Introduction

Fusion's promise as an energy source comes from its inexhaustible fuel supply, and from its potential for almost negligible environmental impact compared to the environmental costs of competing energy sources: the air pollution and carbon dioxide emission from fossil fuel combustion; high-level radioactive waste generation from nuclear fission; and the emissions from the production of the larger quantities of concrete, steel, glass and other materials required to collect dilute solar energy. Significant technical barriers must be overcome before fusion could compete economically with these other energy sources. These notes focus on a specific technology--inertial confinement fusion (ICF)--and outline reasons to be optimistic that the inertial route to fusion energy also promises economical viability. Thus, besides discussing the basics of fusion reactions, how ICF "targets" work, and what major components would go into an inertial fusion energy (IFE) power plant, these notes also present the latest economic estimates for IFE power.

These notes make a rather strong claim: if the new laser-based National Ignition Facility successfully achieves the scientific goal of igniting a fusion reaction in a small mass of fusion fuel, sometime around 2005, then the most important technological hurdle to economical inertial fusion power-plant design will have been passed. The remaining work leading to an economically viable plant can then occur over a few-decade period, with annual research and development investments smaller than those that the United States currently provides for fossil and renewable energy research. In the interim before ignition, much can be done to prepare for the R&D effort that will be needed to develop inertial fusion energy (IFE) power plants after the scientific demonstration of ignition at laboratory scale. The major question will be at what scale IFE becomes economical, either as single plants comparable to current large fossil and nuclear power plants, or in a massive power station with two drivers servicing twenty or more target chambers. Future generations may not choose to build large IFE facilities, but the availability of these designs could be of substantial importance if the worst scenarios for global climate change prove correct. Conversely, if smaller IFE plant sizes prove feasible, IFE power could very well provide a major source of electrical power in the next century.

This claim about the potential economic viability of inertial fusion energy is certainly a strong one, but several inherent characteristics of ICF technology support this viewpoint. In summarizing the arguments for the economic viability of IFE, these notes are written for a broad audience, separating more technical topics from the simpler basic concepts, and invite readers to reach their own conclusions about this claim. Much of the information presented here is drawn from an excellent recent book by 74 researchers in the ICF field, Energy from Inertial Fusion (Hogan, 1994). Readers who find the materials here of interest are strongly encouraged to continue on to this reference, the other references cited here, and the growing body of ICF information now available on the web.

Audience

The Department of Nuclear Engineering at the University of California, Berkeley, along with its sister campuses at San Diego and Los Angeles, support extensive research programs in fusion energy, including efforts directed at transforming inertial confinement fusion (ICF) into an economical power source. At Berkeley, we operate the Rotating Target Neutron Source, currently the most powerful source of fusion-energy neutrons in North America; we support the analysis and design of the target-chamber for the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory; we pursue research in advanced target chamber designs that will be used for the first heavy-ion ignition facility and the first inertial fusion energy (IFE) power plant; and our graduate students work with researchers at Lawrence Berkeley National Laboratory on IFE heavy-ion accelerator and target-design research.

Energy from Fusion

For fusion reactions to occur at sufficiently high rates to produce useful energy, light elements must be confined at sufficiently high density and temperature for a sufficiently long time. In the sun, gravitational forces provide this confinement, while on earth either magnetic or inertial forces must be used. Researchers in the U.S. and abroad now actively study both magnetic confinement fusion and inertial confinement fusion. Because discussion of the magnetic approach can be found elsewhere, these notes focus on the inertial route to fusion energy.

For fusion power plants, the easiest fusion reaction to generate is that of deuterium, the heavy isotope of hydrogen which is readily separated from water, reacting with tritium, a yet heavier hydrogen isotope produced from the inexpensive element lithium carried in the coolant that removes fusion energy from the target chamber. (Click on green-colored text to go to a glossary with definitions of words). The first chapter of these notes, "D-T Fusion: What is it?", provides a simple introduction to this D-T reaction and the conditions required to initiate it.

Fig. 1 - A fuel capsule is compressed in a heavy-ion-heated ICF target, about the size of a nickel.

In inertial confinement fusion, small B-B-size hollow spherical capsules, most likely made of plastic, are filled at high pressure with an equal mixture of deuterium and tritium, and then chilled to cryogenic temperatures, so that the D-T gas freezes as a thin, solid coating on the inside of the capsule wall. Suspended by a thin plastic film at the center of a metal cavity called a hohlraum, these spherical capsules can be injected into the center of a target chamber. There, in a few billionths of a second, lasers, or beams of high-energy heavy ions as pictured in Fig. 1 above, can be used to heat the interior of the hohlraum cavity to temperatures several hundred times the temperature of the sun, vaporizing the surface of the plastic shell into an extremely high pressure plasma. Alternatively, direct-drive targets have no hohlraum, and lasers heat the capsule surface directly.

The capsule, transformed into vaporized plasma, reaches pressures of hundreds of millions of atmospheres. As the plasma expands outward like rocket exhaust, it accelerates the thin layer of D-T radially inward, to velocities of 300 to 400 kilometers per second. The residual D-T gas from the center of the capsule, heated by the denser D-T that surrounds and compresses it, reaches peak temperatures over 100 million degrees Celsius, sufficient to ignite a propagating fusion reaction. Just as a match can light firewood, this hot spot ignites a fusion burn wave that propagates out into the denser D-T. By releasing seventy or more times the energy originally needed to compress and heat the fuel, this dynamic process provides the basis for generating inertial fusion energy. The second chapter of these notes, "How ICF targets work," discusses in greater detail the physical processes that occur in ICF targets, and the issues related to manufacturing inexpensive targets and injecting them with high precision into target chambers.

How to Build an IFE Power Plant

Inertial fusion energy power plants will most likely use steam turbines and generators similar to those used in most coal-fired power plants. The source of the steam for the turbines, however, will be much different. Instead of large boilers, smokestacks, and equipment for unloading 8,000 tons of coal from rail cars each day, a 1,000-MW IFE power plant, making enough electricity for 3,000,000 U.S. households, would have three separate facilities: a target chamber and heat recovery plant, a target fabrication plant, and a driver. Economic studies show that, simply due to the cost of iron, copper, concrete, and other materials, the driver will dominate the cost of these three facilities. Once NIF demonstrates the scientific feasibility of ignition, the viability of IFE as an economical power source will depend on our ability to optimize driver designs, and to design targets and target chambers that minimize the driver energy.

The 192 large laser beams at NIF illustrate the philosophy of ICF driver design. Construction of NIF was approved only after scientists and engineers demonstrated that their laser design works by building a single laser beam line--the Beamlet. Because the cost of a single beam line is so much less than the cost of 192, the NIF laser designers optimized their design to an amazing degree. Like its predecessor NOVA, NIF increases the amplitude of a light beam by passing the beam successively through slabs of neodymium glass. In the case of NOVA, this involved a series of lens between several glass slabs, to increase the diameter of the beam as its intensity increased down the beam line. In each NIF beam line, four arrays of glass slabs have been replaced by a single array, using the exceedingly clever idea of an optical switch. The switch, in effect, transforms a transparent sheet of glass into a mirror so fast that it can reflect a pulse of laser light after it bounces through the slabs exactly four times, extracting most of the energy available in the glass, and then sends the light pulse thorough one more slab array and into the NIF target chamber.

Modularity makes it inexpensive to experiment with numerous ideas, not only in laser drivers but also in heavy-ion drivers, in ICF targets, and in target chambers, allowing ICF designers to explore numerous concepts and optimize the most clever ideas to minimize driver cost and energy. For NIF, the result of the optical switch and other innovations was a laser system 50 times more powerful than the fifteen-year-old NOVA, but only twice as expensive to build.

While the NIF lasers have several advantages as a research tool for studying ICF targets and achieving ignition, international IFE driver research also focuses on additional driver methods. The third chapter of these notes, "Drivers for Inertial Fusion Energy," discusses the use of accelerators, as well as lasers, to drive targets like the heavy-ion target illustrated in Fig. 1 above. Accelerators, currently under development at the Lawrence Berkeley National Laboratory on the mountainside above the U.C. Berkeley campus, and at Sandia National Laboratory and in Europe and Japan, possess several desirable features for driving IFE target chambers at high repetition rates.

At the end of the laser or accelerator driver sits one or more target chambers--physically separate and relatively inexpensive vacuum vessels where targets explode. In research facilities like NIF, target chambers provide rigid target-positioner arms which hold targets with high precision, while in IFE power plants highly accurate gas guns will inject targets to be hit on the fly. The physical phenomena which occur in target chambers provide intriguing topics for research: neutron energy deposition and material response, ablation by x rays, venting and condensation of ablation and target debris, and mechanical response of structures to loading by high-velocity gas and liquid.

While research and development remain to be done, concepts have been developed sufficiently to demonstrate the fundamental feasibility of highly-compact, high-repetition-rate target chambers, as discussed in greater detail in the chapter on Target Chambers for Inertial Fusion Energy . Based on these conceptual target-chamber designs, we already know that target chambers and their support equipment for target injection and tritium and heat recovery will provide a relatively small contribution to the cost of IFE electricity, as long as the target-chamber designs are sufficiently sophisticated and robust so that the chambers operate with high reliability and chamber maintenance does not reduce significantly the amount of time IFE plants can operate.

How Much Will IFE Electricity Cost?

Research investments made decades ago have brought enormous benefits to our present generation, in fields ranging from medicine to electronics to space communications. Today we can anticipate that future generations will live in a world where the cost of fossil fuels will be increasing, rather than decreasing as prices have in the last decade; where human activity generates yet greater stresses on ecosystems and even on the global climate; and where the quality of life will depend strongly on how efficiency and alternatives to fossil energy are implemented. The chapter "The Economics of IFE" discusses the economic characteristics of inertial fusion energy development that make IFE an attractive candidate for research and development investment, particularly when the scientific feasibility of ignition is demonstrated experimentally at the National Ignition Facility.

Economic estimates for IFE have relatively high reliability for a technology at such an early stage of development, for two reasons. First, considerable overlap exists between the equipment in an IFE power plant, and the equipment that would be found in a typical coal power plant for converting steam to electricity. In comparing IFE with its most important competitor, cheap and abundant coal, errors in these costs cancel out and disappear. Second, of the remaining cost of an IFE power plant, the majority, some 65 to 70 percent, goes to purchase a single large piece of equipment, the driver. This has two important implications. First, everything that researchers can do to reduce driver size, from advanced target designs to more compact target-chambers with smaller final-focus stand-off distances, can strongly influence IFE power cost. Second, because we can estimate the quantities of materials required to construct a driver of a given size, say the amount of magnet material (Metglas) and associated materials required to build a heavy-ion accelerator, we can reach relatively accurate estimates of IFE power cost. Current estimates based on current target designs yield electricity costs approaching a competitive level with coal and fission energy, at IFE plants in the 1000-MWe size range, with the cost dropping to beat these current energy sources as larger numbers of target chambers run from a single driver.

A second, perhaps more useful way to view IFE "economics" is in terms of environmental costs. These costs come from the impacts of the energy source on the environment: impacts from accidents, from waste disposal, and from the extraction of the raw materials required to construct and fuel the plant. By radically reducing the inventory of radioactive material that could be released by accident or careless waste disposal, fusion power addresses the primary concerns for nuclear energy sources, while retaining the most important environmental benefit--the extremely sparing use of natural resources. This characteristic of nuclear energy sources, the potential for very high power densities and thus very low resource consumption, provides the primary environmental incentive for its use. Put another way, unlike natural-gas and coal electricity, where the majority of the consumer's electric bill pays for nonrenewable fuel, for nuclear-generated electricity the vast majority of the consumer's bill pays for the salaries of engineers, technicians, and administrative support personnel, for human labor, not natural resources.

Nuclear energy's current competitive disadvantage with natural gas comes primarily from the fact that the taxes consumers pay to employ skilled people to make electricity, including federal and state income taxes, end up totaling over 100%, while the total tax on natural gas remains far lower. A revenue-neutral carbon tax, which would transfer tax burdens from people we would prefer to employ, to fossil fuels we would prefer to conserve, provides a natural way to level the playing field between natural-resource intensive energy sources, and those sustainable sources we would prefer to see developed.

Linkages to National and International Security: Nuclear Stewardship

The United States signed the Comprehensive Test Ban Treaty (CTBT) in 1996, based on the belief that halting nuclear testing globally will strengthen the barriers to the proliferation of nuclear weapons. To support the CTBT, the United States also initiated new programs of research, including the construction of the National Ignition Facility. The CTBT complements the Nuclear Nonproliferation Treaty (NPT), signed in 1973, which gave the promise of civilian nuclear technology to nations in exchange for their legally binding commitments to forgo the acquisition of nuclear weapons. Violation of Iraqi NPT commitments provided the legal basis for inspections and sanctions in Iraq to stop the secret nuclear weapons program there. The NPT provided the legal basis to apply strong pressure to stop North Korea's nuclear weapons program. Conversely, without the NPT barrier, India and Pakistan, who have refused to sign the NPT and CTBT, have recently performed multiple tests of nuclear weapons. Coupled with the START treaties to reduce the size of U.S. and Russian stockpiles, and the Laboratory-to-Laboratory program to help secure Russian nuclear materials and redirect Russian weapons scientists to civilian work, these steps represent major progress in arms control.

For the five nuclear weapons states, the CTBT creates substantial barriers to the development of new nuclear weapons, whether these be weapons with substantially different yields or weights, enhanced radiation, or directed energy. This ceiling on technical improvement makes it easier to proceed with arms-control agreements to reduce the size of nuclear arsenals to much lower levels. However, the CTBT also makes it impossible to demonstrate directly, by detonating nuclear explosions, that existing nuclear weapons remain safe and reliable. For the foreseeable future the United States will maintain some type of nuclear deterrent to neutralize the potential military advantage that could come from the acquisition and use, or threat of use, of nuclear weapons, and to continue to provide security assurances to other non-nuclear weapons states ranging from Canada to Japan to Taiwan. As long as the United States chooses to maintain this nuclear deterrence, a major weight of supporting the CTBT will fall on the shoulders of the three directors of the U.S. weapons laboratories, who must certify the stockpile annually.

ICF capsules will never have military application, because the weight of even the smallest conceivable driver makes the destructive capability of an ICF explosion orders of magnitude lower than any nuclear weapon, or the chemical and biological weapons that terrorists and rogue nations might pursue. The U.S. now funds the bulk of current U.S. ICF research not as a direct replacement for weapons tests, but instead for selected pieces of the puzzle the laboratory directors assemble to make their annual certification. Because these large drivers can heat and compress very small amounts of material to very high temperatures and densities, they allow experimental measurements of the material properties, fluid motion, and energy transport. This experimental information can supply in some, but not all, of the pieces needed to model aging nuclear weapons. More importantly, the enormous challenges provided by 25 years of ICF research have helped create a cadre of talented scientists and engineers at the national laboratories. In the long term, these scientists and engineers, and the new ones that will join them to work and publish in ICF and related areas, will provide the United States an important pool of expertise to draw on for the certification process to support the CTBT.

In the near term, U.S. ICF research provides an important contribution to supporting arms-control efforts, by providing an unclassified yardstick to measure the competence of the scientists and engineers who are also responsible for supporting the certification of the U.S. stockpile. The questions of arms control and deterrence are of great importance, and provide the subject for important debate. In this debate, we can remember that the successful ignition of an ICF target in NIF will have major implications about the potential viability of fusion as a future economic energy source. One can therefore hope that our current defense investments in ICF research will end up having the same enormous impact, by providing economically viable fusion energy, as earlier defense investments that resulted in communications satellites, electronic computers and microelectronics, and the Internet that now brings you this web page.

Conclusions

The scientific demonstration that fusion capsules with a few millionths of a kilogram of fusion fuel can be ignited with drivers providing a few megajoules of energy, the goal of the National Ignition Facility now under construction at the Lawrence Livermore National Laboratory, will have major implications for the potential economic viability of fusion power. By providing a substantially different technical approach and development path, IFE research complements current efforts to develop magnetic confinement fusion. Our current understanding of the requirements for IFE drivers, target chambers, and target fabrication facilities provide strong optimism that following scientific demonstration, engineering demonstration of an economical inertial fusion energy power plant can follow with a phased development program with modest budgets similar to those currently provided to other energy options. In addition, much can be done in the interim period before NIF reaches ignition to lay groundwork for the construction of the first ignition-class driver capable of achieving the repetition rates needed for an IFE power plant. Investigations of drivers, target chamber dynamics, and advanced target designs can all contribute to the understanding required to proceed with the construction of the first high-repetition-rate, ignition-size IFE experimental test facility some time around 2010.

Contents

• D-T Fusion: What is it?
  
  
• How ICF targets work
  
  
• Drivers for Inertial Fusion Energy
  
  
• Target Chambers for Inertial Fusion Energy
  
  
• The Economics of IFE
  
  
• ICF References
  
  
• ICF Links
  
  
• ICF Glossary