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Laser scientists take step towards making fusion energy a reality


Embargoed until 1900 hrs BST
Wednesday 22 August 2001

Scientists from the UK and Japan may have taken us one step further to the reality of fusion energy with a new answer to an old problem. Research by Dr Ryosuke Kodama and his colleagues at Osaka University, Japan and the UK team published in Nature on Thursday 23 August details a new technique for using lasers to start the fusion reaction. The British team comprises researchers from the CLRC Rutherford Appleton Laboratory (RAL) (Dr Peter Norreys) and Oxford University (Professor Steven Rose), Imperial College, London (Bucker Dangor, Dr Karl Krushelnick and Dr Matthew Zepf who is now at Queens University, Belfast) and the University of York (Dr Roger Evans). We have provided the first demonstration that this new scheme of fast ignition can provide an efficient route to fusion energy says Peter Norreys.

Lasers are vital as, according to Peter Norreys Theres no other way of depositing such a vast amount of energy on such a small focal area. The laser beams are focused onto a hollow pellet and produce a plasma almost instantaneously. The combination of high temperature (10 million degrees centigrade) and high density where the laser energy is deposited (1/1000 the density of solid matter) means that the generated pressure on the outside of the pellet is enormous - equivalent to 10 million atmospheres. This causes a rocket-like effect - the shell implodes at high velocity and eventually compresses to super-high density.

In the conventional approach to laser-fusion, the spark to ignite the compressed matter is generated by the simultaneous collapse of a number of accurately timed shock waves, but this requires both precise implosion symmetry and very large drive energy. These can both be relaxed, in principle, in the fast ignition approach. Here a second ultra-intense, short duration laser pulse penetrates the now dense matter to start the fusion chain reaction. The problem is that if you have a ultra-intense laser beam propagating in a plasma then all sorts of instabilities can occur that deflect the laser beam informs Peter Norreys. The team found the answer by inserting a cone inside the pellet that allowed the second laser to pass through the inside. The cone design solves the problem of producing a stable channel that will remain empty long enough for the ignitor beam to travel through and deposit energy in the compressed matter. This is the central theme of the experiment - we are replacing a plasma physics problem - the laser beam instability, with a hydrodynamics problem - how the material behaves in the presence of a cone. Using nine laser beams to implode the pellet using the GEKKO XIII laser at Osaka University, the one millimetre high cone design held up to the rigors of the test as the temperature rapidly rises by approximately 1.4 million degrees centigrade.

The research also implies that less energy is needed than was previously thought which would bring down the cost of fusion power. As Ryosuke Kodama informs, a similar temperature can only be achieved with twice the long-pulse laser energy using the conventional approach. The next step is to increase the short-pulse laser energy level and hopefully see a related increase in temperature. Using new, higher power lasers at Osaka University and RAL the team will continue the research. At the moment the minimum energy conversion efficiency from laser to thermal energy is 20%. We want to see if this maintains itself as we go to higher energy levels when we will actually get ignition asserts a hopeful Ryosuke Kodama. Less energy means lower costs and the laser fusion ignition technique is already looking cheaper than using magnets for reaction control. Magnetic fusion needs a large plasma volume. To create a big plasma you need big money, but with laser fusion the plasma size is very small. So thats another way it might reducethecost, he adds.

A vital part of the new technique is the accurate production of the millimetre high gold cone with extremely smooth sides by engineers (such as Matthew Beardsley) in the mm-wave technology centre at RAL. Using their experience of making extremely small, high precision parts they happily accepted the challenge. We thought it would be very difficult because we had never gold-plated anything like a 175 microns thick layer onto a copper mandrel [the initial mould] before says Matthew Beardsley. The thickness of the wall at the tip of the cone is only 5 microns thick and required detailed instrumentation using a tool much sharper than a razor blade or hypodermic syringe to avoid damaging the soft gold material.

Work will continue at the new peta-Watt laser facilities at RAL and at Osaka University. The work has formed a close bond between the British and Japanese scientists. We started this work together, the first experiment was here at RAL and the second was at Osaka. The third stage will start at Osaka, but will also continue here in the UK says Peter Norreys.

Funding for the research came from the United Kingdoms Royal Society and the Engineering and Physical Sciences Research Council, the Japan Society for the Promotion of Science and the British Council.

Contact details

Mrs Jacky Hutchinson, CLRC Press Officer, Tel: 01235 44 6482, Mob: 0777 55 85 811 Email: j.hutchinson@rl.ac.uk

Dr Peter Norreys, RAL, Mob: 0776 807 1346, Email: p.a.norreys@rl.ac.uk Professor Ryosuke Kodama, Osaka University, Japan, Tel: +81-6-6879-8754, Email: ryo@ile.osaka-u.ac.jp

Dr Karl Krushelnick, Imperial College, Tel: 0207 594 7635, Email: kmkr@imperial.ac.uk

Bucker Dangor, Imperial College, Tel: 0207 594 7634, Email: A.E.Dangor@imperial.ac.uk

Dr Matthew Zepf, Queens University, Belfast, Tel: 028 9027 3123 Email: m.zepf@qub.ac.uk

Dr Roger Evans, University of York, Tel: (at RAL) 01235 44 6344, Email: r.g.evans@rl.ac.uk

Professor Steve Rose, University of Oxford, Tel: 01865 272251, Email: s.rose1@physics.ox.ac.uk

Matthew Beardsley, RAL, Tel: 01235 446562, Email: m.j.beardsley@rl.ac.uk

Useful websites/papers:

  • Experimental studies of the advanced fast ignitor scheme, Physics of Plasmas, Vol. 7, No. 9, Sept 2000, pages 3721-3727 (published by the American Institute of Physics).
  • The National Ignition Facility Project, www.llnl.gov/nif
  • Site covering the inertial confinement fusion concept, fusion.gat.com/icf/concept/

Additional Information

Fusion power has been the energy Holy Grail of science for fifty years or more, potentially offering power without any substantial waste products and efficient energy conversion. As the environmental consequences of our unrenewable energy sources mounts, fusion may offer our energy dependent society the answer.

Fusion is the reaction by which the sun produces energy through the fusing together of certain altered elements called isotopes. The substance preferred for a man-made fusion reactor is the two heavy isotopes of hydrogen: deuterium and tritium. The fusing of these two isotopes produces both helium and radiation in the form of a neutron and energy. The vast amounts of energy released can be harnessed and the radiation rapidly decays to become harmless. Scientists have been battling to reproduce the necessary sun-like conditions on earth with temperatures over 100 million degrees centigrade so that the isotopes fuse together. Additionally, the fuel becomes a plasma (a fourth state of matter where the electrons and nucleus become detached forming a soup like mix of particles) during the reaction and needs to be kept isolated from the enclosure surfaces. Research into controlling plasmas and starting the fusion process has brought two reaction systems to the forefront: magnets and lasers.

The laser implosion technique used by the team is novel and complicated, but the team has managed to pull it off. The fuel pellet is driven down outside of the cone and allowed to stagnate at the end, forming a ball. A multi-beam (nine in total) laser pulse providing 1.2kJ of energy in 1 nanosecond pulses is fired at the pellet to compress the material. Every action has an equal and opposite reaction so in this case the force of the explosion of the plasma on the outside of the pellet is followed by an equally dramatic implosion. The result is a fuel pellet with estimated densities of 50-70 gcm-3 over a core diameter of 40-45 micrometer. The cone design only marginally reduces the compression density when compared to a full spherical implosion and the team is confident that the results are compatible with achieving the higher densities required for controlled fusion energy gain.

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