Physics, P-DO
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Understanding Mix in Inertial-Confinement Fusion
G.A. Kyrala, C.R. Christensen (P-24), M.A. Gunderson, D.A. Haynes, D.C. Wilson (X-2) IntroductionFrom the distant twinkle of the stars to the glowering fire of New Mexico at midday, fusion powers the universe. The Plasma Physics group (P-24) is at the forefront of the quest to capture this clean, safe, inexhaustible energy source here on earth.
Mixed Impurities Reduce Nuclear YieldOne unsolved difficulty standing in the way of achieving fusion power is the problem of mix. During ICF implosions, internal temperatures can be around 140 million degrees Celsius, which is hotter than the core of the sun. The density can be 10,000 times what it would be at atmospheric pressure. Under these conditions, the material that forms the shell of the capsule (as well as the hydrogen fuel) is stripped of its electrons and becomes a highly reactive state of matter called a plasma. If this material mixes into the fuel, it interferes with fusion for several reasons. The heavier atoms of the shell are copious radiators, like a miniature version of the carbon arc searchlights used to light up the sky at grand openings. When heated, the heavier elements release many electrons, increasing the particle density and contributing to the pressure of the hot, dense plasma resisting the force of the lasers. Thus, a capsule with a lot of mix cannot be squeezed down to the high fuel density and temperature that a clean capsule could. In addition, mix dilutes the fuel, so that within the 100 trillionths of a second or so that the burn takes place, fewer of the interparticle collisions are between the fuel nuclei, so the burn gives less yield. For these reasons (i.e., radiating away the energy needed for heating, interfering with compression, and diluting fuel), mix diminishes the amount of energy we get from ICF implosions. To obtain a practical energy source to replace fossil fuels, the fusion reaction must produce enough energy beyond what is required to power the lasers used to implode the fuel capsules so that the scheme will be economically attractive. Mix diminishes energy output, so it’s an important part of the economic equation.
Thus, while it is probably not possible to eliminate mix entirely, it is crucial to understand it. This understanding will enable us to mitigate the problem as much as we can and also to direct future research based on correct predictions. P-24 Implosion Experiments Capture Images of MixRecent experiments performed by P-24 with collaborators from Applied Physics Division use a thin layer of titanium on the inside surface of a capsule (Figure 1) to image mix in the x-ray region. Titanium (22 electrons per atom) is used as the tracer element because it typically keeps at least one electron throughout the implosion, even at the core. Therefore, it still radiates efficiently at these high temperatures. The distribution of different “colors” of x-ray emission depends on the density and temperature in whatever local vicinity the titanium atoms find themselves. Even if we cannot image the individual eddies, our results can still be used to reject models that do not match the energy distribution of the titanium emission versus radius and time.
The time-resolved emission spectrum shown in Figure 4 is produced by a flat crystal located in front of a streak camera. From top to bottom, the radiation is separated by energy, or “color.” This is similar to the bands of colored light seen on a compact disc as it is tilted away from a light source. In our experiments, the different “colors” of light represent actual x-rays of different energies; therefore, a crystal is used to spread out the different energies. From left to right, the image is swept in time. The camera puts the information onto an electron beam, which makes a picture from left to right in the same way that a television screen does. This image enables us to calculate the ratio of the titanium hydrogen-like alpha line (i.e., a transition from the second lowest atomic shell to the innermost shell when the electron making the transition is the last remaining one) to titanium helium-like alpha (i.e., the same transition when the atom still has one other electron that does not participate). The ratio of emission amplitudes in the two x-ray lines is important because the line from the hydrogen-like atoms is radiated only from the hottest part of the core. The ratio as a function of time allows us to watch the mixed material migrating into the center.
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In the inertial-confinement fusion (ICF) approach to fusion research, powerful lasers squeeze tiny capsules (Figure 1) filled with deuterium and tritium (rare forms of hydrogen) to enormous density and temperature to release huge amounts of nuclear energy and harmless helium gas: deuterium + tritium → helium + neutron. This reaction could produce a megawatt of power (the same as a typical power plant) by burning only six millionths of an ounce of fuel per hour. The fuel is separated from seawater and is considered inexhaustible on conceivable time scales.
Mix occurs in ordinary states of matter (liquids or gases) because the molecules or atoms are in ceaseless random movement. The nuclei and electrons that comprise a plasma are in violently agitated motion with the particles traveling long distances without colliding. Thus, mix produced by random motion is greatly enhanced. Unfortunately, this component is only a miniscule part of the problem. Figure 2 depicts the results of a supercomputer simulation at a point in time midway during an ICF implosion (not necessarily our particular experiment). Turbulence develops as the capsule converges under the pressure from the lasers. Anyone who has dropped food coloring into water has seen these turbulent swirls and eddies. Similar behavior can be seen in the smoke from a smokestack or in a pot of boiling water. In ICF, the growth of turbulence is much more virulent, because the interface between a lighter material and a heavier material that accelerates it is inherently unstable. Many theories exist that attempt to model this turbulence behavior. They are difficult to verify in the ICF regime because present diagnostics cannot see the individual swirls looking through the edge of the spherical capsule, which is less than a millimeter in diameter.
Figure 3 shows x-ray framing camera images from our experiments. A framing camera produces a “movie” of the implosion, much like a movie camera produces a reel of film. However, in a framing camera, the film remains motionless, and the sensitive area that forms an image at any instant is determined electronically. The earliest picture is at the top right. As time progresses, the picture moves to the left and then down to the right edge of the next row. The time interval between two successive pictures is 60 ps, which corresponds to 17 thousand million frames per second. These images show the implosion, stagnation, and explosion of a capsule. The titanium shows up as brighter areas on the film. The interpretation is not simple because of the propagation of shocks. The increased noise on the film at stagnation is due to neutrons produced by the burn.
Conclusion