A target for an IFE power plant must compress a few millionths of a kilogram of D-T fuel to a density approximately thirty times the density of lead, and heat a smaller fraction of the fuel to a temperature over 100-million degrees Celsius to ignite a propagating fusion burn. Each millionth of a kilogram (milligram) that burns inside the target releases 340 MJ, the energy equivalent of burning over 10 kilograms of coal. The major challenge for ICF target designers is to ignite a small mass of fusion fuel with a minimal amount of energy from a laser or accelerator, while maximizing the target gain--the ratio of driver energy input to fusion energy output. To achieve this goal, ICF target designers employ sophisticated computer codes, along with some relatively simple design principles.

First designers focus on how to compress fusion fuel to high density with the minimum possible energy input. In theory, ideal compression to the density required for high-gain burn can take as little as 0.008 to 0.035 MJ per milligram of fuel, substantially less than the 340 MJ/milligram obtained if all the fuel burns. Here a simple analogy illustrates the designer's strategy to achieve efficient compression. Imagine a powerful automobile sitting with its front bumper touching a rigid concrete wall. The gas pedal is pushed to the floor, the wheels spin, and little happens besides damage to the paint on the bumper. Next, the car backed away from the wall around the length of a football field, and again the pedal is locked to the floor. Now something much different happens when the car meets the wall.

If a thin-walled, hollow spherical capsule filled with D-T gas is chilled using cryogenic liquid helium, the tritium and deuterium condense as a thin layer of D-T ice on the inside surface of the shell. Even more fortunately, the tiny amount of heat from the radioactive decay of tritium tends to vaporize D-T ice from thick regions and redeposit it in thin regions, producing a D-T ice layer of highly uniform thickness. Figure 1 below illustrates such a hollow capsule, suspended at the center of a cylindrical metal or metal-lined can called a hohlraum, about the size of a dime. In practice, stamping out plastic hohlraum containers with thin metal liners could be done with standard equipment like that used for manufacturing razor blades, while fabricating the high-precision, spherical cryogenic capsules requires sophistication approaching that now achieved by manufacturers of integrated circuits for computers (Woodworth and Meier, 1997)--even more so because the plastic shells burst if the capsules ever warm up fully to room temperature.

Fig. 1 - Schematic illustration of a heavy-ion driven IFE target showing the a hollow plastic capsule containing a layer of D-T ice, suspended inside a cylindrical metal-lined hohlraum.

If the outside surface of the plastic capsule can be rapidly heated to a high temperature--millions of degrees Celsius--the vaporized shell material will reach enormous pressures and this layer of plasma will begin to push the D-T ice layer radially inward, accelerating the D-T just like the car rushing toward the concrete wall, but to much higher speeds--300 to 400 kilometers per second. In direct-drive targets, large numbers of laser beams or heavy-ion beams deposit their energy uniformly on the surface of the target. For IFE power plants, where targets must be shot into a target chamber at high velocities, attention has focused mostly on indirect drive, where the fuel capsule is enclosed inside a hohlraum made from a high-atomic weight metal like lead, gold, or tantalum. When lasers or heavy-ion beams deposit energy inside the hohlraum the inside surfaces are heated, including the outside surface of the capsule which sees radiation from the very hot hohlraum walls. Laser drivers heat hohlraums by entering through small holes in the hohlraum wall, while heavy-ion drivers deposit ion kinetic energy into absorber material as shown in Fig. 2 below. The absorber, heated to a high temperature, radiates energy to other surfaces inside the hohlraum as shown in Fig. 3.

Fig. 2 - High-velocity heavy ions penetrate into absorber material at the ends of an IFE target, starting to heat the absorber to a high temperature. Because most of the ion's energy deposits close to the location where the ion finally stops, most heating occurs deep inside the absorber.

Fig. 3 - Hot absorber material begins to heat the inside surfaces of the hohlraum by radiation, with the hohlraum surfaces reaching temperatures approaching three-million degrees Celsius, and the outside surface of the capsule vaporizing into a high-pressure plasma.

One of the IFE-target designer's goals is to push the D-T ice layer to the highest possible velocity, and to be as efficient as possible in putting kinetic energy into the D-T ice layer. Here the target designer would like to push the layer through the longest distance possible, that is, to make the capsule diameter very large and the ice layer very thin. However, the designer must also fight the natural tendency that a low density fluid (vaporized plastic) has when it pushes on and accelerates a high density fluid (the D-T ice layer). This "Rayliegh-Taylor" instability amplifies the effects of any nonuniformity in the heating and initial imperfections in the surface of the dense material. Over a sufficiently long distance it can force the compressed fuel to take a sausage or a pancake shape, or even punch holes through the dense D-T layer and rip the dense material apart.

Avoiding Rayliegh-Taylor instability requires both very high precision in the target manufacture, and very high uniformity in the heating of the outside of the capsule--that is, very high symmetry. While target designers understand much about symmetry requirements from experiments with the NOVA laser at Livermore, it remains for NIF to experimentally demonstrate adequate symmetry for ignition-size targets. With the best target fabrication tolerances, NIF target designers anticipate that NIF targets can have a capsule radius some fifteen times greater than the D-T layer thickness, and still not have initial disturbances grow too much before the D-T reaches its maximum inward velocity.

Fig. 4 - Expanding material from the capsule surface accelerates the inside layer of D-T radially inward.

Even after achieving a symmetric implosion, life for the IFE target designer remains complicated. The designer must also ignite fusion, by creating a small region at a sufficiently high temperature to ignite a propagating fusion burn. In the classical approach to ICF, the layer of dense D-T ice compresses a small mass of residual D-T gas in the center of the capsule. Because this gas starts out at low density (or higher "entropy") than the ice, when compressed, this gas reaches much higher temperatures, creating a hot spot that can spark ignition.

However, in working to compress the hot spot, once again a low density fluid (D-T gas) pushes against high-density fluid (the originally solid D-T ice layer), again the situation for Rayliegh-Taylor instability. The problem is similar to trying to squeeze a balloon with your hands--the low-density fluid tends to squirt out, and the high density fluid tends to finger into the low-density fluid. This mixing process cools the hot spot, potentially below the threshold temperature required for ignition. Avoiding excessive mix requires capsules with very smooth, highly uniform D-T ice layers. Target designers can not currently predict with certainty that hot-spot mixing can be adequately controlled in NIF, allowing ignition. Considerable evidence exists that mix can be controlled, but proof must await experiments in NIF.

Fig. 5 - The compressed D-T fuel reaches maximum density, and if compressed correctly, does not mix excessively with the hot gas compressed at the center of the mass, allowing ignition and a propagating burn out into the denser compressed D-T fuel.

One approach to eliminating the need for the hot spot is called "fast ignition," where an extremely short laser pulse, heavy ion pulse, or even a beam of antimatter, is injected into the side of the D-T fuel mass at the instant of maximum compression. Shown at the right, a cone provides a clear path for a fast-ignition laser beam to the compressed target fuel. Success with this approach would have major significance, because much lower energy, inexpensive drivers could be used could be used for the simpler task of compressing fusion fuel if no need existed to generate a hot spot. Laser fast ignition will be studied with NIF, where the major technical uncertainty revolves around delivering the fast-ignition laser pulse to the fuel through the debris remaining around the compressed fuel after the compression process. For heavy ion and antimatter fast ignition, the primary uncertainty rests in generating sufficiently short, focused ion pulses.

To be useful for studying ICF targets, heavy-ion drivers must be quite large, with energies above one megajoule, because a large amount of energy is needed just to heat absorber material to a temperature high enough to be interesting. While the physics of this heating process is well understood and simple, heavy-ion drivers have been too expensive to use as ICF research tools. Because lasers can be focused to heat very small spots, even relatively small lasers can achieve high temperatures in hohlraums. For this reason lasers continue to provide the most important ICF research tool. However the interaction of laser light with plasma is not as simple as the interaction of high-velocity ions. In practice, considerable amounts of laser energy can reflect, and laser energy can also accelerate electrons in the heated plasma to velocities high enough to penetrate into the D-T ice, heating it and making it more difficult to compress. Laser-plasma interaction at the very high laser energy of the NIF facility provides the final area of technical uncertainty for NIF. Figure 6 shows 192-beam clusters entering a NIF laser-driven target.

Fig. 6 - The 192 laser beams in NIF will heat the inside surface of a hohlraum with high uniformity. (Credit LLNL)

NIF will answer the three major remaining questions that the National Academy of Sciences (NAS, 1990) has identified for ICF: symmetry, mix, and laser-plasma interaction. If current predictions are borne out, and the answers prove favorable, NIF will achieve ignition, and the extension to the design of ICF targets for power plants will be straight forward.