US researchers say they have achieved a world first by generating more energy from fusion reactions than they put into the nuclear fuel, representing a small but crucial step along the road to harnessing inertial-confinement fusion power.
The ultimate goal – to produce more energy than the whole experiment consumes – remains a long way off, but the feat has raised hopes that firm progress is being made.
Researchers at the Lawrence Livermore National Laboratory (LLNL) in California, said ignition – the process of releasing fusion energy equal to or greater than the amount of energy used to confine the fuel – has long been considered the “holy grail” of inertial-confinement fusion science. A key step along the path to ignition is to have “fuel gains” greater than unity, where the energy generated through fusion reactions exceeds the amount of energy deposited into the fusion fuel.
Though ignition remains the ultimate goal, the milestone of achieving fuel gains greater than 1 has been reached for the first time ever on any facility, the LLNL said in a statement.
In a paper published in the 12 February 2014 online issue of the journal Nature, scientists at the laboratory detail a series of experiments on the National Ignition Facility (NIF), which show an order of magnitude improvement in yield performance over past experiments.
“What’s really exciting is that we are seeing a steadily increasing contribution to the yield coming from the boot-strapping process we call alpha-particle self-heating as we push the implosion a little harder each time,” said lead author Omar Hurricane.
Boot-strapping results when alpha particles – helium nuclei produced in the deuterium-tritium (DT) fusion process – deposit their energy in the DT fuel, rather than escaping. The alpha particles further heat the fuel, increasing the rate of fusion reactions, thus producing more alpha particles. This feedback process is the mechanism that leads to ignition.
As reported in Nature, the boot-strapping process has been demonstrated in a series of experiments in which the fusion yield has been systematically increased by more than a factor of 10 over previous approaches.
The experimental series was carefully designed to avoid breakup of the plastic shell that surrounds and confines the DT fuel as it is compressed by the simultaneous impact of 192 laser beams on its container.
Researchers had hypothesised that the breakup was the source of degraded fusion yields seen in previous experiments. By modifying the laser pulse used to compress the fuel, the instability that causes break-up was suppressed. The higher yields that were obtained affirmed the hypothesis, and demonstrated the onset of boot-strapping, the researchers said.
Researchers believe fusion energy has the potential to become a radical alternative power source, with zero carbon emissions during operation and minimal waste, but the technical difficulties in demonstrating fusion in the lab have so far proved overwhelming. While existing nuclear reactors generate energy by splitting atoms into lighter particles, fusion reactors combine light atomic nuclei into heavier particles.
In their experiments, researchers at the LLNL use a bank of 192 powerful lasers to crush a minuscule amount of fuel so hard and fast that it becomes hotter than the sun.
The lasers are fired into a gold capsule that holds a 2 mm-wide spherical pellet. The fuel is coated on the inside of this plastic pellet in a layer as thin as a human hair.
When the laser light enters the gold capsule, it makes the walls of the gold container emit x-rays, which heat the pellet and make it implode with extraordinary ferocity. The fuel, a mixture of hydrogen isotopes called tritium and deuterium, partially fuses under the intense conditions.