Why such a gloomy introduction to a book about fusion, written by dedicated fusion researchers who are passionately committed to its ultimate success? The reason is to acknowledge, at the outset, an inescapable fact: fusion is difficult. Viewed from that perspective, society's sustained commitment to fusion over so many years is remarkable. The field continues to attract men and women who are motivated to confront the challenges of difficult problems, whether in plasma science, materials science, fusion technology, or engineering, and to have the opportunity to make progress and have an impact on an important enterprise. Fusion receives support from policy makers and funding appropriators in many countries for a host of reasons, but all tied in some way to the realization that the world's need for large-scale clean energy solutions is so great that the fusion's potential must be thoroughly understood, demonstrated, and evaluated. Most importantly, fusion research and development continues to make impressive scientific and technical progress in the face of formidable challenges. That is what this book is about.

Magnetic fusion research crossed a watershed in the first decade of the 21st century with an agreement by the European Union, Japan, China, Russia, India, South Korea, and the United States to build ITER, an experimental project to explore the science and technology of maintaining fusion conditions in a plasma self-heated by its own fusion power, that is, a burning plasma. We can pinpoint a watershed moment on November 21, 2006, the date the ITER agreement was signed in Paris by all seven parties, but in reality the line that was crossed was wide and gray, not sharp and black. The decades leading up to the ITER agreement were marked by research progress across a broad spectrum of fusion topics, much of it achieved through plasma physics experimentation in magnetic fusion machines. As the ITER design came into focus throughout the 1990s, the global fusion community responded by sharpening its research focus on topics that were the most critical for securing the scientific and technical basis for ITER's design. From that perspective, the amassing of a scientific knowledge base sufficient to justify construction of a huge international experiment that will take fusion into the burning plasma regime is an accomplishment that must be viewed as a capstone of the first half-century of fusion research.

Having crossed into what could be called the “ITER Era,” the fusion community has begun to tackle the next set of challenges on the journey to commercial fusion. Most prominent among these challenges, of course, are the construction of the ITER experiment itself and the successful achievement of its burning plasma mission. With the international ITER project team now manufacturing the massive components that will be assembled to form the core tokamak device and developing the supporting infrastructure on the Cadarache, France site, fusion researchers continue to train their experiments toward topics critical to ITER, now prototyping plasma control solutions that will be usable on ITER to optimize its performance and to maintain a burning plasma for several minutes at a time.

In addition, now that ITER is moving forward on its mission to clarify the conditions for controlling a burning plasma, there is already heightened attention to the challenges that lay beyond that mission. Future fusion systems will have to sustain continuous burning plasma operation for months, interrupted only for brief maintenance downtimes. For commercial applications, the fusion energy released from burning plasmas will have to be captured and converted to electricity, and fusion systems will have to breed enough tritium to maintain their own fuel supplies. To address these requirements, a new generation of plasma machines has begun to investigate control strategies for long-pulse to steady-state plasma sustainment. Fusion materials research has expanded, investigating plasma-facing materials that can survive for long periods under exposure to intense charged particle and neutron fluxes, as well as the structural and functional materials required for power extraction and tritium breeding in so-called fusion blankets. The pace of fusion reactor technology development is increasing, with the design of breeding blanket modules now under way in several countries in preparation for being constructed and then tested in ITER. Finally, countries are beginning to plan the next major steps that will follow ITER, namely DEMO machines that will integrate a burning plasma with systems for power extraction and tritium breeding to demonstrate continuous operation, a closed fuel cycle, and net power generation from a fusion plant.

There is no single, unique way to chart the progress in fusion research toward commercial power plants. Experimental machines are nothing more than equipment that researchers use to build the scientific and technical foundations for fusion that will eventually provide society with the knowledge of what is required to obtain net energy from a fusion system safely and economically. Still, it is a reasonable approximation to say that progress in magnetic fusion research has been and continues to be paced by results obtained from a wide array of experimental facilities around the world. This book looks at the field from that perspective, examining 12 major fusion experimental machines in terms of their contributions, actual and planned, to fusion knowledge. Included in the list are two machines that are no longer operating, one that only went into operation shortly before this book went to press, and one still under construction. Of the remaining eight, the oldest has been in operation since 1983, the newest since 2006. Following the watershed metaphor introduced earlier, this book organizes the machines into two groups of six—those whose major contributions helped establish the scientific foundations for ITER (Part Two) and those whose major contributions have come after the ITER design was largely settled and thus are laying the foundations for ITER operation and steps beyond (Part Three). The reader will readily discover that this division is an imperfect one, another approximation, intended only to convey a sense of what was accomplished that made ITER possible, and the new opportunities made available by the decision to go forward with its construction. Preceding the machine chapters are a set of introductions to broad topic areas that make up fusion science—plasma performance, plasma exhaust, power extraction, and tritium self-sufficiency (Part One). The book closes by departing somewhat from experimental machines per se with chapters highlighting key technological developments—plasma heating and current drive, plasma fueling, and plasma diagnostics, and an example fusion power-plant design (Part Four).

The book's coverage, while sufficient for its intended purpose of explaining fusion's march from experiments to power plants, is by no means comprehensive. The reader may wonder about machines that have made important contributions but are not included in Parts Two or Three, important science topics not addressed in Part One, or important technologies or power plant concepts not described in Part Four. It must be emphasized that any perceived omissions were not the result of value judgments made in the book's preparation. The reader will be well served, however, by what is included. The content has been produced by those most qualified for such a task—leaders of the experimental projects that are described, and world-recognized authorities in the fusion plasma physics and technology topics that are covered.

As we have noted, the development of fusion power plants is an enterprise that will span multiple generations. A generation of fusion researchers delivered the accomplishments described in this book, but the accomplishments of the ITER Era will be the responsibility of a new generation. Fusion is in a period of generational transition. With that in mind, this book is written for those who will move the field forward during the ITER Era, researchers who today are either relatively new to the field or are contemplating a career path in fusion. It is hoped that this book will provide them with a useful scientific perspective on fusion's recent history and will help them chart the wisest possible course for fusion in the years to come.