Scientists at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) have successfully delivered more than 1 Megajoule of laser energy to a target in a few billionths of a second, demonstrating the target drive conditions required to achieve fusion ignition. This is about 30 times the energy ever delivered by any other group of lasers in the world.
'Breaking the Megajoule barrier brings us one step closer to fusion ignition at the National Ignition Facility, and shows the universe of opportunities made possible by one of the largest scientific and engineering challenges of our time,' said National Nuclear Security Administration (NNSA) administrator, Thomas D’Agostino. 'NIF is a critical component in our stockpile stewardship programme to maintain a safe, secure and effective nuclear deterrent without underground nuclear testing. This milestone is an example of how our nation's investment in nuclear security is producing benefits in other areas, from advances in energy technology to a better understanding of the universe.'
In order to demonstrate fusion, the energy that powers the sun and the stars, NIF focuses the energy of 192 powerful laser beams into a pencil-eraser-sized cylinder containing a tiny spherical target filled with deuterium and tritium, two isotopes of hydrogen. Inside the cylinder, the laser energy is converted to X-rays, which compress the fuel until it reaches temperatures of more than 200 million degrees Fahrenheit and pressures billions of times greater than Earth's atmospheric pressure. The rapid compression of the fuel capsule forces the hydrogen nuclei to fuse and release many times more energy than the laser energy that was required to initiate the reaction.
This experimental programme to achieve fusion ignition is known as the National Ignition Campaign sponsored by NNSA and is a partnership among LLNL, Los Alamos National Laboratory, the Laboratory for Laser Energetics, General Atomics, Sandia National Laboratories, as well as numerous other national laboratories and universities.
The NIF laser system began firing all 192-laser beams onto targets in June 2009. In order to characterise the X-ray drive achieved inside the target cylinders as the laser energy is ramped up, these first experiments were conducted at lower laser energies and on smaller targets than will be used for the ignition experiments. These targets used gas-filled capsules that act as substitutes for the fusion fuel capsules that will be used in the 2010 ignition campaign. The 1MJ shot represents the culmination of these experiments using an ignition-scale target for the first time.
These early tests have demonstrated that NIF's laser beams can be effectively delivered to the target and are capable of creating sufficient X-ray energy in the target cylinder to drive fuel implosion. The implosions achieved with the surrogate capsules have also been shown to have good symmetry that is adjustable through a variety of techniques. The next step is to move to ignition-like fuel capsules that require the fuel to be in a frozen hydrogen layer (at 425°F below zero) inside the fuel capsule. NIF is currently being made ready to begin experiments with ignition-like fuel capsules in the summer of 2010.
