Laser fusion tests overcome plasma hurdle

Posted by Emma Woollacott

The first laser fusion experiments at Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) have successfully overcome a hurdle that many thought would scupper the project.

Many experts believed that the plasma created by lasers would interrupt the process of fusion. But early tests show that this is much less of a problem than expected.

In inertial confinement fusion (ICF) experiments, the energy of 192 powerful laser beams is fired into a small cylinder called a hohlraum, which contains a tiny spherical target filled with deuterium and tritium, two isotopes of hydrogen.

Rocket-like compression of the fuel capsule forces the hydrogen nuclei to fuse, releasing many times more energy than the laser energy that sparked the reaction.

In the past, the interplay between the laser beams and the hot plasma in the fusion targets, known as laser-plasma interaction (LPI), has tended to scatter the laser beams and dissipate their energy.

But during a series of test shots using helium- and hydrogen-filled targets, NIF researchers have found they can use LPI effects to their advantage to adjust the energy distribution of the laser beams.

The experiments resulted in highly symmetrical compression of simulated fuel capsules – a requirement for fusion ignition.

Using LPI effects to tune ICF laser energy is “a very elegant way to do it,” said Siegfried Glenzer, NIF plasma physics group leader. “You can change the laser wavelengths and get the power where it’s needed without increasing the power of individual beams. This way you can make maximum use of all the available laser beam energy.”

The test shots successfully delivered sufficient energy to the hohlraum to reach the three million degrees Centigrade needed for fuel compression
 
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.

At the end of the tests, the NIF lasers set a world record by firing 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.

NIF’s next step is to move to ignition-like fuel capsules that require the fuel to be in a frozen hydrogen layer inside the fuel capsule. Tests should start this summer.

There's more information in Science Express, the online version of Science.