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Supporting the World's Largest Laser

 

Providing a production part for the National Ignition Facility changed the firm that manufactures it -- a Manufacturing Engineering exclusive.


            
By Ray Chalmers and Russ Olexa
Senior Editors
   

   

 

     

Manufacturing a precision production part is the result of a series of decisions. Manufacturing engineers, many times in a team process, have to decide how a firm can best make the part. Where does it begin, from a casting, forging, bar, or sheet? Is it machined, milled, turned, welded? How is it fixtured, what were the tooling decisions, how was it finished and assembled, and ultimately, did it make money? 

This article is the first in a series of exclusive stories by Manufacturing Engineering highlighting a specific part and the firm that makes it. Production technology will play a large role, but what makes these articles unique is the series of decisions made to produce a part, the people that make these decisions, and how the organization changes as a result.

The story of the frame assembly units (FAU) for the National Ignition Facility, supplied by General Tool Co. (Cincinnati), is just such an example. The National Ignition Facility, currently under construction at the Lawrence Livermore National Laboratory in Livermore, CA, is a US Department of Energy national center to study inertial confinement fusion and the physics of high energy and pressure (see sidebar). Ultimately, what will be the world's largest laser will ignite fusion fuel under controlled conditions, releasing more energy than it took to ignite it.

Frame assembly units are the enclosures housing the glass amplifiers making up the NIF laser line. A prototype FAU bus, consisting of a number of FAUs assembled into a block approximately 35' long and weighing 30,000 lb (13,500 kg), about the size of a school bus, has been built and successfully tested. Forty-eight such buses will make up the entire NIF project, scheduled for completion at the end of 2008.

Since its founding in 1947, General Tool Company (GTC) has been involved in a number of national-class projects, and has previous experience working with Livermore. As one of the largest contract manufacturers in the country, GTC has manufactured parts for the Trident Nuclear Submarine, the Space Shuttle, and the B-1 Bomber.

The story of the frame assembly units begins in March 1999, when GTC received a purchase order from Livermore to build 195 FAUs. Each FAU is shaped roughly like a telephone booth, two "slabs" wide and either two or three slabs long, containing either four or six bays for laser glass, respectively. Each unit consists of a top and bottom section machined from castings, and side units fabricated from extrusions and joined by a custom friction stir-welding machine GTC built just for this project. Livermore specified castings for all FAU components; fabricating and stir-welding the side panels returned more than $200,000 to the project budget, even after building the friction stir-welding machine.

Top and bottom castings have unique features pertaining to aligning and mounting the FAUs. Overall FAU buses are precision-aligned from individual units on rails and suspended from a structure in the National Ignition Facility. Flashlamps and lasing glass for each FAU are inserted from below by automated equipment.

Lawrence Livermore specified a 356-aluminum casting for the top and bottom of the cabinet. The top casting for a two-slab-long unit is approximately 9" (228.6 mm) overall height and the width is 50 X 60" (1270 X 1524 mm). Three-slab-long castings are machined to 50 X 90" (1270 X 2286 mm). These cabinets have either four or six bays for laser slabs, machined so the slabs are on Brewster's angles to the laser beam, meaning there is zero reflectivity of the laser light. Register holes for the upper and lower castings and in the side panels and posts have a true position tolerance of ±0.001" (0.025 mm). "We knew we couldn't hold that tight of a tolerance, and we took exception to it when we bid the contract," says Brian Bishop, GTC project manager. "We designed and built fixtures for the castings that generate holes assuring perfect alignment. This is achieved by drilling the castings and mating parts with the same fixture to guarantee holes are relative to each other." All top holes for mounting come off center-line and other datums confirmed by laser tracking.

FAU side panels are 24 X 24" (609 X 609 mm). and 36 X 24" (914 X 609 mm). These panels are about 6" (152 mm) deep at most, by 60" (1524 mm) long by 74.25" (1886 mm) overall height, but there are also 90" (2286 mm) long ones for the 36 x 24" (914 X 609 mm). Center and end posts that support the side panels are made from extrusions. "It was our choice on how to manufacture them," recalls Bishop. "We could build them from solid blocks, rectangles of aluminum, or we could have them cast or extruded." GTC had three different types of extrusions when it originally started the project. When it bid the job, GTC told Livermore it didn't want to make the side panels out of castings for several reasons. Not only are castings big and clumsy, but the wall was to finish at about 0.400" [10-mm] thick in the center. "With a casting you have 6" [152-mm] posts on the end and the center, while in between, the wall is only 0.400" [10-mm] thick, and you have an extra 0.375" [9.5-mm] envelope of stock over the entire part," explains Bishop. "We thought there had to be a way to make them by using a weldment. We believed we could weld plates together, because we were already getting extrusions for the center and end posts. Extrusions could be used for the angles on the ends and the T-shaped parts for the center of the cabinet. Plates could be placed in between and welded together. This proved to be a successful and cost-effective plan."   

Fixtures to machine the parts are designed from aluminum because it shrinks and expands along with the parts. Originally steel fixturing was going to be used, but GTC changed that before they ever made their first chip. Fixturing was done in-house to maintain the quality that GTC and Livermore needed. "We had to fine-tune the fixtures to get the tolerances needed for the process," Bishop says. "For the center and end-post extrusions, we had a couple of different ways we were going to fixture them. We came up with the idea to do it on a four-axis vertical machining center." A rectangular picture-frame-type fixture was developed with an open center. Three locators for the part to sit on were added, and fixed jaws on one side and vise jaws on the other side function to push the workpiece against the fixed jaws. "We try to catch these extrusions in the free state, because they will have some twist and deflection," Bishop explains. With three-point locating, the fixture keeps the parts in a free state as much as possible, also meaning that GTC can machine all around them. This fixture also can be flipped 90º to make various cuts, and its open structure allows GTC to slab-mill the post's entire length.

According to Tom Spoerl, senior manufacturing engineer at GTC, the company had to create vacuum fixtures to properly hold the side panels for machining. Engineers ended up using vacuum pumps with small, low-cost manifolds that stayed with the vacuum fixture. Operators simply supply air to the manifold and they are good to go. Since most of the machining is done on the inside panel walls, the company had to come up with a qualifying fixture that they would put the friction-stir-welded side panels on in order to machine stock off the flat backside. When they turned the panels around and put them on the vacuum fixture, they had to be positioned accurately.

To fixture the top and bottom castings, GTC uses a cabinet casting that was rejected during the casting process. "We took a casting that was about 8" [203-mm] thick and cut it down to about 2" [51 mm], and put it on 6" [152-mm] risers," says Spoerl. "We mount a rough casting on it, then the counterbored holes are drilled and tapped. We bolt right into where the mounting holes are going to be machined later on. After the part is machined, we flip it over and drill out the tapped holes to the counterbored configuration. We use strips of double-backed tape that are about 0.003" [0.08-mm] thick between the fixture and part that takes up any differences in surfaces and helps dampen vibration for the thin walls."

Ultimately, FAUs are suspended from a structure inside the NIF without lenses or flash units installed. Three points at the top are used to accurately position the hanging cabinet. Three docking blocks at the bottom provide precise positioning for the equipment that insert the slabs and flash lamps. Livermore has some capability to position the cabinet once it is hung, however, FAUs still have to be accurate in the free state. Because they are mounted flush together, turnbuckles pull each cabinet together with seal plates in between them. Once FAUs are mounted to the support structure, Livermore uses a special cart that loads the slabs and flash units up into the cabinets.

Livermore's structural analysis of the cabinets is based on suspending them from the top. "At GTC, we sit each cabinet on its bottom, which is one of its weakest spots," Spoerl explains. "We get some twist with them in the free state, and we also have a lot of trouble with the parts because they move under their own weight. If they aren't supported across their entire footprint they sag. The side panel, if it's hanging over a table about 12" [304 mm], can move about 0.030" (0.8 mm)." Overall weight for the entire assembly is 1783 lb (810 kg) for the small unit, and 2606 lb (1184 kg) for the three-slab unit.

"We've gotten some relief from Livermore on some of the tolerances due to the racking of the cabinets in the free state," Spoerl continues. The side panels and the three posts that go through the center all have to be within 0.005" (0.13 mm) of each other. Center post, end post, and side panel overall machined length is held to 0.002 - 0.003" (0.05 - 0.08 mm) range, "even though we are trying to get to 0.001" [0.025 mm] in the machining process," he says.

"At the time we started machining these parts, we had some new very high-shearing cutters in house from Germany that gave us very good surface finishes," Spoerl explains. They use carbide high-shear polished inserts with very low cutting pressures. Some standard OptiMills were used originally, but GTC had some problems with them pushing the material away and creating chatter. "We realized that by using the high-shear cutters, almost all of our chatter went away," he adds.

To do the walls of the cast parts, GTC uses a 2.5" (63.5-mm) diam carbide-insert end mill. "We have a tooling expert in house, Craig Rowlette, who helped us figure out how to work with these problems," Spoerl clarified. "We developed special form cutters that we grind with very unique shapes. We have special form cutters to produce certain angles to allow the cabinets to be properly positioned. These cutters are brazed carbide form-cutting tools."

Although the tools are designed in-house, an outside company makes the special cutters for the company. On the center and end post, there is a similar design that requires a brazed-carbide form cutter. GTC personnel made some changes to the form cutters they originally designed giving them more shear, less of a shallow rake, and higher positive rake.

When Livermore equipment loads the lenses up into the cabinets, there is a ball detent in the corner, and these ball detents drop into dimples that give a positive registration for the lens assembly. GTC uses a dual-angle head that has two spindles on it to make these dimples.

To eliminate some machining problems, GTC both roughs and finishes cast parts at the same time as they plunge down with a cutter through the casting. This allows them to have greater work support. It also eliminates a lot of chatter on subsequent finishing operations. All of the part-machining programs were done off line and loaded into the machine.

"In the very beginning, we left stock on the interior walls and we polished them out by hand, and we gaged for that," says Spoerl. "We knew we were going to take 0.005" [0.13 mm] per side off to get to a 63 RMS finish that Livermore needed. And we just allowed a little extra stock for this. A 63 RMS surface finish is required on some of the interior finishes of the cabinet. There are two small areas about two inches tall where the seal area is located that requires a 32 RMS."

To machine the parts, GTC had to use special coolants specified in the bid form. Such coolants were authorized and tested by Livermore, because certain types of oil-based coolants couldn't be properly cleaned from the parts. GTC ended up using a water-based coolant for its flood and high-pressure machine-tool systems.

Friction-stir welding became an important development in this project, both for the FAU part itself and for GTC. To reiterate, each FAU has two side panels, specified by Livermore as 356-T6 aluminum sand castings, machined all over. "With the budget for cast side panel production at roughly $2 million, General Tool saw an opportunity to produce these panels more economically by welding plates and extruded sections," says GTC engineer Jack Thompson. These weldments would then be machined to a finished thickness of 0.397" (10 mm) to complete each panel. Three-bay panels could be produced from the same set of materials as two-bay panels by adding a slightly larger plate and another T-shaped extrusion. Fabricating the side panels as opposed to casting them also presented significant ductility and surface cleanliness advantages, vital for their intended use.

       

       

Welding has a notable place in General Tool history. The company has long been associated with welding fixtures for many notable projects, including the external main tank for the Space Shuttle. It began by examining a number of established welding processes for welding the FAU side panels. These included MIG, TIG, and keyhole plasma. Contacts GTC had established with Lockheed Martin's Michoud Assembly facility (New Orleans, LA) with Space Shuttle work introduced the company to the concept of friction stir welding.

A friction-stir weld is formed by plunging a rotating shouldered pin tool with a length slightly less than the required weld depth into the butt-fitted faces of the materials to be joined until the tool shoulder contacts the work surfaces. Now within the workpiece, the rotating pin causes friction, heating the metal and producing a plasticized shaft of metal around the pin. As the pin moves in the welding direction, the leading face of the pin, due to its special profile, forces plasticized material to the back of the pin while applying a substantial forging force to consolidate the weld.       

       


"For the FAUs, we found friction stir welding to be in a class by itself in two critical areas: weld reliability and lack of distortion," Thompson says. If the selected welding process distorted the panels or required significant nondestructive testing and subsequent rework, this would eliminate any economic advantage to welding the panels.   

Next, GTC tried to find friction-stir-welding organizations for subcontracting this work, but only came up with research organizations or friction-stir-welding machine manufacturers, neither of which were motivated by contract work with a demanding schedule. "We considered modifying one of our large milling machines to do friction stir welding, but found it was too valuable in its present role," Thompson continues. "We carefully considered buying a machine from an established builder, but the cost and level of sophistication were simply beyond our means and requirements."

GTC engineers finally decided to build a friction-stir-welding machine themselves. Using as many off-the-shelf pre-engineered components as possible was the best way to meet project needs, they determined, for what was needed was not a general-purpose friction stir welder, but essentially an FAU fixture. GTC planned to build it specifically with a work envelope and workholding table designed to fixture the plates and extrusions for a complete 2 X 2 or 3 X 2 side panel. The table would be manually indexed under the weld head and locked into position for each weld by shot pins. This workholding table was envisioned to be a slightly modified copy of a vacuum holddown fixture already designed and built for machining the side panels.

Next the company developed a design statement of what the machine would and would not be; covering five main areas: spindle, structure, feed system, control system, and workholding.   

Since a friction stir welder's spindle and toolholder carry significant axial and radial forces, the objective was to select rolling element bearings that would last through 900 74" (1.9-m) welds without failure. The spindle needs no precision in the machine-tool sense; anything less than 0.003" (0.0762 mm) TIR would be fine, but a belt drive to maximize flexibility in providing the required spindle speed and horsepower without designing new parts would be essential. Engineers at GTC sought to have the spindle provided as a complete assembly, handling 900 rpm at the spindle at 60 Hz drive frequency, an 1800-rpm motor, and 2:1 drive ratio. GTC would provide a toolholder with minimum thermal conductivity and simple bolt circle attachment to the spindle nose.

For the machine structure, the company needed rigidity in space for the weld spindle in two rotational degrees of freedom perpendicular to the spindle rotational axis, and one linear degree of freedom perpendicular to the machine axis and feed direction. Vertical forces applied to the weld tool should cause zero deflection in the weld tool guided path or axis orientation relative to the work.

The only precision requirements were that the welds start and stop within a 0.050" (1.27 mm) window of repeatability, and that speed be regulated to within about 5%. Maximum feed speed would be less than 20 ipm (508 mm/min) and the weld feed return should take less than two minutes. The machine could stir-weld with one preset feed speed for each axis, and a rapid return speed only would make the machine more productive.

As to controls, a PLC would be adequate for starting and stopping the feed systems with weld program logic and fixed limit switches. A PLC also could command different preset speeds for different motions in a weld program. Speeds, time delays, and range of motion limits were envisioned as the only variables in the weld cycle, eliminating any need for custom interface panels or wiring.

As for workholding, the existing vacuum fixture design with a few additions appeared adequate to clamp the plates and extrusions for welding. Elastomeric seals would need to be specified and placed to endure both transient and long-term temperatures the weldment attains, and they would need to be placed out of the extrusion footprint of the weld tool. Shot-pin holes needed to be placed in a convenient location to lock the work in position under the weld head for each weld. And a simple system of low-friction support and guidance had to allow the table to be manually indexed from one weld position to another, as well as indexing the table out from under the machine for loading and unloading the panels.

This design process yielded a unique machine, one that was constrained by schedule and budget, made conservative by engineering uncertainty, and made robust by confidence and experience, say GTC personnel. "We wanted to buy a spindle assembly, but found spindle manufacturers are too accustomed to runout requirements in the millionths of inches to relax such requirements to supply heads for friction stir welding," says Thomspon. "Though we were quoted $25,000 and 16 weeks of lead time by well-known firms, we built our own for less than a third of that amount in about half the time. Off-the-shelf large, tapered roller bearings met our needs nicely, and heavy, straightforward spindle shafts and housings actually lessened the amount of down force the machine has to supply. A motor with variable-speed drive lets us match motor characteristics with the process, and our toolholder effectively minimizes heat transfer out of the tool and into the spindle through passive thermal design."

The machine's structure provides separate structural paths for the considerable vertical force applied to the machine head. The weld cycle in production applies 11,000 lbf (49,000 N) to the weld spindle block. Any force not reacting at the tool reacts at the pressure rollers. To assure both side-panel plates are equally loaded regardless of their thickness, pressure rollers are mounted in a pivoting truck. Linear ball bearings guide the carriage in the weld-feed direction. The weld spindle block is adjustable by pins and slotted bolt holes in one-degree increments from 0º (perpendicular to the work) to 5º, with a nominal setting at 2.5º.

For feeding, 1.5" (38.1-mm) diam power screw jacks impose horizontal weld feed and vertical tool plunge motions, and variable-frequency AC induction motors drive both axes. The feed-axis drive is direct-coupled, always moving directly by rotation of the drive screw. The vertical-plunge axis is slightly different. The vertical screw nut is axially coupled to the weld head as long as it supports the weld head. When the head is lowered into the work, the screw lowers it until the tool pin contacts the work, then the nut progresses away from the weld head. This leaves a gap slightly greater than the material thickness, de-coupling the head from the screw. The head is then forced into the work with the pressure rollers regulating tool plunge depth as the head moves along the weld. The controlled force is supplied by a constant-force air-actuation system. This approach completely separates regulating tool-plunge distance from machine-frame deflections. It also eliminates any need for dual-mode force-displacement servo systems to map machine-structure force-deflection characteristics.

All weldment components are placed on a flat fixture before welding begins. Each plate and extrusion is placed over a perimeter seal, and a vacuum is drawn between the workholding table and the work. All plates are clamped together with machine vises perpendicular to the faying surfaces to be welded. Pressure rollers also provide a high clamp force moving along the weld on either side of the tool. Tapered-nose shot pins lock the weld table in the proper position under the travel of the weld tool. Silicone rubber, since it can endure the welding temperature range for extended periods, was used for the vacuum seals.

Results have been outstanding. "We began designing our machine in July 1999 and produced our first stir-welded side panel on January 8, 2000," Thompson explains. Weld integrity and consistency has been excellent; transverse "butterfly" distortion is only about 0.050" [1.2 mm] in 74" [1.9 m] in the longitudinal direction. One operator can load the weld tool in about 30 minutes using an overhead crane. Each weld takes 10 minutes, including returning the head to the start position.

With minimal weld distortion, straightness and flatness of the plates and extrusions we start with becomes more significant. GTC found that tapered plugs of 6061-T6 can be hammered into the pinhole of any weld stopped short and stirred over to produce sound welds. In addition, 0.032" (0.81-mm) shims of 6061-T6 have been used successfully between faying surfaces as an aid to dimensional control of the finished weldment, and they also stir into the parent material, producing sound welds.

Producing FAU side panels through friction stir welding as opposed to machining castings will save this job approximately $500,000, calculated after paying for the design and manufacture of the friction stir welding machine and the requisite licensing fees. Moreover, GTC actively looks to provide custom stir welding services as a natural extension of this project.

In the final cleaning of the FAU, leaving no residue on the part is critical. Even after cleaning, there is some coolant left behind on the part. No oils could be used because they imbed into the aluminum and eventually leach out. GTC also had to be vigilant to keep dripping oil, such as from an overhead crane, off the parts. Flashlamps in the cabinet are so intense when they fire that any oil or coolant residue on the metal will sinter from the heat. Sintered material will become airborne and could land on the slabs. When the laser beam hits the sintered material, it explodes, pitting and ruining the glass.

"For in-process inspection, we made some gages that are basically aluminum bars," Spoerl explains. "One has a foot on it, and the other has a dial indicator. Pocket length and width are measured with gages, and the bulk of our inspection is done after machining. One of the things our inspector does before the cabinet goes together is pull all the dimensions on a Mueller gage on all details, and make sure there is less than a 0.005" [0.013 mm] range across all of them before he buttons them together."

To measure the entire cabinet for quality control, a laser CMM with an accuracy of 0.001" (0.025 mm) in 10' (3 m) is used. This allows the inspectors to get inside the part where other types of measuring devices can't go.

Before the assembly process, all FAU components are deburred. Average deburring time per unit is about 200 hours. "During the assembly process, we have dummy hardware that we use," Spoerl notes. "This is what we call a dirty assembly, prior final machining, an acid bath, and cleaning. We machine the outside by first putting it in a standup fixture to machine the four sides, then in a laydown fixture to machine the ends. Then it goes back over to the assembly area, where we completely disassembly by removing the dummy hardware." Final machining brings the overall height, width, and length of the cabinet to size, taking about a 0.063" [1.6 mm] of metal off. This machining is done on a Union TC130 horizontal boring machine with a rotating bed B axis GTC brought over from another one of its facilities.

Next, the cabinet goes through a 16-step cleaning process. First, an acid cleaning is done to the part, then it's soaped down with a special soap called Brulin (recommended by Livermore), washed, and rinsed with deionized water. "We do a gross cleaning to the cabinet, then Livermore does a final cleaning," says Spoerl. "First we rinse and wash the parts, then its on to the acid dip, then back out and washed with soap again. We then do a number of deionized water rinses, then the parts are ready for the clean room, where the clean parts are assembled."   

GTC had to build a Class 10,000 clean room to handle the parts. The company had no experience with clean rooms when it started this job, and worked the price of providing a clean room into the project budget. The clean room is approximately 150 ft2 (45.7 m2). There are four small assembly stations inside with a 15' (4.5 m) overhead crane capability.

"Some of the things we learned during the cleaning process is that when you have an absolutely clean part, everything fits together completely differently compared to when it was dirty," says Spoerl. "The press fit on pins, and the installation and staking of pins is entirely different. When the part is assembled dirty, it has some lubrication, but after cleaning, there is no lubrication, which affects how the part goes back together. Bolts would very easily gall on us, and seize on the inserts. Pins would seize in the part before it going to the bottom of a hole. We've learned over time we had to change some of our part fits, some of our tolerances and processes as we went along, to make sure we can actually put the part together. We use a little alcohol at times for lubrication but that's about it. We never had any exposure to this type of problem before."

In the cabinet, outgassing can plate out over the inside of the cabinet and become ash that can contaminate the surface of the lenses and cause them to fail. Livermore purges the entire laser of any gas and then pressurizes it to keep dust out. They want to control temperature, because when the flashbulbs go off, the cabinets get hot. Slots had to be added to the cabinet for proper airflow and to allow purging. To get certain slots inside the cabinet, GTC had to use a right-angle head cutter with an extension to get deep inside the cabinet and put a radius on the slots. The slots need radii because the airflow is calculated at a given volume.

Spoerl describes the process: "We started out by treating each cabinet component individually. We ran a prototype unit first by making one piece of each part and putting it together to complete one small cabinet assembly. We shipped it to Livermore, and they took a good look at it. They also cleaned it and made sure everything we did up to that point would produce a clean cabinet. It all went together fairly well. We knew it wouldn't be a production-type part. It was going to be a trainer for them. We learned a lot from the first part such as how much the aluminum would move on us, because we had quite a bit of movement, especially on the side panels, mainly due to machining them from a solid instead of an extrusion."

GTC did several straightening operations, and even cryogenically froze a set of panels to help stabilize the material. Freezing offered some modest improvements in the material characteristics, probably about the same produced by heat treating, but the process was not as intrusive to the material as heating. Thermal stress relief on this material would probably require a special fixture. Even with this treatment, GTC still experienced metal movement during machining. "As it turned out," Spoerl further explains, "straightening the panels after friction-stir welding gave us a stable part to machine."

Livermore personnel helped GTC resolve such problems as replenishing the clean room acid tank. Livermore said it just needed replenishing twice a year, but in reality, it needed replenishing every month. GTC added 18 extra bath changes, which was about 700 gallons (2653 L) of acid, so GTC had to resubmit a cost increase to Livermore to cover the cost of extra chemicals.

Today, GTC is quite a different company from when they began designing the production processes for these parts. The company has added a clean room, friction stir welding, and gained much experience in dealing with thermal movement of aluminum, including cryogenic freezing.

  

The National Ignition Facility      

Currently under construction in Livermore, CA, the National Ignition Facility (NIF) project (total estimated construction cost: $2.2 billion) will comprise the world's most energetic laser system, a total of 192 laser beams all aimed at a target the size of a BB-gun pellet. Scheduled for first light in 2003, with completion at the end of 2008, NIF experiments will allow scientists to study physical processes at extremely high temperatures (100,000,000ºC) and 100 million times atmospheric pressure. Such conditions exist only in the interior of stars and in nuclear weapons explosions.

All this power will ignite a small fusion target and liberate more energy than is required to initiate the fusion reaction, a process known as inertial confinement fusion. A plastic sphere about 2 mm in diam contains fusion fuel (deuterium and tritium) that will be compressed and heated ("driven," in NIF language) either directly or indirectly by the 192 lasers. When driven indirectly, the fuel capsule is contained in a hollow cylinder a few millimeters wide and about twice as long. The laser beams enter through the cylinder ends, strike the cylinder walls, and create X-rays that crush and heat the fuel pellet. When the fuel is driven directly, the lasers strike the fuel capsule itself, compressing and heating it. In either process, the fuel capsule's surface blows off like a rocket, causing a smooth inward implosion on the fusion fuel. Pressure can reach 100 million times the earth's atmosphere at 100,000,000ºC, igniting the fusion fuel. All this occurs in a few billionths of a second.

Achieving fusion ignition will help NIF establish a scientific and technical framework for generating electrical power through inertial confinement fusion. Such power production on a large scale would be substantially more benign than burning coal or using uranium. Fusion fuel could be "mined" from water, inertial confinement fusion produces no greenhouse gases, and the radioactive waste is projected to be a thousand times less than current fission-power plants, with a shorter half-life.

Since the NIF will be the world's largest laser, other technical goals include advancing other US high-technology industries such as optics, semiconductors, and high-precision manufacturing.       

The laser system is the heart of the facility. Together, the 192 laser beams will produce 1.8 million joules, approximately 500 trillion watts of energy, for three billionths of a second. Each beam is optically independent, providing flexibility for design of NIF experiments.

The frame assembly units (FAU) that General Tool provides will be assembled into a "bus" approximately 35' (10.7-m) long, weighing 30,000 lb (13,500 kg). A total of 48 FAU buses will be included in the final project. Each will contain seven or 11 amplifier modules, consisting of powerful flashlamps and slabs of laser glass.

The prototype FAU bus was successfully tested in June 2000. In addition to holding a precise alignment, each FAU bus had to be designed to work with robotic transport vehicles that will move this equipment. "Testing the prototype FAU bus required testing its alignment, the performance of the assembly equipment, and the safety and repeatability of the installation process," says Buzz Pedrotti, lead NIF engineer for amplifier mechanical systems. "We packed the unit up and drove it around to simulate the transport route the rest of the units will undergo. After the transportation test, it was still aligned."

In September 2000, the NIF assembly facility was certified as a Class 100 clean room, and production FAU bus assembly began. By July 2001, the first 12 buses were installed in the NIF facility, and production continues at approximately two buses per month. Learn more about the project a the NIF home page, www.llnl.gov/nif/nif.html.

 

This article was first published in the March 2002 edition of Manufacturing Engineering magazine. 


Published Date : 3/1/2002

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