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. The research, which was published in the August 23 issue of Nature, details a new technique for using lasers to start the fusion reaction. The Japanese researchers are Ryosuke Kodama and colleagues at Osaka University, Japan. The British team comprises researchers from the CLRC Rutherford Appleton Laboratory (Peter Norreys) and Oxford University (Steven Rose), Imperial College, London (Bucker Dangor, Karl Krushelnick, and Matthew Zepf, who is now at Queens University, Belfast), and the University of York (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 of fuel 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 – the equivalent of 10 million atmospheres. This causes a rocket-like effect in that 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,” says Norreys, &/38220;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.” 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 related to 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 1-mm-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. According to Ryosuke Kodama, “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. The team will continue the research using new, higher-power lasers at Osaka University and the Rutherford Appleton Laboratory (RAL).
“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,” notes 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. “This was a very challenging project; we�ve never gold-plated anything as thick as 175 microns of gold onto a copper mandrel mould before,” says RAL engineer Matthew Beardsley. “The thickness of the wall at the tip of the cone is only 5 microns. This is a very demanding task that requires specialised 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.
“�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. “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 Norreys.
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.