Mark Tillack and Dave Petti

Technology research in the US Fusion Energy Sciences program serves two distinct roles, both of which are central to the development of an attractive fusion energy source. "Plasma enabling technologies" are needed to support the creation and control of reactor grade plasmas, whereas "fusion energy technologies" are needed to create a useful product that meets economic, safety and environmental requirements. In both cases, innovation and improvement are essential if the vision of an attractive fusion energy source ever will come to fruition.

In the same way that improved fundamental understanding of plasma physics is expected to help create more desirable characteristics of burning plasmas, improved fundamental understanding of the engineering sciences is expected to help create a desirable end-product. A broad array of engineering sciences is invoved in fusion technology, including chemistry, solid state materials science, nuclear science, and fluid mechanics, to name only a few. In this document, we attempt to enumerate several of the more important disciplines and expain how advances can serve not only to accelerate the development of an attractive fusion energy product, but also to push the frontiers of basic engineering sciences.

The restructured fusion energy sciences program has adopted a strategy in which exploration of alternate confinement schemes has been embraced as a means to seek potentially more attractive concepts for fusion energy applications as well as to increase our overall understanding of plasma confinement. Analogous changes have been implemented in the fusion technology program, where new ideas for both plasma enabling and fusion energy technologies have been encouraged. In order to provide a framework for evaluating and comparing progress for alternative technologies, and to establish guidelines for major programmatic decisions, clearly-defined stages of development are needed.

Fortunately, an applicable framework already has been articulated by the FESAC Panel on Alternate Concepts [1] and the FESAC Panel on Criteria, Goals and Metrics [2]. The major stages of development include "concept exploration", "prrof of principle", "performance extension" and finally "fusion energy development". This document seeks to apply and extend the FESAC methodology for specific relevance to advanced fusion technology concept development.

Because the fusion technology program contains science, engineering science and applied engineering facets, we begin by defining what we mean by the advancement of science for fusion. Contributions that technology makes to advancing science include:

Here we discuss specific aspects of the technology program that fit one of these definitions in the areas of:

There are numerous examples where fusion materials science research has had a positive impact on the broader engineering/materials science fields. For example, a bainitic (Fe-3Cr-W-Ta) steel with superior toughness and strength to existing steels was developed by fusion researchers which have potential applications in numerous commercial systems (e.g., fossil energy.). In the area of composite materials, fusion materials research has led to improved oxidation resistant interphases in SiC/SiC composites. Fundamental research on ceramics by fusion researchers has led to the first known experimental measurements of point defect (interstitial) migration energies in SiC, alumina and spinel.

Experimental and theoretical analysis of neutron-irradiated metals is providing an improved understanding of the fundamentals of mechanical deformation, which has far-reaching impact on numerous engineering disciplines. For example, it appears possible to obtain the constitutive equations for twinning (which is one of the 6 possible deformation mechanisms in solids) from an analysis of neutron-irradiated metals. Most of the present-day understanding of fracture mechanics (essential for all advanced engineering structural applications) is derived from early studies on neutron irradiated metals. Further analyses of on-going fusion materials experiments with neutron-irradiated materials can provide fundamental understanding related to the physical mechanisms of flow and fracture of deformed metals. Significant advances in the science of mechanical deformation of refractory metals are being provided by fusion research on vanadium alloys and other refractory metals.

Better understanding of the complex interactions between plasma physics, surface science, solid state physics and fluid mechanics that occur at the plasma-to-material interface has led to improved control of edge density and impurity influx and thus confinement device performance in the last fifteen years. This understanding has been achieved through a combination of laboratory experiments, experiments and measurements on confinement devices, and first principles modeling of the observed phenomena, and is the scientific basis that underpins our ability to "engineer" the plasma edge in fusion devices.

The scientific basis needed to engineer the edge requires understanding the underlying chemistry, solid state physics, and fluid mechanics that controls the transport behavior of plasma particles back to the plasma, in the edge itself and in plasma facing materials. Fusion experimental studies in the laboratory and on existing fusion devices and theoretical analysis on impurity transport in the plasma contribute to the fields of plasma physics and atomic physics by providing fundamental data to describe transport in the edge at fusion relevant temperatures and densities. Extensive surface science experiments are being conducted to understand the mechanisms controlling the release of particles from surfaces, such as sputtering and evaporation chemical erosion as a function of ion energy, angle of incidence and level of impurities for both liquid and solid materials. Surface science measurement of uptake and subsequent behavior of deuterium in plasma facing components contributes to the field of mass transport in solid materials. Fusion measurements add to the database on the diffusivity and solubility of hydrogen in solid materials and the influence of radiation damage on the overall permeability of the material.

Liquid metal MHD is a large and growing scientific discipline with applications in a diverse set of fields including metals processing, geophysics (e.g., geodynamo studies), boundary layer control and turbulence studies. Fusion researchers developed the first general 3D numerical method for the solution of the inviscid core flow equations (including vorticity generation and decay) and provided some of the most detailed data on MHD fluctuations at high field strength (well beyond the transition to bulk laminar flow). The latest efforts are providing improved understanding of MHD flows and turbulence at free surfaces.

For example, heat transfer at the free surface of a non-conducting liquid wall is dominated by phenomena of rapid surface renewal by turbulent eddy structures generated either near the free surface due to temperature gradient driven viscosity variations, or near the back wall or nozzle surfaces by frictional shear stresses. The intensity of these turbulent structures and their effectiveness in cycling energy from the free surface into the bulk flow of the liquid wall depends heavily on the velocity of the main flow, the stability of the free surface, the distance from back wall and nozzle surfaces, the degree of damping by the magnetic field and even the magnitude and distribution of the surface heat flux itself. This is a challenging interdisciplinary scientific problem, with relevance to fields such as oceanography, meteorology, metallurgy and other high heat flux applications like rocket engines.

The picture is different for a liquid metal, which may be fully laminarized by the magnetic field, but is still likely to be highly wavy or possess two-dimensional turbulence-like structures with vorticity oriented along the field lines. Surface waves and 2D turbulence increase the area for heat transfer and have motion that helps to convect heat into the bulk flow. Understanding the relative importance of these terms to the dominant conduction and radiation transport effects, and judging the effectiveness of using turbulence promoters such as coarse screens, is required to assess the feasibility of liquid metal walls from the heat transfer point of view. The complicated hydrodynamics is now heavily coupled to the applied magnetic fields and the motion of the plasma through Ohm’s law and Maxwell’s Equations. The solution to these systems is of similar complexity to the MHD fluid motions in the plasma.

Pulsed energy deposition processes dominate the engineering of IFE chambers, and are of primary importance in predicting disruption effects on plasma-facing surfaces. Comprehensive models have been developed for energy transport, deposition and interaction with evolving species from the wall. An example where improved understanding of pulsed energy deposition can benefit fields outiside of fusion is vapor shielding. Evolution of materials and the subsequent interaction with the incident beam is one of the factors limiting the resolution of microfabrication with lasers.

Determination of the inertial fusion chamber environment following a target explosion is another example of complicated, interdisciplinary scientific exploration. The x-rays, neutron and debris emanating from the exploded target must be absorbed by the chamber and a reasonably quiescent condition must be re-established before the next shot can take place. The phenomena that must be understood and modeled include photon transport in gases and condensed matter, time dependent neutron transport, ionized gas dynamics and radiation hydrodynamics, ablation and thermo-physics of rapidly heated surfaces, dynamics of large scale free liquid flows, and the condensation heat and mass transfer. Simulation tools have been developed or adapted to model these different processes, and work proceeds towards integration into a code that can simulate all relevant physics of the chamber.

The mobilization of induced activation in fusion materials under air and steam ingress conditions is an important safety issue in all fusion designs. Chemical reactions between fusion materials and air or steam change the chemical form of the structure and in the case of steam can lead to the production of combustible quantities of hydrogen. In addition, the presence of air or steam changes the oxygen potential of the system resulting in vaporization of radioactivity from the structure. Experiments and extensive materials characterization are performed to measure the chemical reactivity of fusion materials and the concurrent vaporization. Basic thermodynamic information on the reactions of interest and gas phase and solid state mass transport models are used to understand the results of experiments. The data on the behavior of fusion materials in high temperature air or steam environments adds to the database on chemical thermodynamic behavior of materials that has applicability for high temperature and chemical processes in the aerospace, defense, chemical, nuclear and environmental remediation industries.

Plasma-wall interactions in magnetic fusion (e.g., erosion, disruptions) and target implosion in inertial fusion result in the production of particulate. The presence of particulate could impact operation of the facility. Particulate deposited on mirrors and diagnostics could impair these systems. In addition, the particulate has important safety implications; it could be radioactive, toxic and chemically reactive. Thus, characterization of the particulate in fusion systems is needed to better understand the safety hazard and operational limits that such material could impose on the system. Particulate production is a broad area of aerosol science that has important industrial applications including for example cloud formation, pollution control, precipitation technologies, and production of nanophase material. State of the art aerosol characterization technologies are used to measure the quantity, size and chemical form of the particulate in both magnetic and inertial fusion facilities. In addition, theoretical modeling of the plasma and the relevant vapor condensation, nucleation and growth phenomena are used to understand the basic mechanisms responsible for particulate production in magnetic and inertial fusion systems. This work is expected to add significantly to the understanding of aerosol nucleation phenomena and the transition from plasma to supersaturated gas to particulate.

In order to demonstrate the scientific, technological and environmental attractiveness of a technology concept for the utilization of fusion energy, several stages of development are anticipated. Technology stages of development can be defined in terms analogous to stages of development for confinement concepts that previously have been articulated by FESAC [1,2], including:

These stages of development of a concept actually represent points on a continuous scale. However, pragmatically, the boundaries between various stages usually represent quantum changes in the cost of program, in the level of commitment to that concept, and in the focus of the program. In each stage, the research program contains experiments, theory and modeling, and design studies elements. The mix of these elements varies in each stage, but at least one main experiment is needed together with an array of supporting modeling, power-plant and design studies, and supporting plasma physics that may be needed for a concept.

Technology research in the US Fusion Energy Sciences program serves two distinct roles, both of which are central to the development of an attractive fusion energy source. "Plasma enabling technologies" are needed to support the creation and control of reactor grade plasmas, whereas "fusion energy technologies" are needed to create a useful product that meets economic, safety and environmental requirements. This document seeks to apply and extend the FESAC methodology for specific relevance to advanced technology concept development as it relates to either of these two program elements.

As with the development steps, a set of metrics and criteria also has been established by FESAC, including (see Appendix A):

For each of the first three development stages, we describe the technical and programmatic features and key metrics for defining success and deciding on the readiness for a concept to proceed to a given step. The decision to proceed from one stage to the next should be based on the maturity of the concept in order to be reasonably confident that: 1) the next stage of the program will be successful; and 2) the anticipated benefits of the next stage of the research justifies the increased level of effort.

Concept exploration programs are aimed at innovation and basic understanding of relevant scientific and engineering phenomena. They consist of experiments costing typically less than $1M/year and/or theory and modeling, and strive at establishing: 1) the basic feasibility of a concept (e.g., for a first wall/blanket system, these issues include establishing underlying thermal hydraulic and thermomechanical characteristics, tritium breeding potential, safety and environmental features, power conversion and power density limits, and compatibility with plasma confinement concepts); and/or 2) exploring certain phenomena of interest and benefit to multiple concepts. Power plant scoping should be limited to demonstration of feasibility and identification of potential advantages and disadvantages, since reliable scaling information for extrapolation to fusion conditions might not be available.

Many independent experiments and modeling activities are preferred at this level and can be attempted in parallel, each focusing on a small set of issues. High risk, large payoff research is desirable and should be encouraged. Activities should be of short duration (less than 3 years, requiring renewal after a 3 year period) in order to allow for a high turnover rate.

The major benefits of these programs are in encouraging innovation and advancing general and fusion engineering science.

The proof-of-principle (PoP) stage is the lowest cost program aimed at developing an integrated and broad understanding of the basic scientific and engineering aspects of the concept which can be scaled with confidence to provide a basis for evaluating the potential of the concept for fusion energy applications. The basic prerequisite for embarking on an engineering PoP stage is that (1) its scientific and engineering basis looks promising and that (2) it will lead to an attractive energy utilization embodiment. As with the concept exploration stage, the PoP stage is a combined effort involving experiments, modeling and theory.

At this stage, supporting power plant studies should be carried out to evaluate the potential of the concept, including in-depth physics and engineering analysis to identify key technology issues. PoP experiments are usually not in a regime of fusion-relevant parameters in absolute terms, but are sufficient to provide scaling relationships useful to develop a predictive capability for evaluating the concept. Experimental facilities at the engineering PoP stage usually can be expected to be in the range of $1M to $10M.

There are a number of criteria that should be applied to the engineering PoP stage:

The construction, operation, and analysis of a Proof-of-Principle-class experiment takes roughly five to ten years, which sets the lower bound on the duration of a Proof-of-Principle program. Furthermore, substantial resources are necessary to operate a Proof-of-Principle-class experiment. These programs, therefore, should be national endeavors, drawing expertise from many institutions. Sufficient resources should be committed both to the Proof-of-Principle-class device as well as the supporting smaller experiments, theory and modeling, and power-plant studies in order to ensure a healthy return on the investment of the talent as well as resources in such an activity.

It is beneficial for the Proof-of-Principle program to include Concept-Exploration-class experiments which focus on certain key issues of the concept and help promote further innovations. Theory, modeling, and benchmarking with experiments should be vigorously pursued in order to provide a theoretical basis for scaling phenomena and evaluating the potential of the concept. Power-plant studies, including in-depth physics and engineering analysis, should be carried out to identify key physics and technological issues and help define the research program. Any technological issue specific to the concept should also be addressed during the Proof-of-Principle stage.

The major benefits at this stage are advancement of fusion energy science with some contribution to fusion energy development and power plants.

Performance extension programs explore the engineering behavior of the concept at or near the fusion-relevant regime in absolute parameters, albeit without a burning plasma.

This stage aims at generating sufficient confidence so that absolute parameters needed for a fusion development device can be achieved and a fusion development program with a reasonable cost can be attempted. At this stage, the predictive capability and scaling information is refined further, new phenomena in fusion-relevant regimes are examined, and the performance of the concept is optimized. Because of the demand on absolute performance, a large single device ($5-10M per year) may be needed and equipped with a variety of auxiliary systems for control and operational flexibility as well as extensive diagnostics providing complete coverage in space and time. This program should contain Concept-Exploration-class and possibly Proof-of-Principle-class experiments to help in optimization of the concept. Extensive theory and modeling activities should exist to analyze the experimental results on all issues and start providing a predictive capability for the concept. Both power-plant and design studies, including in-depth physics and engineering analyses, should be carried out to focus on critical issues, help in optimizing the physics regimes, and evaluate the potential of the concept for fusion development and power plants.

Key questions for this stage include the importance of neutrons and the need for coupling to a plasma confinement device.

As with the Proof-of-Principle program, this must be a national endeavor, which should include expertise from many institutions and sufficient resources allocated for supporting activities. The major benefits at this stage are contributions to fusion energy development and power plants, and advancement of fusion energy science and technology.

[1] Report of the FESAC Scicom Alternate Concepts Panel, July 1996.

(http://aries.ucsd.edu/SCICOM/AC-PANEL)

[2] FESAC Panel on Criteria, Goals and Metrics (in preparation).

1. Quality of Science. Is the proposed research program of very high quality; does it have scientific and technical credibility; is it based on an understanding appropriate for its stage of the program?

2. Confidence for Next Step. Will the proposed program provide reasonable expectation for a knowledge base to proceed to the next stage?

3. Engineering Science/Technology Benefit. What is the benefit to advancement in general engineering science issues?

4. Issue Resolution. Does the research resolve key issues and provide the basis for a decision to advance to the next stage, to re-direct within a stage, or to terminate the concept?

5. Leading Edge.

Is the research at the leading edge in the context of the national and international fusion programs?

- In which areas would the proposed research contribute at the leading edge?

- In which areas would the proposed research be behind the leading edge?

- What are the opportunities for leveraging broad knowledge bases?

6. Energy Vision. What is the overall attractiveness of the energy vision for this concept?

- Have the important outstanding issues been identified?

- Can the issues be addressed in the context of the broader national and world programs?

- What is the proposed activity to contribute to this effort?

- What is the potential for energy applications?

- What is the definition and impact on development pathway: costs, schedule and risks?

7. Program Issues. The following program issues should be considered:

- What are the construction and operating costs and their basis?

- Are there adequate resources to accomplish proposed program goals?

- Are there opportunities to be a national research facility?

- Are there opportunities to leverage existing facilities?

8. Portfolio Balance. Does the proposed program maintain a balanced portfolio of research opportunities?

9. Science/Technology Goals. How does the proposed program contribute to broad based national science and technology goals and educational opportunities?

10. Milestones. What are the key milestones to accomplish the proposed program?