Assessment of Inertial Confinement Fusion Targets (2013) / Chapter Skim
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2 Technical Background
2 Technical Background
Pages 12-32
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From page 12... ...
, and target physics. INERTIAL CONFINEMENT FUSION AND INERTIAL FUSION ENERGY Nuclear fusion -- the process by which the nuclei of atoms such as deuterium or tritium combine to form a heavier nucleus, such as that of helium -- can release a significant amount of energy.
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From page 13... ...
At the same time, sudden increases in the driver power profile both accelerate the implosion and send shock waves into the center of the fuel, heating it sufficiently that fusion reactions begin to occur.1 The goal of ICF is to initiate a self-sustaining process in which the energetic alpha particles emitted by the ongoing fusion reactions heat the surrounding fuel to the point where it also begins to undergo fusion reactions. The percentage of fuel that undergoes fusion is referred to as the "burn-up fraction." The fuel gain G (defined as the ratio of the total energy released by the target to the driving beam energy impinging upon it)
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From page 14... ...
Additional discussion of the program is provided in classified Appendix D BASICS OF ICF TARGET PHYSICS AND DESIGN Target Design: Direct and Indirect Drive, Z-Pinch There are two major concepts for ICF target design: direct-drive targets, in which the driver energy (e.g., in the form of laser beams, particle beams, or magnetic field pressure)
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From page 15... ...
FIGURE 2-2 In the case of indirect drive, driver energy incident on a hohlraum is converted to X-rays, which then impinge symmetrically on the fuel capsule, causing it to implode. This figure shows the laser beam geometry used in the National Ignition Campaign (NIC)
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From page 16... ...
This relatively large gain is in large part due to avoiding the losses that occur during the conversion of laser beams or particle beams to X-rays in the hohlraum, discussed in detail in the next section. Avoiding these losses results in a higher percentage of driver energy absorbed by the capsule in direct drive, increasing the efficiency and potentially decreasing the size of the driver required.
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Recently LLE developed SSD with multiple phase-modulation frequencies (Multi-FM) and proposed using this technique to modify the NIF for polar direct drive.
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From page 18... ...
A direct drive capsule must tolerate four major sources of perturbations to ignite and burn: drive asymmetry, inhomogeneous capsule surface finish, ice roughness in the layer between the cryogenic DT and the DT gas; and driver imprint.3 The effects of the driver imprint and drive asymmetry are reduced for indirect drive. In addition, without a hohlraum to protect the capsule from the high temperatures in the chamber, and if there is no buffer gas to protect the chamber walls from emitted alpha particles, alternative methods must be found to address these threats.
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From page 19... ...
This results in lower calculated potential gains for indirect-drive fusion targets. As with direct drive, although its primary development historically has been with laser drivers, indirect drive has been used in IFE system designs with other drivers (e.g., heavy ions and early Z-pinch schemes)
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From page 20... ...
, sur rounded by a larger mass of dense but colder fuel that has sufficient areal density (>300 mg/cm2) to trap alpha particles and initiate bootstrap heating.4 The primary reason for utilizing hot-spot ignition is to minimize the driver energy requirements.
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SOURCE: Juan Fernandez, LANL, "Inertial Confinement Fusion Targets at Los Alamos National Laboratory," presentation to the panel, May 2011.
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From page 22... ...
The magnetized, preheated fuel is then imploded at a lower implosion velocity than is used in hot-spot ignition to minimize driver energy requirements. The magnetic field is applied to inhibit fuel cooling during the slow implosion process (i.e., inhibit cross-field transport)
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From page 23... ...
For the same driver energy, direct drive delivers more energy to the fuel than does indirect drive. Implicit in this yield-scaling is the fact that the increasing fusion energy output comes from burning more fuel.
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From page 24... ...
All designs try to use driver energy efficiently; thus, they implode a cold mass of fuel isentropically and a small amount of fuel to high temperature -- either by hot spot ignition, fast ignition, or shock ignition. Instabilities can limit the propagation of burn from the ignition region to the remaining fuel.
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These challenges are discussed in the following paragraphs. A necessary condition for achieving the optimal energy output from each target is that the target be uniformly compressed by the laser beams.
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showed that a surrogate target could be repeatedly placed within 10 mm of target chamber center, where a final engagement system does the final pointing. For the indirect-drive targets currently under development, the target is required to be within 100 μm of the focus of the laser beam,7� which appears to be within the capabilities of the system developed by the HAPL program; however, one difference between the direct- and indirect-drive approaches to fusion is that the indirect-drive approach has a higher gas pressure in the reactor chamber that may affect the repeatability of the injection process (Norimatsu et al., 2003)
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From page 27... ...
The heavy-ion fusion energy concepts originated as a variation of laser-driven concepts in which the driver energy is supplied by heavy ions accelerated by a linear accelerator. Subsequently, a variety of target-design concepts have been proposed: an indirect-drive design (3-4 GeV Bi+1)
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From page 28... ...
In this section, the key challenges are outlined for the production of these targets for laser drivers, pulsed-power drivers, and heavy-ion drivers. Targets proposed for each of the fusion energy concepts have equal mixtures of deuterium and tritium as the fuel.
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From page 29... ...
SOURCE: S.A. Slutz, SNL, "Design and Simulation of Magnetized Liner Inertial Fusion Targets," presentation to the panel on May 10, 2011.
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Additional outer layers may be needed to provide greater protection to the target when it is injected into the reactor chamber. The DT fuel is diffused into the plastic shell, and the target assembly is cooled to form the uniformly thick ice layer.
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From page 31... ...
15 S.A. Slutz, SNL, "Design and Simulation of Magnetized Liner Inertial Fusion Targets," presenta tion to the panel on May 10, 2011.
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From page 32... ...
Additionally, tritium is diffused into the capsule instead of flowing through a hole, which takes 2 to 4 days because of the fragility of the target and the quantity of fuel that has to be added.18 The process for forming the ice layer adds about 12 hours to the production cycle, which is the same process that the indirect-drive concept will use if it is not possible to subcool the liquid layer sufficiently to achieve the desired gas density. Two main contributors to the total tritium inventory of an IFE plant will be these: • The amount of tritium that is trapped inside the target during the target assembly phases and • The amount that is entrained in the tritium-breeding and recovery processes (from the gaseous effluent from the reaction chamber)
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