colored line), was a few microns thinner at both the upper and lower steps but the step height was correct Thickness (mm) Position (mm) Fig. 6: Absolute wall thickness variation plotted as a 7th degree polynomial fit of wall thickness data with 40 discreet points along fit of ablator targets W2 and W3. These two targets are slightly different in thickness but equal in angle. American Society for Precision Engineering 145

5 Thickness (µm) Position (mm) Fig. 7 Top and bottom contour data from a 150 µm wide slice of the original 3D cloud of the entire target area. A 6th degree polynomial fit of ablator wedge target W2 is included for contour visualization purposes Thickness (mm) Position (mm) Fig. 8 Absolute wall thickness variations plotted as a 6th degree polynomial fit of wall thickness data from 150 µm thick slice of 3 different ablator step targets. Red = S1, Blue =S2 and Green =S3 targets Winter Topical Meeting - Volume

6 Discussion The primary objective of this metrology was to determine wall thickness variations in the specific area of the ablator target imaged by the ultra-violet (UV) and X-ray diagnostics during the shock breakout experiment at Omega laser in Rochester, N.Y. Step heights and wall thickness dimensions, as measured using this the contour method, varied from target to target. The results of this metrology were used to verify target dimensions, pre-select the best target assemblies, customize shot parameters for individual targets and aid in the interpretation of the shot breakout results. Prior to developing and using the contour method, wall thickness variations on these small targets could not be measured by conventional metrology. Step heights could be determined on the free standing target using a scanning white light interferometer that provided surface contour information, however the sensitive measurement for these shock breakout experiments was not step height per se but wall thickness change which can not be determined accurately by the single surface contour methods. There is also a real advantage to the experimentalist of having a full 3D record of the as-built target. Many times the shock breakout or x-ray burn though data from a highly experimental shot is not fully predicted by the design codes, so no one knows for certain whether they metrologized the target well enough to explain all the results. The ability to revisit the target and ask different dimensional questions after it was destroyed is a very powerful tool. This is an exciting new frontier in meso-metrology for fusion related physics targets. References 1. Sebring, R., Anderson, W., Bartos, J., Garcia, F., Randolph, B., Salazar, M. and Edwards, J. Non-Contact Optical Three Dimensional Liner Metrology, Proc. 28th IEEE International Conference on Plasma Science and The 13th IEEE International Pulsed Power Conference, Las Vegas, NV, June 17-22, 2001, (2001) 2. Kaplan, H., Laser Gauging Enters a Submicron World, Photonics Spectra, 31 (6), (1997). 3. Gasvik, K. J., Optical Metrology, 2nd Edition, John Wiley & Sons Ltd., Wes Sussex, England (1995). 4. Dainty, J. C., Current Trends in Optics, Academic Press Ltd., San Diego, CA (1994). Key words: NIF, metrology, confocal laser, contour method, wall thickness, meso-metrology, shock American Society for Precision Engineering 147