Basic Principles of Direct-Drive Ignition Target Design

LLE Review, Volume 106 83 Introduction Inertial confinement fusion (ICF) is an approach to fusion that relies on the inertia of the fuel mass to provide confinement. The confined fuel must reach a high temperature and density to produce enough D + T $ a(3.5 MeV) + n(14.1 MeV) reactions so that the total energy released is much larger than the driver energy required to compress the fuel. The capsule in an ICF implosion, which is a spherical cryogenic deuterium–tritium (DT) shell filled with DT vapor, is irradiated directly by laser beams (direct-drive approach) or by x rays emitted by a high-Z enclosure (hohlraum) surrounding the target (indirect drive).1 Only a small portion of the fuel is heated to ignition conditions in a typical ignition target. This part of the fuel forms a hot spot that initiates a burn wave that ignites the remaining fuel. In the direct-drive approach, the following stages of an implosion can be identified: At the beginning of the laser pulse, the outer portion of the pellet heats up and expands outward, creating a plasma atmosphere around the pellet. Then a critical electron density n mc e L 2 2 2 cr = r m is established outside the cold portion of the shell, where m is the electron mass, c is the speed of light, e is the electron charge, and mL is the laser wavelength. The laser energy is absorbed in a narrow region near the critical surface via the inverse bremsstrahlung, and the absorbed energy is transported, mainly by electrons, toward the colder portion of the shell. The cold material, heated by the thermal conduction, expands outward. Such an expansion creates an ablation pressure that, similar to the rocket effect, compresses the pellet. At the beginning of implosion, the ablation pressure launches a shock wave that propagates ahead of the thermal ablation front and increases the fuel entropy. Then, as the first shock breaks out at the rear surface of the shell, the transmitted shock is formed. It converges through the vapor to the capsule center. After reflection from the center, the shock moves outward and interacts with the incoming shell. At this point, the velocity of the inner portion of the shell starts to decrease, reversing its sign at stagnation. This is a crucial point of the implosion since no more “pdV” compression work can be done to the hot spot after the stagnation, and the only remaining heating source is the energy deposition of a particles produced by fusion reactions inside the hot spot (a heating). Basic Principles of Direct-Drive Ignition Target Design