PROGRESS TOWARD IGNITION AND BURN PROPAGATION IN INERTIAL CONFINEMENT FUSION To achieve efficient inertial confinement fusion one must produce a small hot spot within the imploding target from which thermonuclear burn can ignite. John D. Lindl, Robert L McCrory and E. Michael Campbell For the past four decades, scientists throughout the world have pursued the dream of controlled thermonuclear fusion. The attraction of this goal is the enormous energy that is potentially available in fusion fuels and the view of fusion as a safe, clean energy source. The fusion reaction with the highest cross section uses the deuterium and tritium isotopes of hydrogen, and D-T would be the fuel of choice for the first generation of fusion reactors. (See the article by J. Geoffrey Cordey, Robert J. Goldston and Ronald R. Parker, January, page 22.) Development of an economically viable fusion reactor would literally give us the energy equivalent of oceans of oil. Because seawater contains about 40 g of deuterium and 0.1 g of lithium per tonne, every barrel of seawater contains the energy equivalent of almost 30 barrels of oil in deuterium fuel and about one-fifth of a barrel of oil in D-T fuel (where tritium is obtained from neutron reac- tions on lithium). A volume of seawater equal to the top meter of the Earth's oceans would yield enough fuel to supply D-T fusion reactors for thousands of years of electricity production at today's rate of usage. The two primary approaches to developing fusion are magnetic confinement fusion, reviewed in the January issue of PHYSICS TODAY, and inertial confinement fusion, 1 reviewed in this article and the article by William J. Hogan, Roger Bangerter and Gerald L. Kulcinski on page 42. Significant elements of the work presented here were carried out under classified Department of Energy pro- grams and have been only recently declassified. In its review of ICF 2 carried out in 1990, the National Academy of Sciences found the DOE classification guidelines for ICF John Lindl is the ICF target physics program leader at Lawrence Livermore National Laboratory, in Livermore, California. Robert McCrory is director of the Laboratory for Laser Energetics at the University of Rochester, in Rochester, New York. Michael Campbell is deputy associate director and ICF program leader at Lawrence Livermore. to be excessive and recommended that DOE review them and schedule further declassification of target physics. DOE is continuing to review its classification policy. For D-T fuel, both the magnetic and inertial ap- proaches require a fuel temperature in excess of 100 million K and a fuel particle density n and confinement time r such that TIT = 10 14 -10 15 sec/cm 3 . Magnetic con- finement fusion operates in a regime with r~l sec and n=:10 14 cm" 3 . For magnetic confinement fusion, the density is limited by the maximum magneticfieldthat can be generated, which is determined by the strength of the material of the confinement vessels. Inertial confinement fusion relies on the inertia of an imploding target to provide confinement. 1 Confinement times are less than 10 10 sec, and particle densities in the fuel are typically greater than 10 25 cm 3 . Implosion ond burn of ICF forgets High-gain ICF targets have features similar to those shown in figure 1. These capsules consist of a spherical shell filled with low-density (S1.0 mg/cm 3 ) equimolar deuterium-tritium gas. The shell is composed of an ablator and an inner region of D-T, which forms the main fuel. Energy from a driver is rapidly delivered to the ablator, which heats up and expands. As the ablator expands outward, the rest of the shell is forced inward to conserve momentum. The capsule behaves as a spherical, ablation-driven rocket. The fusion fuel is imploded with a typical efficiency of 5-15%. That is, 5-15% of the total absorbed energy goes into the fuel. In its final configuration, the fuel is nearly isobaric at pressures up to about 200 gigabars but consists of two effectively distinct regions: a central hot spot, containing about 2-5% of the fuel, and a dense main fuel region (the "cold fuel pusher"). Fusion begins in the central hot spot, and a thermonuclear burn front propa- gates rapidly outward into the main fuel, producing high gain. The efficient arrangement of the fuel in this configu- 32 PHYSICS TODAY SEPTEMBER 1992 © 1992 Americon Insrirure of Physics