Development of Direct Drive, High-Gain Capsules for Inertial Fusion: A Materials Challenge
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corona). From that point, the laser energy is then deposited in the plasma and conducted via thermal electrons to the pellet surface. The outer surface of the pellet is removed (i.e., ablated) at an explosive rate. This rocketlike blowoff of surface material produces a reaction force that causes the remainder of the pellet to implode. The center, or core, of the imploded DT fuel is the "spark plug" that ignites during the final stage of compression. The fusion "burn" then radially propagates out from the spark plug region into the remaining fuel. Further details of the physics of the implosion process are given in several excellent review articles. When deuterium and tritium atoms fuse they produce an alpha particle (He2), a neutron, and 17.6 MeV of energy that is kinetic shared by these particles. Most of the kinetic energy (14 MeV) is contained in the neutron which, for power production, would be captured by a blanket of flowing material inside a reactor chamber. This heated blanket material then flows to a heat exchanger where the energy is extracted for use in a conventional electric power production cycle. The ratio of the fusion energy released by the capsule to the laser energy used to
drive it is called the capsule gain. The success of the ICFconcept requires that the gain be greater than 100; capsules capable of performing at this level are commonly referred to as "high-gain" capsules. For ICF to become a reality, several crucial material issues must be solved. Currently, one of the most pressing materials research needs is the development of precision high-gain ICF capsules containing cryogenic DT fuel. The materials research challenges these capsules present is the subject of most this article. Evolution of Single-Shell Capsules In the early phases of the ICF program, the implosion capsules were thin-walled glass microspheres filled with DTgas. This capsule design, the "exploding-pusher" (Figure 2), is the simplest fuel capsule to fabricate. The name "exploding-pusher" refers to the physical response of the capsule as it is illuminated with a short (~ 20-100 ps) high-power laser pulse. Because the energy deposition in the glass shell is so rapid it explodes, sending about half the shell mass outward while driving the other half inward to compress the fuel. This capsule design can produce high DT fuel Continued
INERTIAL CONFINEMENT FUSION CONCEPT
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Figure 1. Conceptual drawing of the four main stages in a spherically symmetric ICF implosion driven with converging laser or particle beams. For the DT fuel to ignite,it must be compressed to about 1000 times liquid density and a temperature of about 10 8 degrees. The duration of the laser or ion beam pulse is about 10"a seconds; the final ignition and burn occurs
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