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FEATURE LASER FUSION


Seen from above, each of NIF’s two identical laser bays has two clusters of 48 beamlines, one on either side of the utility spine running down the middle of the bay


more efficient, but may also be harder to engineer. Other planned facilities such as Dipole (see box overleaf) were designed initially to use a process called fast ignition that requires much lower powered lasers. The casing is heated the same but with lower energy lasers before a very high- powered petawatt laser delivers a single burst of energy to one side. This, however, is yet to be put into practice.


The laser system at NIF is actually based on quite an old technology; the amplification is achieved by passing the light through neodymium doped glass that has been excited by an array of flash lamps. The glass lases at a wavelength of around 1µm.


Only a certain


amount of added energy can be gained each time the laser travels through the glass


Only a certain amount of added energy can be gained each time the laser travels through the glass. The amount of energy per unit area that can be gained from a solid-state material, like glass, is around 10 joules per square centimetre. ‘You can get a few joules of energy out of a rod but its gain is only about a factor of 10,’ Smith explained. Increasing the efficiency is difficult: ‘The saturation fluence (energy per unit area) is a fundamental atomic property of the


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material decided by lasing and transition rates. This is easy to make worse but hard to make better,’ he said. ‘This immediately outlines a problem with laser fusion: if you can get 10 joules per square centimetre for a laser and you want a megajoule or more, you are going to need a lot of area,’ explained Smith. In order to improve the efficiency of the process the laser is passed through the glass multiple times in a process called regenerative amplification. A relatively small rod is pumped with light from the flash lamps and by passing the laser through the rod multiple times, meaning more energy can be extracted. Once the beam has increased in


energy it is passed to a slightly bigger rod where the process is repeated, and repeated again in larger and larger rods until the beam is pumped into giant slabs of glass. However, the glass gain medium collects a great deal of energy during this process and as the size of the glass and the amount of energy passing through it increases, problems are caused for the engineers. They need to avoid thermal lens


focusing which occurs when the glass gets too hot causing the refractive index to change. This will focus the laser prematurely and cause large amounts of very expensive damage to the system. To combat this they use glass slabs because it allows them to insert coolants between the surfaces and reduce the temperature to a manageable level. The system also requires a few hours to cool down in between shots. Once the laser has reached an acceptable level


it is frequency doubled, or sometimes tripled, to reach the correct wavelength for laser fusion. The frequency manipulators used at NIF are made using potassium dihydrogen phosphate (KDP) crystals provided by Gooch and Housego and are the largest of their type to be grown. The laser is then diverted and aimed through the holes at the top and bottom of the hohlraum where, hopefully, fusion occurs and ignites a sustained set of reactions that produce more energy than was initially put in. However, these are proof of concept experiments with most of the interest being dependent on the nuclear weapon and nuclear safety aspect of successful fusion. The question has to be asked: ‘How do we get energy out of this?’ In the UK, the people that are asking this are the Inertial Fusion Energy Network (IFEN), headed by Smith. They are a group of experts that are trying


FEBRUARY 2014 l ELECTRO OPTICS 23





Lawrence Livermore National Laboratory


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