Overcoming Obstacles to Fusion Energy
Winter 2003/2004
SAIC's Executive Science and Technology Council (ESTC) promotes high-quality technical work by presenting yearly awards for papers published in peer-reviewed journals. A recent EST Award winner discusses award-winning research in fusion.
In attempting to generate fusion energy, scientists mimic the sun, sending superheated nuclei of hydrogen isotopes crashing into each other to release prodigious amounts of energy. Unlike its fission cousin, fusion produces no chemical by-products or long-term radioactive waste. What's more, hydrogen nuclei are basically extracted from water - Earth's inexhaustible resource. If fusion can be successfully harnessed, it will satisfy the entire world's energy needs for millions of years with no carbon emissions, risk of global warming, or meltdown.
Unfortunately the scientific challenges of fusion appear to be as vast as its potential advantages. After almost 50 years, a solution continues to elude fusion physicists. SAIC's Nathan Metzler comments, "There is much work yet to be done, but we continually make strides toward practical generation of fusion energy."
Metzler has worked in the field of inertial confinement fusion (ICF) for nearly 30 years. In his award-winning research, he explores a new approach to overcoming one of the primary obstacles to successful ICF.
Inertial confinement fusion begins with a spherical pellet or target of frozen thermonuclear fuel that is only a few millimeters in diameter. Multiple laser beams simultaneously transmit a burst of energy directly on the target to heat and compress it, imploding the fuel at a velocity in excess of 300 kilometers per second! This process creates two different layers of fluid within the target: A hotter, lighter fluid on the exterior that pushes on a colder, denser fluid in the interior.
"Imagine the instability of a layer of cold water being supported by a layer of steam. The slightest vibration would of course set the fluids into turbulent motion to reverse the unstable situation," explains Metzler. "In ICF, we have a similar, yet even more dramatic hydrodynamic instability. Imperfections in laser illumination and target fabrication introduce perturbations, which are magnified by this instability. We must find ways to minimize perturbations, or the target will disintegrate rather than implode," warns Metzler.
In Metzler's previous research, he found that a target with a smoothly graded density profile was very effective in reducing laser perturbations. However, materials with a graded density profile are difficult to manufacture. With this research, Metzler suggests using a foam-plastic coating on the target and a short laser pulse, called a "shaping pulse," prior to the main set of lasers or "drive pulse." The shaping pulse sends a shock to the foam-plastic layer of the target, dynamically "shaping" its density to a smoothly graded profile. "The 'shaping' of the foam-plastic's density reduces laser perturbations, which can make the difference between success and failure," Metzler states.
Numerical simulation shows that Metzler's approach is two to three times more effective in smoothing laser perturbations than hard-plastic and foam-plastic targets without a shaping pulse. Now the next phase of research is underway; Metzler and his colleagues are preparing to test the method on a flat target to see whether fusion energy generation is indeed one step closer to fruition. Requiring less than one percent of the energy of the drive laser, the shaping laser promises to be a practical solution.
Metzler's research was sponsored by the U.S. Department of Energy through a contract for the Naval Research Laboratory (NRL). The paper, "Laser Imprint Reduction with a Short Shaping Laser Pulse Incident upon a Foam-Plastic Target," appeared in Physics of Plasmas.
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