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Finally, in 1984 a project named Nova managed to produce 30 000joules, a miniscule amount when compared with the latest system,which is expected to provide 4-million joules of ultraviolet energy,possibly enough to create a tiny star with a positive power output.To reach this sort of power, the National Ignition Facility uses morethan 3 000 pieces of neodymium-doped phosphate amplifier glass toincrease the power of the laser beams. The amplifier glass slabs areenclosed in airtight containers within the laser bay.Each beam zooms through the power amplifier and, when it reachesthe main amplifier, a special optical switch called a plasma electrodePockels cell traps the light, forcing it to travel back and forth fourtimes through 11 sets of laser amplifier glass slabs before it exits themain cavity.High-power flashguns (similar to those used in an ordinary camera)but much, much larger are used to excite the lasers. When the beamstarts out it is about as strong as an infrared beam in a laser pointerbut when these are all aimed together and amplified they push outabout 500 terawatts of energy in two billionths of a second – morethan 500 times the entire peak power output of the United States.This can be achieved because the laboratory’s giant bank ofcapacitors store a reservoir of energy. However, the bank is dangerousand is placed in complete lock down state because of the risk of highvoltage arcing.Without this switch, the main laser building would have to be about228 metres longer than it actually is. The beams then make a final passthrough the power amplifier before speeding into the target chamberwhere deformable mirrors, each with 39 actuators, are used to shapethe beam’s wavefront and compensate for any flaws before it enters anarea known as the switchyard.In the switchyard, a complex system of special mirrors in two tenstoreysteel structures, rearrange the parallel, linear array of 192 laserbeams into a spherical configuration so that the beams can be focusedinto the centre of the target chamber.This switchyard is one of the stiffest structures ever built with wallsof just over a metre thick. Each beam passes through the final opticsassembly, where it is converted from infrared to ultraviolet wavelength,and is focused through the final focus lens onto the target in thechamber’s centre.From beginning to the end, the beams’ total energy grows from onebillionthof a joule to four million joules – a factor of more than aquadrillion – in less than two-millionths of a second.
Superconducting quadrupole electromagnetsare used to direct the beams to fourintersection points, where interactionsbetween accelerated protons will take place.The Seven Wonders of NIFThe National Ignition Facility is one of themost remarkable science and technologyfacilities in the world – alongside, perhaps,the Large Hadron Collider – and is due tostart a range of experiments next year. Themost important of these is to create nuclearfusion. To create the NIF, scientists, industrialpartners and the NIF itself had to overcomea daunting array of challenges. The sevenmost important achievements were:• Faster, inexpensive laser glass production.Laser glass is fundamental for the NIFlaser system as it amplifies the laser lightto the energies required for the scientificexperiments. The system uses phosphateglass that has a chemical additivecontaining atoms of neodymium. Thereare 3 070 large plates of laser glassthat, if stacked end-to-end, would forma continuous ribbon of glass about 2,5kilometres long. To produce the glass, theNIF joined forces with Hoya Corporationand Schott Glass Technologies anddeveloped a continuous productionmethod that melts and pours glass, whichis then cut and polished to specification.• Large aperture optical switches. A keyelement of the amplifier section is anoptical device called a plasma electrodePockels cell or PEPC. It contains a plateof potassium dihydrogen phosphateand, in conjunction with a polariser,acts as a switch, allowing laser beamsinto the amplifier and then rotating itspolarisation to trap the laser beamsin the amplifier section. A thin plasmaelectrode that is transparent to the laserwavelength allows the high electric fieldto be placed on the crystal, causingthe polarisation to rotate. The trappedlaser beams can then increase theirenergy more efficiently using multiplepasses back and forth through theenergised amplifier glass. After the laserbeam makes four passes through theamplifiers, the optical switch rotatestheir polarisation back to its normalconfiguration, letting them setoff alongtheir path to the target chamber.• Stable, high-gain amplifiers. The NIFuses 48 preamplifier modules, each ofwhich provides laser energy for four NIFbeams. The preamplifier modules receivea low energy (one billionth of a joule)pulse from the master oscillator roomand it amplifies the pulse by a factorof a million to one millijoule. It thenboosts the pulse again to a maximumof ten joules by passing the beam fourtimes through a flashlamp-pumped rodamplifier. The preamplifiers can performspatial shaping, spectral shaping andtemporal shaping of the input laserbeams.• Deformable mirrors are an adaptive opticsystem that uses an array of actuatorsto bend the surface of the mirror tocompensate for wavefront errors. Thereis a deformable mirror for each of the192 beams.• Large rapid-growth crystals serve twofunctions, the frequency conversion andthe polarisation rotation. Using specialtechniques the NIF is able to grow thecrystals in just two months and these aresufficiently large to allow plates to be cutfrom each crystal and fitted to the glass.• The millimetre-sized target for thelasers must meet extremely exactingspecifications with regard to density,concentricity and surface smoothness.Components are machined to an accuracyof one micrometre. To complicate matters,the extreme temperatures and pressuresencountered by the targets make themhighly susceptible to imperfection infabrication.• The NIF has installed one of the mostsophisticated computer controls systemsever assembled in government serviceor in the private sector. Every NIFexperimental laser shot means that60 000 control points for electronic,high voltage, optical and mechanicaldevices including motorised mirrors andlenses, energy and power sensors, videocameras, laser amplifiers, pulse powerand diagnostic instruments must all besynchronised, monitored and controlled.The computer control system will resultin the propagation of 192 separatenanosecond-long bursts of light over aone kilometre path length. Each one ofthe beams must have optical pathlengthsequal to within nine millimetres so thatthe pulses arrive within 30 picosecons(trillionths of a second) of each other atthe centre of the target chamber. Theymust strike to within 50 micrometreson a target measuring less than onecentimetre. To put this into perspective,tht like throwing a baseball pitch at abatter from 565 kilometres away.June 2009 35
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