Introduction

We have been working on the research of magnetic fusion energy and science for more than 50 years. One cannot fail to notice that its development path has similarity to the support, design and construction of cathedrals a few centuries ago. Both are multiple-generation quests. The building of cathedrals has the noble goal towards the glorification of God and the manifestation of regional prosperity. To be successful it required the combination of the need as perceived by the people in power and/or who could direct large financial resources for dedicated architects and artisans to design and build the many magnificent cathedrals with materials available at that time. As an example, it took a couple of generations before the successful construction of the elegant Brunelleschi's Dome of Florence even with the example of the Roman Pantheon, which was already built more than a thousand years before. Compared to our quest of fusion energy in the 20^th and 21^st centuries, we have a pragmatic goal of fusion energy, but the requirements of programmatic support and the challenges of seemingly impossible design goals are similar to the building of cathedrals. This comparison tells us that there is precedent in human quests and financial restriction is not paramount, yet substantial support will have to be developed if the market force alone is not sufficient. On the other hand, there is a significant difference between the building of cathedrals and magnetic fusion devices besides the obvious, which is that of communication. We in the 21^st century are able to investigate and distribute ideas in much more rapid pace as demonstrated by a note like this. We have the privilege of utilizing rapid communication but at the same time in order to get public and political support we have the responsibility of trying our best to find the most efficient technical pathway to achieve our goals while recognizing and overcoming various programmatic constraints. With the present situation of ITER getting toward its final decision phase of being constructed and the upcoming Snowmass meeting we have the opportunity to work towards a direction for fusion development that we can be proud of in making timely scientific progress with a complementary machines program, which will then help to justify the trust and support that our government has given to us.

In terms of the development path, it is clear that ITER will provide significant scientific and technical progress in the areas of long burn DT tokamak physics of hundreds of seconds and operating advanced superconducting magnets, which is an obvious approach to minimize the re-circulating power when compared to normal conducting magnet designs of the same fusion power. However, since it is superconducting, which is also a contributing factor to the present size and cost of ITER, it will have limitations in operational and research flexibility when compared to normal coil designs. To complement ITER, we will need in parallel to provide other less costly machines for materials development and continue the improvements of finding a robust high performance tokamak physics operation regime. From the plasma physics point of view, it does not matter whether superconducting or normal conducting magnets would be generating the confinement field. With less shielding distance needed for the central column, which translates into a smaller machine, a normal conducting machine costing in the range of 1 $B can be envisioned, an example of which is FIRE. Ignitor will surely also provide invaluable data on the physics of ignited plasma. A longer burn normal conducting coil machine will also be complementary to Ignitor. As a development path we should emphasize the complementary nature of this three tokamaks approach. A US normal conducting coil burning plasma machine could become another international machine suitable for scientific exchanges and collaborations. With this three tokamaks approach it becomes logical to develop a tokamak advocacy attitude.

If we were given a job to select and recommend the next burning plasma machine, which most likely would be a tokamak, there is no reason that we should not take an advocacy position, which means that in addition to satisfying the burning plasma physics requirements we should review our fusion program goals and the fundamental concerns about the utilization of the tokamak to produce fusion energy or for other applications (e.g., hydrogen production and fission waste destruction), before we make our recommendation. This advocacy approach is a way to focus our attention in a positive manner, but it should be in no way diminish the US programmatic attention to alternate concepts as represented by IFE and other innovative fusion concepts. Once we take the advocacy position for tokamak research, to be responsible, we would then ask the following two questions: 1. What would be the most suitable normal conducting machine that we should recommend? 2. What are the critical issues that will need to be addressed in order to enhance the success of the tokamak concept to produce energy? To answer these questions we should surely take advantage of the technical progress already made in the tokamak research.

To answer the first question in the search for the most suitable normal conducting tokamak the defining geometric parameters are major radius Ro, aspect ratio A, elongation K, and to a lesser extent the triangularity d. The impacts from Ro are clear. The impacts from A and K are actually well established from equilibrium physics. They have strong impacts to ß_N, plasma stability, coil design and cost. However, their parametric impacts are less represented from experimental results. The parameters of Ro, A and K also have direct impact on the volume to surface area ratio (V/A) of the toroidal plasma. This seemingly simple V/A parameter can be used to provide a feeling on the relative performance contributions from the core physics, scrape-off-layer physics, and the corresponding interaction between the plasma and material interface, and impurity transport from the chamber wall. Relatively, higher V/A should have less performance impact from the edge or surface effects. It can also provide an indication to the relative cost of electricity for power reactors, since the coil cost, representing a large fraction of the cost of the fusion power core, is mostly affected by the plasma surface area. In general for plasma performance and cost, we should go after higher V/A, which means the increase in Ro, reduction in A and the increase in K. In the case for the variation of A, if we consider machines with A from about 1.3 to 4 as tokamaks, they are already represented by NSTX and MAST as low A=1.25, DIII-D as medium A=2.5 and TFTR and others as higher A>=4 machines. However, this direction of reducing A to get higher physics performance and lower cost is only part of the story for the burning plasma device, especially when we take into account the coil design.

When we consider the normal coil design for the burning plasma machine, the observation on A is not as simple. For A<1.6, due to the restricted geometry of the central column, when radiation damage is taken into consideration, the TF coil central column design becomes very difficult and may be impossible. If adequate inboard shielding is applied, the device would become large and the fusion power and capital cost would become unacceptably high. At the other hand of the spectrum, for A>3, because of the lower ß_N, and the 1/R dependence of the toroidal field, the B-field at the coil will be high, which translates into high coil stresses and when coupled to the minimization of re-circulating power, these characteristics would limit the performance of the machine. This is also why FIRE has to use cryogenically cooled coils, which may not be necessary with lower A designs. These observations give us an indication that medium A in the range of 2 to 2.5 and K in the range of 2 to 3 may be appropriate for the burning plasma machine. These ranges may turn out to be suitable in terms of improved physics performance while meeting different technology restrictions, and the design would then be based on a reasonable extension of present knowledge and database while providing challenges in achieving high performance regimes. Interestingly, this ranges of A and K could also be suitable for both superconducting and normal conducting coil reactor designs [1].

To address the question of fundamental issues for tokamak designs I would like to post a few recognized issues. But before I do that, it is important to note that up to this point of tokamak research we have been operating with tokamak experiments and most of the approach has been to push for the highest performance that the machine can deliver and naturally pushing the envelope of specific machine parameters. Therefore, we become acquainted in studying instabilities of various types with very restrictive flat-top duration of <10s. Different transient events like disruption and type-I ELMs become very worrisome when they are extrapolated to larger and higher performance machines. On the other hand, as demonstrated by the TRIAM-1M experiment, that the plasma could be operated for hours with lower plasma performance. The reason for me to mention this is simply using this as a reminder that some of the concerns that we have could be relaxed with a larger machine and/or operating further away from the operating limits of the selected machine. Therefore in trying to address critical issues for the tokamak concept, we have to distinguish issues that could be relaxed and issues that could get worse with a burning plasma. The latter could be represented by the issue of plasma material interaction with DT plasma including effects from radiation damage.

There are colleagues much more qualify than myself to expound on the physics requirements of burning plasma devices. However, as we plan for a $B class tokamak burning plasma experiment it should also be used to address some of the critical technology issues related to the successful development the tokamak as an acceptable energy producing concept. The following is a short list of tokamak issues that we should pay attention to when we are designing the machine.

Disruptions are the fundamental mental barrier for the acceptance of the tokamak concept for power production. But it is clear that disruptions can definitely be avoided if we don't push for the operational limits of the machine. We are beginning to understand the fundamentals of disruption physics, avoidance, detection and mitigation of disruption impact. Since it has significant impact on the machine design, we have to make sure that advanced DT burning plasma operation will have minimum disruptions, which could mean the possible approach of over design by increasing Ro and by proper selection of A, among other remedies.

This area is another feasibility area for the tokamak concept and we will learn a lot more from DT burn experiments. But the necessity of hot spot and peak particle flux control is obvious and various scenarios like radiative divertor and mantle radiation should be assessed for the burning plasma design.

Directly related to the topic of heat and particle flux control are the areas of particle transport, scrape off layer physics and plasma wall interaction under the burning plasma conditions. These areas will also directly help to define the burn time and corresponding availability of the tokamak concept. Development of the predictive modeling and the identification of acceptable first wall and divertor plasma facing materials and designs will become critically important.

As we are beginning to understand the evolution of high frequency "grassy" ELMs to the unacceptable low frequency type-I ELMs for high performance discharges, we have to identify burning plasma regimes that would allow us to predict the acceptable physics operation regime for heat removal and for steady state power producing reactors.

We have to find means that require low input power, to provide the necessary density and temperature profile controls that would allow the plasma to operate with high bootstrap fraction, which is the only way to reach the necessary power balance for any tokamak power reactor. This also means the development of high efficiency profile control approaches.

When we ask the question of optimum configuration for the normal coil burning plasma machine, the impact of coil design should definitely be included. Feasible coil design assessment involves complicated trade-offs between allowable coil current density, OH coil requirements, Ro, A, K and d, fusion power output, operating scenario; and innovative coil designs, etc. This area of design should be treated as equal in importance to the search for the most appropriate burning plasma physics regimes.

As an experimental burning plasma device, we would expect the need for frequent maintenance and components change out. Gradual implementation of fully remote maintenance approach should be part of the design assessment. But the basic approach has to be defined early, even at the pre-conceptual design phase.

Other generic issues for any burning plasma device to be addressed are fusion material irradiation tests, development of radiation resistance diagnostics, acceptable power balance and capital cost, just to name a few.

The reason for me to list the above is not to expose my ignorance, but simply to illustrate the result of taking the advocacy position for tokamak development, which then very easily leading to the observations of the necessary integrated approach of going after the burning plasma machine, with a goal of resolving most of the critical tokamak issues on the pathway towards the development of an acceptable fusion energy source.

It should be emphasized again that the tokamak advocacy approach is a means to focus tokamak research, and it should be by no means detract from the programmatic need of developing alternate concepts like the IFE and innovative alternate concepts.

My recommendation is that we should make use of the Snowmass meeting as suggested in doing a sincere physics and technology assessment of the ITER, Ignitor and FIRE machines as points of focus. However, we have an opportunity to make significant progress in fusion. It is necessary for some of us to take the tokamak advocacy position to find the most suitable normal conducting burning plasma machine for the US and possibly offering it as another international machine to complement ITER and Ignitor. The recognition that the quest for fusion energy development is a multi-generation research should relax the need for making hasty decisions, yet at this age of democratic decision making and advanced communication, we have the responsibility to provide the most suitable technical development path. The maturity of tokamak experiments has led us to the next stage of development, which would become more cross discipline in nature and the integrated approach of research will be crucial to meet the energy goal of tokamak development. Considering the trade-offs between physics performance and magnetic coil design constraints, A in the range of 2 to 2.5 and K in the range of 2 to 3 should be assessed. This assessment would become a significant challenge for the US fusion community to select and justify the most suitable machine by the year 2004. However, I believe that this generation of fusion scientists/technologists/engineers are up to the challenge and will be ready to pass the torch to the next generation of fusion power producing machine builders.

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