P A R T 1

By Lynn Yarris

The joke has made the rounds. Fusion, the nuclear process that lights up the sun, is the energy source of the future and always will be. The joke arose (unfairly) from the technical difficulties encountered by a strategy for achieving fusion energy called "magnetic confinement." There is, however, a second strategy for achieving fusion energy, one that has only recently been thrust into the r&d spotlight. This second strategy is called "inertial confinement," and its future is now.

Fusion is the melding together of lighter atomic nuclei to form heavier nuclei, an action that releases roughly one million times the energy released by the burning of oil. The reason for the continued pursuit of fusion as a commercial source of energy is no joke. The citizens of this country and the rest of the world currently depend upon fossil fuels-oil, gas and coal-for the energy that lights, heats and cools buildings, powers machinery, and drives industrial processes. One day these fuels will be exhausted. Most projections show that severe shortfalls between global energy demands and fossil fuel supplies will begin to be felt toward the end of the 21st century. New sources of energy before then are a must.

Fusion is an excellent candidate. It is safe: unlike fission, where atomic nuclei are split, fusion cannot sustain a chain reaction and, with proper design, does not produce high-level radioactive by-products that must be carefully stored for thousands of years. Nor is there a solid fuel core that could melt down-the so-called "China Syndrome." It is clean: unlike the burning of fossil fuels, fusion would not contribute to the greenhouse effect, acid rain, or the depletion of the ozone layer. And it would last forever: the fuel for a fusion power plant would be deuterium and tritium, the two isotopes of hydrogen. Both of these isotopes can be obtained from ordinary water-deuterium is extracted directly, and tritium is produced from the element lithium. Enough fusion water to supply a year's worth of electrical power to a metropolitan city could be delivered in a pickup truck.

Ironically, the holdup in the development of fusion as a commercial energy source has been an energy barrier. The analogy has been made that igniting a fusion reaction is like trying to light a wet match. Fusing two nuclei together requires overcoming the electrostatic repulsion that drives them apart (try forcing a pair of bar magnets to touch at the same poles). This electrostatic energy barrier can only be overcome by forcing the nuclei together at extremely high speeds, which means heating them to a temperature of about 100 million degrees Celsius (180 million degrees Fahrenheit). Furthermore, the heated nuclei must be confined long enough to interact which poses another problem since heated nuclei tend to fly apart. To date, this has only been achieved in the hydrogen bomb which uses an atomic bomb as its trigger. Clearly, an alternative is needed for a commercial fusion energy plant.

Starting from the early 1950s, the largest fusion energy research effort has been directed at magnetic confinement, also known as magnetic fusion energy or MFE. In this scheme, thermonuclear fuel is contained as an ionized gas, called a plasma, by a powerful magnetic field. This magnetically confined plasma is then heated to ignition. Although experimental efforts have shown tantalizing promise (hence the joke), the technical obstacles in scaling up from small laboratory experiments to commercial-size reactors have persisted. MFE experiments continue to consume more energy than they produce, though the gap has been shrinking and could be erased should the proposed International Thermonuclear Experimental Reactor ever go online.

In the case of inertial confinement, also known as inertial fusion energy or IFE, no magnetic field or any other external confinement system is required. Inertia is the tendency to resist movement that is universal to all forms of matter, even atomic nuclei that have been heated to the surface temperature of the sun. In an IFE system, a pea-sized spherical capsule made of plastic or a light metal encases a hollow shell of frozen deuterium and tritium which is filled with deuterium and tritium vapor. The outer layer of the sphere is rapidly heated so that it vaporizes and begins to "ablate" or expand away from the frozen fuel at an incredible speed. The ablating material implodes the frozen fuel inward with enormous force (like the blast of gases that propel a rocket forward), compressing its density about a thousand times and causing the vaporized fuel in the center to ignite. The ignited fuel in the central "hot spot" in turn ignites the cooler solid fuel like the spread of a grass fire. The fuel burns fast enough to remain confined by its own inertia. How fast is fast? The entire process takes place inside one billionth of a second. By the time the fuel's reactants have overcome their resistance to movement, they've already been consumed.

In contrast to MFE, the capacity of IFE to produce far more energy than is consumed has already been vividly demonstrated by hydrogen bomb detonations. The question is whether the technology can be scaled-down to useful size. Traditionally, downscaling a technology has always been easier than scaling up, but this is not to say there aren't substantial technical challenges facing IFE researchers. One of the most critical issues is how best to ignite the thermonuclear fire. Should the fuel be heated directly or indirectly and what should be the source of this heat, a laser or a particle accelerator that produces beams of heavy ions? (A heavy ion is defined as a positively charged atom with more than three protons in its nucleus.)

As would be expected, in direct heating, energy is applied directly onto the spherical capsule until the fuel inside ignites. Sounds simple enough, but in practice this is quite difficult because the energy deposited on the sphere's surface must be extremely uniform (less than a 2-percent variation). It means that multiple beams of energy must be precisely applied to the sphere-called the "target" in IFE-speak-from many different angles at once. The energy required to achieve ignition of the fuel has been calculated to lie between one and two megajoules, which is about the energy required to make two pots of coffee. However, because this energy is delivered in pulses of less than a billionth of a second, the power (defined as energy divided by time) poured onto the target is staggering-about 500 trillion watts or 1,000 times the total electrical generator capacity of the United States. Manipulating such power onto such a tiny target with the precision demanded for direct heating has convinced some IFE researchers to investigate indirect heating as an easier alternative.

In the most popular indirect heating scheme, the spherical fuel capsule is mounted inside a cylinder that is about the size of a large paper clip. This cylinder is called a "hohlraum," which is German for "cavity" and it is usually made of some heavy element such as lead. Energy beams are shined through holes at the end of the hohlraum, vaporizing its inside surface and releasing a burst of x-rays. These x-rays bounce around inside the hohlraum, heating the fuel capsule much like heat from an oven bakes bread. Indirect heating achieves a highly uniform compression and heating of fusion fuel without the precise positioning of incoming energy beams required for direct heating. On the other hand, the extra step in the heating process means that energy is lost: direct heating is estimated to be at least twice as efficient as indirect.

As to the question of whether to use laser or heavy ion beams to heat the fuel-this energy source is called a "driver" in IFE-speak-again each has its pros and cons. High-powered lasers hold one big edge in that their underlying technology is better understood. The laser of choice for driving a fusion reaction is one that uses glass which has been doped with neodymium, a rare-earth element, as its lasing-medium. Neodymium-glass lasers can store a lot of energy over a relatively long period of time, and their beams are relatively easy to aim and spot focus onto tiny targets. For this reason, such lasers have been used in the bulk of fusion target physics studies. None of these past efforts, however, heated the target to ignition temperatures. A new project is now underway that is designed to take that critical next step forward.

The project is called the National Ignition Facility (NIF) and it involves a collaboration between the Lawrence Livermore National Laboratory (LLNL), where it will be located, Los Alamos and Sandia national laboratories, and the University of Rochester's Laboratory for Laser Energetics, home to the National Laser Users' Facility. All of these laboratories have notable on-going IFE research programs in which solid-state lasers have been used as the driver. None, however, has ever approached the scale of NIF. Housed inside a building the size of an indoor football stadium, NIF will generate 192 neodymium-glass laser beams in pulses of 1.8 megajoules of energy lasting slightly more than three-billionths of a second. Estimated to cost approximately $1.1 billion, NIF's primary purpose will be to provide scientific support for the Department of Energy's Stockpile Stewardship and Management Program. This is the program aimed at maintaining and managing the nation's remaining stockpile of nuclear weapons in the absence of underground testing and a ban on new weapons design. However, NIF will also provide critical information that can answer questions about the potential use of lasers as drivers for a commercial fusion power plant. Since the target physics for indirect heating are the same whether the driver is a solid-state laser or a heavy ion particle beam accelerator, NIF can provide answers to those questions as well.

What NIF cannot do is answer questions about the use of heavy ion beams in a direct heating scheme. In a classic case of the last not being the least, the one area of research that NIF cannot address could well be the most important: it is the general and consistent consensus of fusion energy experts, including panelists from the National Academy of Sciences and the 1996 Fusion Energy Sciences Advisory Committee, that the most likely driver in a commercial power plant will be an accelerator of high-powered beams of heavy ions, such as xenon, mercury, or lead.

The strengths of heavy ion particle accelerators are energy efficiency, reliability, durability, and pulse-repetition rates. In a research laboratory, even one as large as NIF, there might be half-a-dozen fusion reactions triggered in a single day. In a commercial power plant, there will have to be half-a-dozen fusion fires ignited every second. Lasers must be energized or "pumped" to emit light. For glass lasers, only about one percent of this energy is emitted as light, the rest is given off as heat. Other types of lasers have shown more promise, but inefficiency remains a problem. This poor efficiency also means that every time a laser is fired, it must be allowed to cool off before it can be fired again, which means its repetition rate is limited.

Heavy ion beams, on the other hand, are readily pulsed at rates much higher than the repetition rates required of a commercial fusion reactor. Also, heavy ions deposit their energy at high voltages over a short distance which means that beams can be readily set up to produce the conditions in a fusion target needed to ignite its fuel. As for efficiency, it has been calculated that for a fusion power plant to be economically viable, the targets must release at least ten times as much energy as the driver consumes. This takes into consideration all of the other gains and losses that occur during the process of converting thermal into electrical energy. At present, the combination of target energy gain and driver energy efficiency is beyond the optimal performance of any laser but well within the proven capabilities of a heavy ion accelerator. The reliability and durability of heavy ion accelerators are well-documented through their extensive use in high-energy physics and nuclear science research.

Continue to part 2 of this story.