Scientists at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) have delivered a historic level of laser energy – more than 1 MJ – to a target in a few billionths of a second and demonstrated 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. 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 US at any given time. Laser bay 1 was commissioned in September 2008. It holds half of the NIF's 192 beams. (Images: Jacqueline McBride/LLNL) “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 program 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.” To demonstrate fusion, the energy that powers the sun and the stars, the NIF focuses the energy of 192 powerful laser beams into a pencil-eraser-size 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 ºF 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 program to achieve fusion ignition is known as the National Ignition Campaign and is sponsored by NNSA in partnership with LLNL, Los Alamos National Laboratory, the Laboratory for Laser Energetics, General Atomics, Sandia National Laboratories as well as numerous other national laboratories and universities. A metallic case called a hohlraum holds the fuel capsule for NIF experiments. The NIF laser system, the only megajoule laser system in the world, began firing all 192-laser beams onto targets in June 2009. To characterize 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 1 MJ shot represents the culmination of these experiments using an ignition-scale target for the first time. These early tests have demonstrated that the NIF's laser beams can be effectively delivered to the target and can create sufficient x-ray energy in the target cylinder to drive fuel implosion. The implosions achieved with the surrogate capsules also have been shown to have good symmetry that is adjustable through a variety of techniques. The next step is to move to ignitionlike fuel capsules that require the fuel to be in a frozen hydrogen layer (at 425 ºF below zero) inside the fuel capsule. “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.” NIF Meets Fusion Ignition Requirements The NIF conducted its first experiments, demonstrating a unique physics effect that bodes well for its success in generating a self-sustaining nuclear fusion reaction. The facility is the first expected to achieve fusion ignition and energy gain in a laboratory setting. Dante Diagnostic: Many instruments – detectors, oscilloscopes, interferometers, streak cameras and other diagnostics – surround the target chamber to measure the system's performance and record experimental results. By characterizing the x-rays generated during NIF experiments, including the latest laser-plasma interaction experiments, the Dante soft x-ray power diagnostic helps scientists understand how well the experiment performed. In inertial confinement fusion (ICF) experiments on the NIF, the energy of 192 powerful laser beams is fired into a pencil-eraser-size cylinder called a hohlraum, which contains a tiny spherical target filled with deuterium and tritium, two isotopes of hydrogen. Rocketlike compression of the fuel capsule forces the hydrogen nuclei to combine, or fuse, releasing many times more energy than the laser energy that was required to spark the reaction. Fusion energy is what powers the sun and stars. The interplay between the NIF’s high-energy laser beams and the hot plasma in NIF fusion targets, known as laser-plasma interactions, or LPI, has long been regarded as a major challenge in ICF research because of the tendency to scatter the laser beams and dissipate their energy. But during a series of test shots using helium- and hydrogen-filled targets last fall, NIF researchers used LPI effects to their advantage to adjust the energy distribution of the NIF’s laser beams. The experiments resulted in highly symmetrical compression of simulated fuel capsules – a requirement for the NIF to achieve its goal of fusion ignition and energy gain when ignition experiments begin later this year. This artist's rendering shows an NIF target pellet (the white ball) inside a hohlraum capsule, with laser beams entering through openings on either end. The beams compress and heat the target to the necessary conditions for nuclear fusion to occur. “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.” 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.” Glenzer, MacGowan and their NIF colleagues said that “self-generated plasma-optics gratings on either end of the hohlraum tune the laser power distribution in the hohlraum, producing symmetric x-ray drive.” Glenzer said the gratings act like tiny prisms, redirecting the energy of some of the laser beams just as a prism splits and redirects sunlight according to its wavelength. An NIF technician checks the target positioner, which precisely centers the target inside the target chamber before each experiment and serves as a reference to align the laser beams. Glenzer attributed the new LPI phenomenon to the size of the test hohlraums, which, although somewhat smaller than actual NIF ignition targets, are two to three times larger than hohlraums used in previous ICF experiments at other laser facilities. He said the increased amount of the high-temperature, low-density plasma in the areas where the laser beams enter the hohlraum was responsible for the spontaneous generation of the plasma gratings. The technique of slightly shifting the wavelength of some laser beams to control the transfer of energy between the beams and equalize the laser power distribution in the hohlraum had been predicted and modeled by NIF scientists using high-fidelity 3-D simulations. In last fall’s experiments, an initially asymmetric target implosion with a “pancake” shape was changed to a spherical shape by the wavelength-shifting technique, validating the modeling results. The first laser system experiments were conducted at lower laser energies and on smaller targets than will be used for ignition experiments. These targets used cryogenically cooled gas-filled capsules that act as substitutes for the fusion fuel capsules that will be used in the ignition campaign that begins this summer. Before the wavelength-shifting effects were tested, the only way to adjust the laser energy reaching the walls of the hohlraum, where it is converted into x-rays that heat and ablate the outer surface of the fuel capsule and cause the compression of the fuel inside the capsule, was to adjust the relative energy of the laser beams in the early stages of a shot, during preamplification. By taking advantage of the LPI effects in the target, as the beams crossed at the entrance of the hohlraums, the scientists could make use of minute wavelength adjustments, ranging from a fraction of an angstrom to a few angstroms (an angstrom is one ten-billionth of a meter, about the size of an atom). With the LPI scheme, “you can run every beam at maximum power and have another distribution mechanism to achieve symmetry,” Glenzer said. The test shots proved the NIF’s ability to deliver sufficient energy to the hohlraum to reach the radiation temperatures – more than 3 million ºC – needed to create the intense bath of x-rays that compress the fuel capsule. When NIF scientists extrapolate the results of the initial experiments to higher-energy shots on full-size 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. Since setting a world record of 1 MJ of laser energy, the NIF’s next step is to move to ignitionlike fuel capsules that require the fuel to be in a frozen hydrogen layer (at 425 ºF below zero) inside the fuel capsule. The NIF is currently being made ready to begin experiments with ignitionlike fuel capsules in the summer of 2010. For more information, visit: www.lasers.llnl.gov