this confinement: (1) magnetic confinement fusion (MCF), in which magnetic fields are used to confine the plasma, and (2) ICF, the topic of the current report, in which a driver delivers energy to the surface of a pellet of fuel, heating and compressing it. Potential drivers include lasers, particle beams, and X-rays, among other concepts.
In ICF, energy supplied by the driver is applied, either directly or indirectly, to the outer layer of a fuel pellet that is typically made up of an ablator material (e.g., beryllium, doped plastic, or high-density carbon) that explodes outward as it heats. This outward explosion of the surface layer forces the remainder of the fuel (typically light elements such as deuterium and tritium) to accelerate inward to conserve momentum. The timing of the inward fuel acceleration is controlled carefully in order to compress the fuel using a minimum of energy. 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) depends on the burn-up fraction, and gains greater than about 10 will need to be demonstrated to validate the target physics of any approach to a practical IFE power plant.
Important target physics includes processes that deflect or absorb driver energy within the target; the transport of energy within the target; capsule preheat; conversion of energy to the inward-directed implosion by ablation; fuel compression and heating; thermonuclear reactions; transport and deposition of neutron and alpha-particle energy, resulting in bootstrapping thermonuclear reactions; and hydrodynamic disassembly and output. Models exist for all of these processes, but some are more predictive than others. Some processes are difficult to simulate, such as laser-plasma interactions, the generation and transport of hot electrons in self-consistent magnetic fields, nonlocal-thermal-equilibrium atomic physics, hydrodynamic instabilities, mix, and debris generation. These models continue to evolve to keep pace with experiments. Other processes, such as large-scale hydrodynamics, thermonuclear reactions, and X-ray-, neutron- and alpha-particle transport appear to be simulated adequately using standard numerical models.
The Department of Energy (DOE) is funding multiple efforts to investigate the physics of ICF; many of these efforts have the potential to inform current
1 What is described here is known as hot-spot ignition; other potential concepts for ignition are being considered and are introduced briefly later in this chapter.