In their quest to be the first to trigger a nuclear fusion reaction using lasers scientists at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) have delivered more than one megajoule of laser energy to a target. The peak power of the laser light, which was delivered within a few billionths of a second, was about 500 times that used by the United States at any given time and demonstrates the target drive conditions required to achieve fusion ignition.
The historic level of laser energy delivered is about 30 times the energy delivered by any other group of lasers in the world and was achieved by focusing the energy of 192 powerful laser beams into a pencil-eraser-sized cylinder containing a tiny spherical target called a hohlraum (pictured below). 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.
For the 1 MJ shot the target consisted of gas-filled capsules acting as substitutes for the fusion fuel capsules that are filled with deuterium and tritium, two isotopes of hydrogen, that will be used in the 2010 ignition campaign. The 1 MJ shot using an ignition-scale target for the first time represents the culmination of experiments that began in June 2009 when the NIF laser system first began firing all 192-laser beams onto targets.
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 degrees Fahrenheit 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.
“This accomplishment is a major milestone that demonstrates both the power and the reliability of NIF’s integrated laser system, the precision targets and the integration of the scientific diagnostics needed to begin ignition experiments,” said NIF Director Ed Moses. “NIF has shown that it can consistently deliver the energy required to conduct ignition experiments later this year.”
The NIF at LLNL is the world’s largest laser facility and the only one megajoule laser system in the world. It is expected to be the first facility to achieve fusion ignition and energy gain in a laboratory setting.