The quest to create a controlled fusion reaction is underway at the Lawrence Livermore National Laboratory’s National Ignition Facility (NIF), with scientists reporting early progress ahead of ignition experiments which are due to start later this year. The ultimate aim of the world's largest laser - which is the size of three football fields - is to develop carbon-free, limitless fusion energy.
Inside the NIF, a beam of concentrated light charges up by bouncing back and forth over the distance of a mile and is then split into 192 beams which are concentrated on a tiny spot of deuterium and tritium (reactive isotopes of hydrogen that can be extracted from seawater) called a hohlraum.
When the laser is fired the fusion reaction will be more than 100 million degrees Celsius (hotter than the sun), and exert more than 100 billion atmospheres of pressure. The resulting fusion reaction will also release many times more energy than the laser energy required to initiate the reaction.
One of the key challenges for researchers in initial experiments is to overcome the tendency of the laser beams to scatter and dissipate their energy when they hit the hot plasma in the fusion targets. The researchers have now demonstrated control over these so-called laser-plasma interactions (LPI) to achieve highly symmetrical compression, an important step towards fusion ignition and energy gain.
“Laser-plasma interactions are an instability, and in many cases they can surprise you,” said ICF Program Director Brian MacGowan. “However, we showed in the experiments that we could use laser-plasma interactions to transfer energy and actually control symmetry in the hohlraum. Overall, we didn’t find any pathological problem with laser-plasma interactions that would prevent us generating a hohlraum suitable for ignition.”
When NIF scientists extrapolate the results of the initial experiments to higher-energy shots on full-sized hohlraums, “we feel we will be able to create the necessary hohlraum conditions to drive an implosion to ignition,” said Jeff Atherton, director of NIF experiments.
During these early experiments, the NIF lasers fired more than one megajoule of ultraviolet energy into a hohlraum – more than 30 times the energy previously delivered to a target by any laser system.
“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.”
Later this year the researchers will 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.
In addition to the quest for nuclear fusion, the NIF is used to ensure the reliability and safety of the U.S. nuclear weapons stockpile without live testing and will also be used to conduct astrophysics and basic science research.
The initial experiments are described in an article on Science Express.