The design of early target chambers, like the NOVA chamber pictured below, was relatively simple and focused primarily on providing precise beam pointing and target positioning. Target-chamber design has become more complicated for the National Ignition Facility, because instead of the 50,000 joules produced by NOVA's ten lasers, the NIF target chamber must accommodate 1.8-million joules (1.8 MJ) of laser energy, plus a design fusion yield of 20-million joules (20 MJ). While target chambers provide only a small fraction of the total cost of an ICF facility (for example, NIF's target chamber costs $30 million, out of a $1.2 billion total construction cost), target chambers provide some of the most interesting, fundamental research and design problems in ICF technology. Due to the relatively low cost of target chambers, considerable flexibility exists to optimize designs. NIF has been designed to accommodate a second target chamber, and IFE researchers anticipate studying a series of three or more target chambers with progressively greater sophistication following construction of the first high-repetition rate, ignition-class driver. The design issues for the NIF target chamber, and the more challenging issues for high-repetition-rate IFE chambers, center on accommodating the energy and debris released by ICF targets.

Neutrons. Eighty percent of the energy from the D-T fusion reaction is released as high energy neutrons. Because neutrons have no electrical charge, most escape from the target, and deposit the majority of their kinetic energy over large distances by large numbers of collisions. For objects close to targets, neutrons can deposit enough energy so that the relaxation that occurs after the pressurization caused by the rapid heating can actually shatter the material; for example, in NIF it is anticipated that that the first few centimeters (1 inch = 2.54 cm) of the thin stalk holding the target will shatter into high-velocity shrapnel. At greater distances neutrons generate more gentle heating, which a liquid coolant can remove in some IFE chamber concepts. Because the neutrons cause damage at the molecular level by knocking atoms out of location, liquid shielding is valuable to reduce structural damage rates. After large numbers of collisions, the majority of the neutrons eventually stop by being absorbed in the nucleus of some atom, changing the atom's atomic weight. In some cases the new isotopes formed by neutron absorption are radioactive. Careful selection of structural and coolant materials can ensure that the these radioactive isotopes decay very quickly, minimizing radioactive waste generation so that IFE power plants create only small amounts of low-level waste.

X rays. As target disassembly occurs, the target surfaces can reach temperatures of half a million to a few million degrees Celsius. At these temperatures, instead of radiating energy primarily in the visible spectrum like incandescent light bulb filaments, the target debris radiates x rays. For targets with hohlraums, roughly ten to fifteen percent of the fusion energy ends up as x-rays, absorbed by the surfaces which face the target. These x rays vaporize a surface layer thinner than the thickness of a sheet of writing paper. Particularly for surfaces close to targets, the resulting blow off of ablated material can create large pressure loading, as well as an important source of debris.

Debris. The high temperatures reached by ICF targets generate enormous pressures, which accelerate the debris of the ignited target outward to velocities of tens of kilometers per second. Radiant heat from the expanding cloud of hot target debris causes additional ablation from surrounding surfaces, and combined with ablation debris, can generate strong mechanical loading on liquid and on structures in the target chamber. In IFE power plants, rapid condensation of the debris must occur to allow subsequent shots.

In the National Ignition Facility, where shots occur a few times per day, the primary target-chamber design goal centers on maintaining cleanliness of the transparent glass shields which protect the final-focus lenses of the 192 laser beams, and create the transparent vacuum barrier between the chamber and the laser assemblies. To prevent excessive ablation of the wall surface, NIF designers selected a large target-chamber radius--5 meters (16 ft)--to reduce the intensity of x rays at the chamber wall. Concepts for laser-driven IFE power plants call for similar methods to maintain a transparent vacuum interface, potentially using liquid-metal mirrors to allow the optics to be kept out of sight of target neutrons. For simplicity, this chapter does not discuss in greater detail the work that has been done for laser target chambers, and instead focuses on concepts for targets driven by heavy ions.

In accelerator-driven IFE power plants, concerns about contamination of final optics are eliminated by the use of magnets for final focusing. However, the requirement to operate at high repetition rates, and to recover fusion energy to make steam to drive turbines, introduces new design goals. Over the last two decades, researchers have developed a number of innovative IFE target-chamber designs for both laser and heavy-ion driven targets. For heavy-ion targets, much recent research has focused on "liquid-protected" designs that allow highly compact target chambers. For heavy-ion IFE, significant benefits come from reducing target-chamber size. Smaller chambers use fewer construction materials and have lower cost. More importantly, smaller chambers allow the final-focus magnets to be closer to the target, which helps to reduce the focus spot size and in turn the size of the driver, with a large, positive economic impact on the cost of IFE electricity.

Figure 1 shows an example of a liquid-protected target chamber for heavy-ion targets. The vessel is only three meters (10 feet) in radius, yet with targets injected six times per second, it generates enough fusion energy to make 1,000 MW of electricity. The key feature of the target chamber is the jets of liquid injected from the top that form a shielded liquid pocket where the fusion microexplosion can occur.(Moir et al., 1994) The nozzles that inject the liquid--some stationary, and others oscillating--create a liquid geometry that provides passages for target injection and driver beams, while making sure that in every other direction neutrons coming from the target must pass through at least half a meter (20 inches) of liquid before reaching any of the chamber's metal structure. This reduces the number and energy of neutrons reaching the chamber structure sufficiently to permit target-chamber construction using ordinary stainless steel, and still the target-chamber can last without replacement for the entire 30-year life of the plant and then have such low levels of short-lived radioactivity that it can be disposed of as low-level waste.

Luckily, an excellent liquid coolant exists for compact IFE target chambers. The liquid, called Flibe (Li₂BeF₄), is a molten salt with twice the density of water, and roughly the same viscosity, at the 650 °C (1140 °F) temperature desired for making high-pressure steam for driving turbines. Flibe has interesting properties. Besides being nonflammable and compatible with stainless steel, Flibe is composed of lithium, fluorine, and beryllium. The lithium ⁶Li isotope absorbs neutrons, each reaction producing one atom of helium and one of tritium. When the high-energy fusion neutrons strike beryllium nuclei, commonly one of the beryllium nuclei's neutrons is torn lose, providing a source of additional neutrons to make up for the fusion neutrons that miss lithium nuclei and are absorbed elsewhere. By controlling the liquid geometry, and taking advantage of the extra neutrons from beryllium, slightly more tritium can be made than the IFE power plant consumes as fuel.

Fig. 2 - Oscillating liquid jets enter the HYLIFE-II target chamber, forming a liquid pocket for the fusion microexplosion. Debris vents through slots in the pockets, to be condensed by sprays of liquid droplets. A grid of vertical and horizontal stationary liquid jets shields each end of the pocket while allowing driver beams to enter.

The nozzles inject the liquid jets into the chamber with a high velocity, as much as 12 meters per second (27 mph). However, when the target ignites it emits x-rays and disassembles so rapidly, with velocities approaching tens of kilometers per second (~100,000,000 mph), that the liquid might as well be moving at the speed of a glacier. Venting debris can begin to reach the chamber wall in as little as sixty millionths of a second. U.C. Berkeley has developed the capability to model these gas dynamics processes inside complex chamber geometries, and to predict where and how fast the debris will go, and how much mechanical loading it generates on the liquid jets and target-chamber structures. Figure 3 shows calculations made with the UCB code TSUNAMI for debris venting from the HYLIFE-II liquid pocket. In this case, the venting occurs through large numbers of open slots formed by oscillating rectangular sheet jets, while the driver beams enter through a grid formed by stationary liquid jets at each end of the pocket. Other pocket-venting geometries are also under active study, as research continues to further refine IFE target-chamber designs and reduce final-focus-magnet stand-off distances.

Fig. 3 - TSUNAMI code predictions for ablation and target debris venting from the HYLIFE-II liquid pocket for a 350-MJ target (Credit: C. Jantzen, UCB).

Active research continues to better refine our understanding of IFE target-chamber phenomena. Experiments at U.C. Berkeley are currently creating scaled stationary liquid jets in a large vacuum tank, and in the near future experimental studies of oscillating liquid jets under IFE conditions will begin. Progress continues in other areas as well, for example, researchers at LBNL (Petzoldt, 1998) studying high-velocity target injection, using laser measurements of moving targets to predict their final position, recently demonstrated accuracies of 0.1 millimeter (0.004 inch) for target injection, well inside the precision needed for IFE power plants, 0.4 mm for combined target position prediction and beam pointing (Moir et al., 1994).

Target-chamber research has provided excellent synergy with driver development efforts. Models and computer codes like TSUNAMI (Fig. 3 above) that were developed during the early 1990's to study IFE power-plant target chambers have been used to design the target chamber for the National Ignition Facility, to model the x-ray ablation that will occur from surfaces close to targets and to a more limited extent from the chamber wall for high-yield shots, and the transport and deposition of target and ablation debris. This has allowed calculations of the rate of contamination accumulation on the transparent debris shields that protect the 192 final focus lenses, which in turn controls the 1-week maintenance frequency for the NIF target chamber.

NIF in turn, with its ignition shots, will generate transient conditions closely approaching those anticipated for IFE power-plant target chambers like HYLIFE-II. Experiments in NIF, using systems like the mini-chamber illustrated in Fig. 4 below, will allow target-chamber researchers to study the x-ray ablation, gas dynamics, venting and structural loading processes that will control how compact future IFE target chambers can be.

Fig. 4 - Mini-chamber experiment inserted inside the NIF target chamber to study ablation and target debris venting under prototypical IFE target-chamber conditions.