Readiness and Innovation in Technology
Richard Nygren (Sandia National Laboratories)
and Sam Berk (US Dept. of Energy)
This paper frames the goals of technology R&D in the Magnetic Fusion Energy Program using the themes of innovation and readiness. Both are necessary aspects of development. Examples are drawn from R&D on Chamber Science.
There is inevitably a tension between near term goals and long term goals for development of technology. Hardware is needed for new and ongoing projects. And we must demonstrate a sound nuclear technology for fusion reactors at some point to give credibility to the program's goals. Each requires effort and, with a constrained budget, decisions when and how to proceed.
Be ready! At the Snowmass Workshop, the two elements of this tension were captured well in the introductory remarks. Bob Conn described the strong influence on the fusion program of external events, such as the oil embargo of the 1980's; he advised the fusion program to develop and maintain a position so that it can take full advantage of such externally prompted opportunities.
Innovation and vision! Other speakers emphasized the need for a clear vision of fusion energy and the importance of innovation in the program.
Fig. 1 illustrates the general roles of innovation and readiness. Innovation brings new ideas or applications into the program and enables our vision of the future by surpassing the state-of the-art. Research and development (R&D) is the pathway of realization from innovation to readiness. Readiness is the capability to deliver proven products as required by the near term goals of the program.
Two example of "readiness" in fusion technology are (1) the preparation for D-T shots in TFTR and (2) development of the divertor for ITER. Both cases were combined efforts in science and engineering and in physics and technology.
In readying TFTR for its D-T campaign, an important question loomed - How much tritium would be collected in the graphite walls? The objective here was a number -- a technically well supported answer was needed to develop operating scenarios that met stringent safety requirements for approval of this experiment. An interdisciplinary working group of plasma physicists and researchers in plasma surface interactions from across the US (TFTR D-T Physics Group) discussed the problem, peformed lab experiments, analyzed new and old data, and developed the answer [and enabled TFTR's successful D-T campaign].
Development of the ITER divertor was a challenge in both engineering and physics. The development path involved innovations in physics, as in the application of radiating edge plasmas, and innovations in technology, such as the US-developed "tungsten brush" armor that advanced the performance of water-cooled plasma facing components. Iterations in the design specifications narrowed the heat and particle loads to a reasonable range. This was a massive interdisciplinary effort by researchers worldwide and was perhaps the most technically challenging aspect of the ITER project.
Much was done to ready an ITER divertor for production. The R&D involved the development of concepts and the modification, improvement and ultimately the demonstration of a technology. In some cases, this included fabrication of a prototype and its qualification in a suitable acceptance test. An example is the complicated precision casting, with many internal cooling passages, of a prototype divertor cassette body done for ITER. This casting of the prototype extended the technology and demonstrated a fabrication method that would save considerable cost by reducing the final machining needed.
Let us now extend the previous figure so that time can be included. Fig. 2 shows several innovative contributions from technology in the development path for a fusion reactor. There are three important points in Figure 2.
First is readiness. The two examples above represent technologies, such as tritium processing and robust PFCs (e.g., the water-cooled "brush armor" developed for ITER) that may be needed soon for the next generation fusion experiment and have proceeded well along the R&D path toward realization.
Second is the intermediate R&D that brings the realization of innovative ideas. The development of liquid walls is an interesting example that is revisited later.
The third point in Fig. 2 is how our vision of the fusion program is defined by the "long term" technologies. How would magnets with superconducting strand that is half the present cost, flowing liquid walls, minimization of radioactive waste, etc., enhance our vision of the realization of fusion energy?
The "long term" technologies have a significant near term objective
in helping to define our vision of the fusion program.
For example, US policy on radioactive waste is important for siting of any facility that will use tritium and produce radioactive waste. While some ramifications of this policy will be important for the next step device and others are "longer term," many long-term issues are also very important right now in defining our vision for the future. To the degree we ignore these aspects, the resulting less-complete vision can imperil our progress.
Our vision for the future is a partnership that involves all parts of the fusion program. We all know that partnership between "physics" and "technology" is necessary; we also recognize a tension between near term goals, driven largely by physics, and long term goals for technology development. Our common goals include:
1. finding a satisfactory balance between physics and technology programs, and
2. promoting strategies that (a) lead to effective development paths for the technology programs and (b) foster the productive partnerships among research programs in fusion.
We expect that that some innovative technology concepts will have a significant impact in defining our vision of fusion energy. That vision should in turn help guide the overall fusion program in the planning of experimental programs and facilities. Figure 3, while still emphasizing the role of technology, also includes the complementary and interactive relationship with physics in developing reactor concepts and defining a vision for fusion energy.
The potential of liquid walls to alter our vision of magnetic fusion is a good example. We expect that in a fusion device with liquid wall concepts, the plasma behavior at the edge would differ from a device with solid walls. [The liquid walls would pump particles from the plasma at a faster rate. This would significantly raise the edge temperature, and also the required fueling rate, and overall, the temperature profile of the plasma would differ from that expected in a device with solid walls.] To learn whether we can indeed produce and utilize such a plasma, we will need to do experiments. Likely tasks include laboratory experiments, modeling, early experiments in small fusion devices and the introduction of liquid metal divertors in a fusion device with moderate to high power.
Experiments with liquid surfaces present several challenges and will not be easily incorporated into magnetic fusion devices with high-power plasmas. However, this clearly must be done. The suggested intermediate goal of deploying a liquid metal divertor has a dual physics and technology mission. It could provide (a) heat management in existing or near term devices and (b) experience with MHD effects on conductive liquids flowing in a high magnetic field that is necessary for the future development of flowing liquid walls and difficult to obtain in any other way.
This development path will take several years. The main point here is that (parallel) development in magnetic confinement concepts should be coordinated with and influenced by our vision of advances in technology. [The reverse should also be true.]
The final punch line here is that our definition of readiness must be expanded. To be ready, in the sense communicated by Bob Conn, our technology programs need several attributes.
Still the first priority is the ability to develop and deliver hardware for fusion projects. This is clearly necessary but is not sufficient.
Second, there must be sufficient activity with innovative advances in technology to explore the benefits and challenges of applying these ideas to fusion. There must be enough real work (experiments, modeling, analysis of applications, etc.) that we can distill whether these ideas can be realized, and with what benefit and cost. Such work shows progress toward (and may modify) our vision of fusion and is necessary to confirm our progress to ourselves and others.
Third, our programs in technology must have sufficient depth and balance that they are "poised to jump" when an opportunity for advancing the fusion program presents itself. Sufficient breadth means the range of available talent can enable some rapid redirection of the program's goals. Sufficient depth means there is enough ongoing work and planning that we can shift gears to an enhanced program with greater and more immediate objectives without the delay of perhaps 3-5 years required to implement an essentially new program.
This last attribute implies something beyond a collection of narrowly targeted R&D activities and budget limitations will certainly be an important limiting factor.
Still, a technology program with the attributes above is the appropriate goal. Our challenge is to find a way to develop such a program. One important avenue for such opportunities is collaborations with international partners. Another is shared missions and stronger partnerships between physics and experimental technology on US confinement experiments. Neither of these is new, but we must use them to the best advantage with a shared future vision of fusion.