Fusion power has been the energy Holy Grail 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 substances preferred for a manmade fusion reactor are 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.
Now, scientists from the UK and Japan may have taken us one step further toward 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 today details a new technique for using lasers to start the fusion reaction.
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 a 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.
The British team includes 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 Norreys, “There’s 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 an ultra-intense laser beam propagating in a plasma then all sorts of instabilities can occur that deflect the laser beam,” Norreys says.
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 millimeter high cone design held up to the rigors of the test as the temperature rapidly rose by approximately 1.4 million degrees centigrade.
The research implies that less energy is needed than was previously thought, which would bring down the cost of fusion power.
As Ryosuke Kodama declares, “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, one hopes, 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,” says 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 that’s another way it might reduce the cost,” he adds.
A vital part of the new technique is the accurate production of the millimeter-high gold cone with extremely smooth sides by engineers at RAL. Using their experience of making extremely small high precision parts, one of these engineers, Matthew Beardsley, and his colleagues happily accepted the challenge.
“This was a very challenging project; we’ve never gold-plated anything as thick as 175 microns of gold onto a copper mandrel mold before”, says Matthew. “The thickness of the wall at the tip of the cone is only 5 microns. This is a very demanding task that requires specialized tooling and machinery which are accurate at the micron level."
"We had to make and use a tool much sharper than a razor blade or hypodermic syringe to avoid damaging the soft gold material as it was being machined," he added. “Once the gold cone was machined on the copper mandrel, it was placed into nitric acid, which attacks and etches away the copper, leaving the tiny gold cone intact.”
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.
Funding for the research came from the United Kingdom’s Royal Society and the Engineering and Physical Sciences Research Council, the Japan Society for the Promotion of Science and the British Council.
(Reference: Nature, Thursday 23 August 2001. See also "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
The inertial confinement fusion concept
[Contact: Dr. Peter Norreys, Professor Ryosuke Kodama, Matthew Beardsley, Mrs. Jacky Hutchinson]