Amplifiers are NIF's crown joules
Amplifiers are to NIF, what the heart is to the human body; they pump energy into the system. NIF amplifiers increase the optical energy of each of the 192 laser beams from one joule to 20,000 joules. NIF is 40 times more powerful than its predecessor, Nova, due to its multipass amplifier architecture.
Because amplifiers are so critical to the system, and because researchers want to minimize NIF’s down time for maintenance and repairs, the amplifier was designed as a Line Replaceable Unit (LRU) that cleanly slides into its many specific locations in the NIF system. LRUs are self-contained packages that consist of replaceable components that can be assembled and tested off-line, in a clean room, and quickly installed or replaced in NIF as a unit, while preserving their highly clean and pre-aligned state. There are 3,500 amplifier LRUs in NIF.
The LRU is able to slide into place because of a specially created framework, or box, referred to as a Frame Assembly Unit (FAU). Earlier this summer, NIF began installing the first frame assembly units. By the end of July, Jacobs iron workers and Lab riggers had installed the first 12 main and power amplifier buses. This is a major step toward completing the NIF laser and achieving "first light" in 2004.
Making a Frame Assembly Unit
General Tool Co. (GTC) of Cincinnati, Ohio, manufactures the largest pieces of the amplifier bus, the FAU. There are 192 FAUs in NIF. These are assembled to form the structural backbone of the 48 amplifier buses. GTC assembles the bottom, top, side panels and posts into a unit that resembles a box kite, five-feet-square, eight-feet-high and weighing 1,700 pounds.
The FAU is then finish machined as a unit to achieve the required tolerances for LRU mounting points and sealing surfaces, and to obtain the required smooth uniform surfaces for cleaning. Each FAU is then disassembled into its components and cleaned to exacting standards. Once the components of a disassembled unit are cleaned, they are reassembled into their final form.
When the FAUs arrive at Astro Pak, they are as clean as a dinner plate, but not clean enough for NIF. So Astro Pak personnel precision clean the FAUs and the other amplifier bus parts in their clean room facility located just off-site and then assemble the parts into microbuses.
To assemble the amplifier buses, FAU microbuses are hung on 36-foot-long rails that are exact duplicates of the flat, straight, level rails on which the amplifier buses will hang 12-feet off the ground in NIF. The rails are aligned and pre-loaded to simulate the weight of the amplifier bus so the parts can be aligned precisely as they are assembled.
After the microbuses are aligned and bolted together, the flash lamp windows (which isolate the flash lamps from the expensive laser glass in the adjacent cavity) are installed, bottom covers are attached, and the amplifier bus is sealed to ensure cleanliness.
Then NIF assembly team conduct three tests on the amplifier bus. One test ensures that it is properly aligned with respect to its mounting points. A second test, for leaks, ensures the amplifier bus will contain the necessary cooling gases. And a third test is conducted to ensure that it is electrically isolated from everything else in the system.
When building a sports arena-size, high-tech tool that’s never been built before with cleanliness requirements similar to those achieved in the semiconductor industry, and tolerances that push standard fabrication technology, it is natural to anticipate challenges along the way.
Precision positioning and alignment was one challenge to overcome when building the special boxes that will hold the amplifier components. These amplifier boxes are the size of a small school bus and yet their parts within must be positioned to better than half a millimeter. The challenge can be illustrated by comparing this activity to locating the seats on a school bus properly to the precision of a grain of sand.
To meet this challenge, four years ago mechanical engineer Don Bartel figured out how to use low-power, off-the-shelf lasers to align the FAU components as they are being assembled in the clean room. This allows the assembly team to align the parts precisely, quickly and cheaply.
Another challenge facing the amplifier engineering team is managing the many vendor and supplier contracts to ensure that parts and supplies are manufactured properly and arrive on time. Some of the parts are off-the-shelf, while others require special manufacturing processes to meet NIF specifications.
Less than two years ago, Tom Kohut joined the NIF Project as the production manager of the Amplifier Assembly Area, a Class 100 clean room in the high bay of Bldg. 381. He hired and trained a team of 15 technicians to assemble the amplifier buses. Kohut is also responsible for scheduling delivery of the hundreds of parts required to assemble an amplifier bus.
During this entire process of assembly, transport and installation of amplifier buses, a key challenge is preserving the cleanliness of the inside surfaces. In the jargon of the trade, the surfaces are maintained at a Level 83 clean or better; no more than 900 particles bigger than five microns per square foot of flat surface. This is 2,000 times cleaner than the average coffee table that’s just been dusted.
The amplifier buses are maintained at the Level 83 cleanliness or better, throughout the assembly process because once they’re installed, they cannot be readily cleaned in place. These large amplifier buses are the cleanest objects of their size in the world.
Another challenge when making an amplifier bus is the limited set of materials that are able to maintain cleanliness and structural integrity and survive in the amplifier environment where "surface of the sun" temperatures occur when the flash lamps fire.
"The challenges get easier as time goes on and we assemble more amplifier buses," says Kohut.