The current objective of the US Fusion Energy Sciences Program is the development of the science of fusion plasmas toward an eventual goal of demonstrating fusion's potential as a practical source of energy. Progress in the field has been such that the natural next step for the program is a burning plasma (BP) facility wherein the fusion process itself provides a significant fraction of the power necessary to heat the fuel to fusion conditions. Although such an achievement would constitute an important step in both the scientific community's and the broader public's acceptance of fusion, the question must be addressed as to the role and contributions of such an initiative given the US program's current scientific focus. This question has several components:

• What are the important scientific questions that can equally or better be addressed in non-BP facilities and, if so, how to avoid their pursuance being threatened by a BP initiative?

• What scientific questions require BP conditions in order to be addressed?

• What can be done to maximize the scientific return of such an experiment?

• To what extent would the science learned in a BP experiment, which today would unquestionably be a tokamak, be applicable and transferable to another magnetic configuration, should one be found to have superior power-plant characteristics?

• How could the institutional participation in a BP experiment be structured so as to assure the widest possible scientific participation?

Any BP initiative must answer these questions to the satisfaction of their interested stakeholders, and their combined answers contribute necessary scientific elements to a plan for proceeding on a burning plasma initiative. An important side benefit of addressing them well in advance of the start-up of a BP facility is the wider interest (and support for) such an initiative that could be expected to be generated.

Non-burning-plasma Issues

Scientific progress in understanding and predicting plasma behavior has been dramatic in recent years, especially in the areas of equilibrium and macro-stability, heating, current drive, edge and divertor physics, etc. However, two broad areas remain to be completed.

In the first area, while there has been much progress in understanding the mechanisms of heat transport via low-level turbulence, especially through the ion channel, there remains much to be done for the electron channel and in particle and momentum transport. Indeed, completing this task is one of the principal scientific challenges remaining in non-burning plasma physics. Needed are both adequate experimental time to exploit existing measurement techniques and resources for the development of new techniques, again, especially to measure turbulence on the electron time and length scales.

Although the underlying phenomena would likely be modified somewhat under BP conditions, they are inherent to all heated, magnetically confined plasmas. Having to address them under burning plasma conditions would greatly -- and unnecessarily -- complicate the task. This reality provides two elements of a plan for any BP initiative undertaken:

• Current non-BP facilities must continue to be made available to pursue the important topic of the role of turbulence in the transport of key confinement quantities, with assurance both for adequate experimental run time and for resources for the necessary new measurement instruments.

• The initial operation of a new BP facility must allow sufficient operation under non-BP conditions to permit the testing of earlier findings in the conditions of the new facility, e.g., smaller rho-star.

Continued access to existing U.S. facilities such as DllI-D and Alcator C-mod, together with resources for diagnostic development, should satisfy the first requirement. The second can be satisfied with appropriate allocation of runtime on the new BP facility before the start of deuterium-tritium, and even before deuterium-only, operation.

The second area of unfinished non-BP business has been called "concept optimization" or "alternates" and refers to the important task of assuring that the magnetic geometry of an eventual power plant is optimal within the constraints of the underlying physics. While the tokamak has certainly proved to be the most capable experimental device both for understanding fusion science and for reaching fusion conditions, it does not immediately follow that it would be the best approach for a fusion power plant, since a broader set of criteria apply. A well-balanced fusion development strategy protects the efforts to find a better product while pressing the frontiers with the best means presently available, and that strategy has become well engrained in the program over the last few years.

A third area deals with long-pulse/steady-state issues entailing a sequence of longer time scales. This area has both burning and non-burning aspects. Non-BP phenomena evolving over the shorter time scales can be explored in existing devices, but truly long-pulse phenomena, e.g., requiring equilibrium with the walls, will require a new facility on the world fusion scene. (The JAERI proposed JT60-S/C, for example, is motivated by the long-pulse requirement.) Long-pulse BP phenomena, of course, require a BP facility, and the achievable length of its pulse compared to important time scales becomes an important capability measure for any proposed BP device.

Combining these areas, we conclude that

• Any BP initiative must assure that the uncompleted non-BP fusion research in the area of concept optimization continues, meaning that funding for a BP initiative must be additive to, and preferably separate from, the current base program.

Scientific Questions Requiring BP Conditions

While a list of issues requiring BP conditions is quite extensive, it can be broken into two rather broad classes of phenomena.

In the first class are those effects that arise because of the existence of an energetic alpha population, such as Alfven modes driven by the fusion-generated alpha particles. While these can be studied to an extent under simulated conditions having an energetic, externally generated species in a non-burning plasma, many questions concerning their properties, and in particular their effect on the alpha confinement, will require realistic BP conditions.

In the second class are those effects that arise owing to the new feedback loop(s) provided by the self-heating generated by the extant plasma conditions. These are both more difficult to simulate on non-burning plasmas and are the effects most likely to produce surprises beyond our current capability for prediction. Important in this class is the issue of the competition between the goal of self-heating of the plasma, as well as self-driving of current through the bootstrap effect, with that of control of plasma profiles so important to advanced-tokamak operation. The precise fraction of self-heating needed to address these two classes of issues varies somewhat, although the generally accepted fraction is at least 50%, or an overall energy gain (Q) equal ten or greater.

Maximizing the Scientific Benefit of a BP experiment

An important evolution in recent years has been the opening of our major facilities to use by multiple institutions and/or or teams. This has made available to many university and laboratory groups the high-power and costly facilities needed to pursue advanced fusion research, and it has greatly enriched the research performed. As the principal MFE facility of a generation of fusion scientists, any BP facility will require thoughtful planning and similar organization of its research program so as to optimize its utility as a widely used scientific instrument.

Several features would be needed to assure the scientific benefit of a BP device: sophisticated instruments, or diagnostics, for measurements of important physical quantities and phenomena; detailed modeling capability for testing theoretical understanding in realistic geometry; adequate experimental flexibility and run-time; scientifically motivated experimental planning; access to data, etc. Sophisticated, high-quality, time- and space-resolved diagnostics have proved invaluable over the past several years in the rapid development of our understanding of magnetic plasmas behavior. Used together with theoretically based computer modeling, they have been indispensable in the maturation of the underlying science, enabling it to be applied broadly across MFE plasmas.

Clearly, diagnostics will likewise hold the key to unraveling the secrets of a BP. However, performing measurements on a BP having the quality of those currently in use on non-BPs will be made most demanding by the intense neutron flux and/or fluence realized under BP conditions. While some experience in the challenges to be met in this area has been gained in TFTR and JET operation, these facilities provide only a foretaste of what will be necessary in high-gain BP conditions. Development of diagnostics that will provide the measurements needed under BP conditions will be a daunting task and one which must be initiated well before a BP were to come into operations. Having an adequate diagnostic capability goes to the very heart of the scientific rationale for a BP. Assuring their availability should be given high priority throughout the project. We conclude, therefore, that:

• Intensive efforts should be initiated as soon as practicable to assess and design the measurement instruments necessary to pursue the science of a BP. This must entail not only developing neutron-tolerant instruments to measure most of the quantities routinely measured in today's experimental facilities but also those instruments needed for measurements of the new BP phenomena.

As suggested earlier, conducting measurements on a BP which are sufficient to unravel the underlying science may prove very difficult or, in some cases, even impossible. It was largely for this latter reason that it would be necessary to complete our understanding of plasma phenomena under non-BP conditions as benchmarks against which to compare related phenomena under burning conditions.

The complications raised by neutrons for diagnostic implementation have an institutional dimension, as well. Since diagnostics have provided the principal vehicle for university and other smaller groups to participate in current non-BP experiments, it is natural to assume that they should do the same for a BP. However, these are just the institutions that generally lack the expertise required to meet the added complications. To facilitate the access of these smaller groups, one such vehicle can be suggested:

• A centralized (possible "virtual") center of expertise be established for the purpose of working with teams developing BP diagnostics to solve the special problems raised by the neutron environment.

Because physical access to the experimental hall of a BP will be much more limited than current facilities, the rationale for "remote site(s)" capability becomes all the more compelling. Remote operation of diagnostics, and even entire experiments, has already been demonstrated successfully by exploiting high-speed, high-bandwidth links. With this technology continuing to improve, this capability provides yet another vehicle for multi-institutional participation. If the BP were international, e.g., the ITER-FEAT, the concept of remote operation takes on a whole new dimension by enabling round-the-clock experimental operation.

Broader Applicability of Tokamak BP Results

By a wide margin, the tokamak is recognized today as the only MFE confinement concept ready to proceed to the BP step, based both on the level of its performance demonstrated and on the understanding of its behavior achieved. In fact, it is that understanding, expressed in suitable computer modeling and design codes, that provides many of the tools for analyzing other configurations offering potentially superior characteristics to the tokamak as a power plant. There is every reason to expect that the transferability of BP physics should be as successful as non-BP physics, provided appropriate measures are taken to assure it. Key among these measures are diagnostics for measuring details of BP phenomena and the theoretical and modeling tools for analyzing and applying their results. If properly planned and implemented, BP science results from a tokamak will be every bit as valuable to the broader fusion confinement community as non-BP results have proven to be in the past.

Conclusion