New 'Super Clean' Laser
 |
|
High standards of cleanliness dictated that nickel-containing
S30400 stainless steel be used in the target chamber of the NIF laser.
|
|
 |
|
The US$2.5-billion laser will be 200 metres long and 85 metres wide.
|
|
 |
|
More than 11,500 tonnes of stainless steel have been used to date. The facility is designed to last at
least 30 years.
|
|
 |
|
In some places the beams travel through circular stainless steel tubes 25 to 30 centimetres in diameter; in others square stainless steel beam tube assemblies have been constructed to accommodate the 40-cm-square laser beams. |
|
|
|
|
|
|
Nickel stainless steel is a critical material in what will become the world's largest, most
powerful laser. By Steve Dawson
Nickel magazine, February 2003 -- A 192-beam laser system is being designed
at the Lawrence Livermore National Laboratory in Livermore, California, U.S.A. and, upon completion in
2008, will be installed at the nearby National Ignition Facility (NIF).
Designed to unravel the intricacies of fusion, the mammoth laser will also play a role in the United States Department of Energy's science-based Stockpile Stewardship Program. The mandate of the program is to assure the safety and reliability of the country's nuclear weapons. Experiments involving the NIF laser will advance our knowledge in the fields of astrophysics, hydrodynamics and material properties, while also advancing the pursuit of fusion as a possible source of energy.
Budgeted at US$2.5 billion, the laser will be about 200 metres long and 85 metres wide, roughly the size of a modern sports arena. More than 11,500 tonnes of nickel-containing stainless steel have already been used, primarily to support the target chamber. The laser's ultimate goal is to direct 1.8 million joules of ultraviolet laser energy in a few billionths of a second on to targets to study inertial fusion and high-energy-density physics.
In inertial fusion studies, NIF will direct its laser beams into a hollow gold cylinder called a hohlraum, situated in a 118,000-kilogram aluminum target chamber 10 metres in diameter. The hohlraum contains a fusion capsule 2 millimetres in diameter, filled with deuterium-tritium fuel. X-rays generated from the interaction of the intense laser beams with the inside walls of the hohlraum will rapidly heat the outer surface of the capsule to more than 1 million degrees Celsius, blowing off the outer surface. The resulting reaction drives the fusion fuel inward, compressing it and resulting in the ignition and fusion burn of the fuel, as well as liberating about 10 times more energy than what was required to initiate the reaction.
More than 1,000 scientists, engineers, designers and technicians have worked on the laser's construction,
which began in 1997.
The NIF laser system begins as a single laser pulse on the order of a nano-joule, which is then further
amplified and split into 192 ten-joule pulses. The pulses enter the main laser system, where each laser beam
is transported in a fully enclosed beam-path containing mirrors, lenses, amplifiers, switches and spatial
filters.
S30400 stainless steel, containing 8% nickel (minimum), plays an important role in maintaining
cleanliness inside the laser at Class 10, Level 80, or better.
Clean room Class is currently defined by International Organization for Standardization (ISO) Standards
14644. The numerical value of the Class refers to the number of particles within a specific size range
suspended in the air inside a clean room. Class 10 means that there are less than 10 particles greater
than 0.5 micrometre in size per cubic foot in the air and is comparable to the best clean rooms
used by the semiconductor industry. For comparison, the air in a typical office building might be Class
100,000.
The higher the airborne concentration of particles, the faster they will settle out onto surfaces. Level 80
is an indication of the number of particles on surfaces. Like the airborne concentration, the surface
cleanliness Level is defined as the number of particles per square foot on a surface with a particle size
exceeding a stated size. A Level 80 surface would have about 2,200 particles greater than 1
micrometer per square foot and is considerably cleaner than what can be seen with the unaided eye even
with excellent lighting conditions.
In its 450-metre journey, each laser light pulse reflects off the equivalent of 54 mirrors and goes through a
total of 2 metres of glass. Special, adaptive optic deformable mirrors are used to correct aberrations that
accumulate in the beam. Additional filtering of the beams is done using large vacuum telescopes that focus
the laser energy through narrow pinholes to remove high-frequency noise, resulting in additional filtering of
the beams.
Once the beams have completed their passes through the main laser system, they proceed to two switch-yards on
either side of the target chamber. The switch-yards take the 192 beams, which up to now have been traveling
in bundles of eight beams, four high and two across, and split them into quads of two-by-two arrays of beams.
The quads are directed into a radial, three-dimensional configuration around the spherical target chamber.
Just before entering the chamber, each quad of pulses passes through a final optics assembly, where the
pulses are converted from infrared to ultraviolet light and focused onto the target. The entire journey takes
1.5 microseconds.
Lou Bertolini, one of NIF's mechanical engineers, stresses that
S30400 stainless steel is a critical component in keeping the laser system free of contaminants.
"Cleanliness of NIF's optical components is crucial to prevent damage from the intense laser energy inside
each beam enclosure," he says.
Passivated stainless steel can be washed with an organic solvent or cleaned with very clean water and a
detergent. In addition, iron oxide, which could damage the laser glass, does not form on passivated
stainless steel.
Various configurations of stainless steel have been designed to protect the beam path and the optical
components. In some places the beams travel through circular stainless steel tubes 25 to 30 centimetres in
diameter; in others square stainless steel beam tube assemblies have been constructed to accommodate NIF's
40-cm-square laser beams. The largest enclosures, which are the size of railway boxcars, are used to protect
the spatial filters. Recently the beam path enclosure received Class 100 Clean Room certification, and when
the laser begins operations, these cleanliness levels are expected to improve significantly.
The first eight-beam bundle of lasers is to be commissioned in 2003, and additional eight-beam bundles will be installed every few months thereafter until the NIF system is fully operational with 192 lasers in 2008. Experiments in support of basic science and stockpile stewardship missions will be performed soon after the first laser beams are commissioned on NIF. Experiments can be carried out concurrently with NIF construction.
By the time NIF is completed, more than 1,000 experiments will have been carried out. The system is expected to perform between 750 and 1,000 experimental shots per year.
NIF and its various components, including stainless steel enclosures and structural support, is designed
to last 30 years.
Steve Dawson is a Toronto-based freelance science writer.
Photos: LAWRENCE LIVERMORE LABORATORY
 David Schwoegler Principal PIO Lawrence Livermore National Laboratory 7000 East Avenue Livermore, California U.S.A. 94550-9234 Tel: 1-925-422-6900 Fax: 1-925-424-2780 E-mail: newsguy@llnl.gov Website: www.llnl.gov |
