Fusion reaction

The gold container used in the experiments.

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. This milestone has been reached for the first time ever. In a paper published in the February 12 online issue of the journal Nature, scientists at Lawrence Livermore National Laboratory (LLNL), California, reported the results of a series of experiments at the National Ignition Facility (NIF) that showed 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 the 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 and thus producing more alpha particles. This feedback process is the mechanism that leads to ignition. The boot-strapping process was demonstrated in a series of experiments in which the fusion yield was systematically increased by more than a factor of 10 over previous approaches. The experimental series was carefully designed (the laser pulse used to compress the laser pulse was modified) to avoid the break-up of the plastic shell that confines the DT fuel as it is compressed. It was hypothesised that the break-up was the source of the degraded fusion yields observed in previous experiments. The higher yields that were obtained affirmed the hypothesis and demonstrated the onset of boot-strapping.

The method scientists used to optimise the ignition process involved hitting the inside of a cylindrical gold container with 192 lasers in order to produce X-rays. That radiation then heats and blasts away the outer layer of a spherical capsule suspended at the centre of the cylinder. The reaction force sends the remainder of this target inwards, compressing some 170 micrograms of frozen fuel at the centre.