This page contains a Flash digital edition of a book.
FEATURE LASER FUSION


Making energy out surpass energy in


Laser inertial confinement fusion has the potential to solve much of the world’s energy needs, once it can generate more energy than is put in. Tom Eddershaw investigates the current state of laser fusion projects and asks how the research can benefit industry


L


ast October the Lawrence Livermore National Laboratory’s National Ignition Facility (NIF), which houses the largest laser in the world, achieved an important


step towards the commercialisation of fusion energy – for the first time a fuel capsule gave off more energy than was absorbed by the fuel. This isn’t quite the goal of ‘ignition’, whereby the fusion reaction generates as much energy as is put in to the whole system, but it’s getting close. By the end of 2014, a very similar facility, the Laser Mégajoule (LMJ), will start experiments at the Commissariat à l’Énergie Atomique’s Cesta Centre, near Bordeaux, France, although its prototype Ligne d’intégration laser (LIL) has been open to the scientific community since 2005. Gareth Jones, CEO of Gooch and Housego, and Professor Roland Smith, head of Plasma Physics at Imperial College London, both agree that once the proof-of- concept has been achieved by one or other of these projects, Laser Inertial Confinement Fusion (ICF) which is the technique used by both facilities, will be all over the front pages and become of critical importance to both governments and to industry. A large part of the driving force behind these


research facilities has been the Comprehensive Nuclear Test-Ban treaty which was adopted in 1996 by the United Nations General Assembly. It means that no nuclear weapons tests can be carried out by any state that has signed the treaty, so nuclear nations have had to find alternative methods of testing fusion reactions. This has led to


22 ELECTRO OPTICS l FEBRUARY 2014 A NIF target contains a polished capsule about 2mm in diameter, filled with cryogenic (super-cooled) hydrogen fuel


facilities being commissioned that are supported by governments which aim to better understand fusion reactions and to be able to create them in safe, manageable environments. Nuclear weapon development is not held in too high regard by large parts of the public, meaning that spending taxpayer’s money on the large facilities required to increase a weapon’s effectiveness is not always popular. NIF is quoted to have cost $3.54 billion and took 15 years to build and was started just a year after the test ban, but the advances that could be made here could be used for more than weapons of mass destruction. The most commercially valuable application is in the energy sector where the market constantly asks for cleaner, less expensive and safer methods of producing electricity.


Maximising the energy out


The basic ICF method employed at facilities such as NIF is to take a large amount of energy and heat a pellet, also known as a target, of deuterium and tritium to similar temperatures to that of the core of the Sun in order to get energy out. The aim is to eventually generate more energy from the process than the energy put in. The fuel is readily available and abundant: sea water provides the deuterium, and the tritium can


be produced from the by-products of the fusion reaction. NIF bases its laser inertial confinement fusion on a process called indirect drive, whereby heat is generated from a fuel pellet imploding. The 192 laser beams at the NIF don’t heat the pellet directly, but focus their energy onto a hohlraum – essentially a housing made of gold or uranium holding the pellet. This generates soft X-rays which break down the coating of the fuel pellet in such a way as to cause the pellet to implode and ignite the deuterium-tritium fuel core to release energy. The smooth and even expansion of the casing is crucial to ensure that the pellet compresses uniformly, as Dr Smith explained: ‘Imagine squeezing a ball of jelly in your hands – it doesn’t squeeze elegantly and goes very unstable, things get messy.’ This uneven ‘squeezing’ means the whole process becomes very inefficient at heating the fuel and almost definitely results in a failed attempt at getting the elements to fuse. Indirect drive is less than one per cent efficient and so very energetic lasers are required. Here, a single relatively low laser pulse is split and directed to separate laser bays where they are pre-amplified and then split again until 192 separate parallel beams are created. These beams are sent to the main amplifier units. Direct drive, whereby the pellet is heated directly by a laser, is potentially


@electrooptics | www.electrooptics.com


Lawrence Livermore National Laboratory


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44