An Assessment of the Department of Energy's Office of Fusion Energy Sciences Program (2001) / Chapter Skim
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Scientific Progress and the Development of Predictive Capability
Scientific Progress and the Development of Predictive Capability
Pages 14-44
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From page 14... ...
A measure of the maturity of this knowledge base and, also, of the quality of the science is the ability to predict the performance of experiments from a fundamental understanding of the characteristic dynamics of plasmas in the laboratory and in nature. Such predictive capability goes beyond simple performance metrics such as the energy containment time, temperature, or the plasma pressure, which, although important, do not necessarily reflect the degree to which the fusion program has broadly impacted plasma science and related fields.
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From page 15... ...
These techniques are being used to control pressure, current, and flow profiles and thus to optimize plasma performance in present-day large experiments, but techniques applicable to future high-pressure plasmas require further development. Diagnostics for remotely measuring important equilibrium-related quantities such as plasma density, electron and ion temperatures, and magnetic field are now available in major plasma experiments.
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From page 16... ...
The existence of a density limit for plasma stability is a robust feature of magnetically confined plasmas and is well described by a simple scaling law (for tokamaks)
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From page 17... ...
Although many instabilities can be present in modern plasma experiments, one particular instability driven by the ion temperature gradient has been identified as the dominant mechanism for ion thermal transport in the core of tokamak plasmas. The nonlinear behavior of this instability requires further investigation.
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From page 18... ...
Similarly, highly developed techniques for measuring important equilibrium quantities such as the plasma density and temperature and the magnetic field are now standard on plasma experiments. Diagnostic tools to measure electric fields and associated equilibrium plasma flows are, however, less well developed.
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From page 19... ...
Very early in the fusion program, Thomson scattering techniques were developed to measure the electron temperature and density profiles. Later, when time-dependent measurements were required to understand macroscopic instability and magnetohydrodynamics (MHD)
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From page 20... ...
Over the past several years there has been an accumulation of evidence that plasma flows, driven by electric fields that point perpendicular to the magnetic flux surfaces, can have a strong influence on plasma stability with respect to both large-scale and small-scale disturbances. These electric fields can be directly measured by a beam of heavy ions injected into the magnetized plasma.
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From page 21... ...
The basic physics of bulk heating and current drive has been established for the most common applications of both approaches, but techniques for detailed control of pressure and current profiles and applications to plasmas with high dielectric constants are still in development. In neutral beam heating and current drive, energetic neutrals injected across the magnetic field deposit energy and directed momentum in the confined plasma.
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From page 22... ...
As a new generation of large plasma devices with improved confinement was deployed in the 1970s and 1980s, tests of wave-plasma interactions were carried out at the megawatt power level, confirming the predictions of ion and electron bulk heating. Over the past decade, moreover, the use of radio-frequency waves to drive plasma currents at the mega-ampere level has been demonstrated at efficiencies consistent with theoretical predictions.
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From page 23... ...
MHD energy principles in multidimensional geometry, has become a routine component of all plasma fusion experiments during both design and operation. Modern high-temperature experiments, however, have operational limits that arise not only from ideal MHD stability but also from nonideal (nonzero resistivity)
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From page 24... ...
The U.S. fusion program strongly supported the development of codes to evaluate local and global plasma stability from the ideal MHD energy principle.
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From page 25... ...
For example, ideal MHD equilibrium and stability calculations that predicted stable, very-highpressure equilibra in the spherical torus (a U.S. contribution)
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From page 26... ...
In the FRC, magnetic reconnection is in fact required to convert the initially open magnetic system to the final state with closed field lines. In reversed-field pinches and spheromaks, the system remains in a dynamical state in which the diffusion of magnetic flux due to finite plasma resistivity is balanced by a dynamo magnetic field generation process in which the plasma evolves to a minimum energy state through a complex reconnection process.
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From page 27... ...
, magnetic reconnection was widely studied by fusion scientists, especially in the early days of the program. The theory of the "tearing mode," which is the manifestation of reconnection when the amplitude of the magnetic perturbations is small, was developed for a resistive plasma.
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From page 28... ...
The recent construction of a dedicated magnetic reconnection experiment and its exploration in some of the emerging new experiments signals a rebirth of scientific interest in this area in the fusion program. Although magnetic reconnection and resistive instabilities have historically emphasized the plasma current and its associated magnetic field as the source of free energy, there is increasing observational and theoretical evidence that the plasma pressure gradient can also drive magnetic islands and reconnection.
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From page 29... ...
The development of a predictive capability for these phenomena is complicated by their sensitivity to initial finite-amplitude perturbations and by the absence of a welldefined set of nonlinear equations with which their nonlinear dynamics can be explored. An ongoing issue in the fusion program has been the degree to which the resistive MHD equations are a valid description of the dynamics that characterize modern high-temperature fusion plasmas.
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From page 30... ...
Improved diagnostics to measure fluctuations at the plasma edge are also likely to be required. It is probably too early to assess the transferability of theories for density limits observed on tokamaks to other types of confinement devices.
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From page 31... ...
A greater challenge is the development of a predictive capability for transport due to collective instabilities. There has recently been rapid growth in the understanding of how energetic particles interact with linear modes, causing instabilities that have been observed and manipulated in experiments.
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From page 32... ...
On the theoretical side, this effort is complicated by the difficulty of understanding, in complex magnetic geometries, the linear growth and nonlinear saturation of instabilities that evolve into a fully turbulent state: bv the tremendous range of space- and timescales associated with this turbulence: , , , ~ ~ , 1 ,1 , · , r,1 r1 , ,~ 11 1 1 1- 1 , ,1 ret ret 1 1 by the extreme an~sotropy ot the fluctuations parallel and perpendicular to the confining magnetic field; and by the essentially collisionless character of the plasma dynamics in the core of most confinement systems. On the experimental side, the difficult issues are how to remotely measure the spectrum of small-scale fluctuations in the density, temperature, and electric and magnetic fields; how to determine the linear growth and nonlinear mode coupling properties of a fully saturated turbulent plasma; and how to visualize this turbulence (as has been so effectively done in experimental studies of fluid turbulence)
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From page 33... ...
The dramatic advances in the ability of scientists to model turbulence in complex magnetic geometries, combined with the failure of the empirical approach to account for phenomena such as the spontaneous formation of transport barriers, has caused the empirical approach to come under fire and has provided increased confidence that a true predictive capability may be achievable. Another driver for the development of confinement predictability is the sensitivity of performance projections to uncertainties in scaling laws.
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From page 34... ...
Nevertheless, scaling laws for describing energy containment carried more and more qualifiers, requiring that only discharges with, say, matching shapes or density profiles could be compared. The growing complexity of anomalous transport from the experimental perspective, coupled with the opportunities provided by an array of control techniques, undermined the scaling law approach to confinement predictability.
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From page 35... ...
While electrostatic turbulence has been the focus of study in the high-temperature interior of a plasma, magnetic field perturbations have been explored in two contexts: at the plasma edge and in very-shortscale turbulence driven by the electron temperature gradient. At the low-temperature plasma edge, since collisions are important, three-dimensional models of turbulence based on the collisional fluid equations were derived.
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From page 36... ...
Magnetic field perturbations and the required dynamics of the very-high-speed electrons parallel to the magnetic field have also been included in studies of short-wavelength turbulence driven by the electron temperature gradient in the hot plasma core. Since the characteristic scale-length of this turbulence ranges between the electron gyroradius and the electron electromagnetic skin depth and is therefore very small compared to the ion gyroradius, static behavior for the ions is a valid approximation.
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From page 37... ...
The result from experiments on the DIII-D tokamak that anomalous ion transport was completely suppressed over the entire cross section of the plasma cemented the current view that turbulence and transport can in fact be controlled. The theoretical picture of transport barriers now needs to be tested in sufficient detail to achieve predictive capability.
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From page 38... ...
are responsible for creating the equilibrium magnetic field, is currently a major focus of the European and Japanese fusion programs owing to its intrinsically good stability and steady-state nature. In both European and Japanese stellarator experiments, the H-mode transition has been observed.
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From page 39... ...
A number of important, fundamental discoveries have been made in the effort to understand plasma turbulence and transport. These discoveries include the identification of the short-wavelength instabilities driven by ion temperature gradients as the most probable mechanism for anomalous ion transport; the self-generation of zonal flows as the primary mechanism for saturating the linear growth of instabilities driven by magnetic field line curvature; the creation of avalanches, associated with fast radial propagation of heat pulses and cold pulses; the role of velocity shear fields in stabilizing local turbulence; and the self-generation of transport barriers, which locally suppress turbulence and transport.
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From page 40... ...
Suppression of these zonal flows in numerical simulations causes the transport to rise by an order of magnitude; accordingly, the importance of zonal flows in controlling the fluctuation levels is widely accepted. Experimental efforts in visualization can help confirm or challenge theoretical predictions.
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From page 41... ...
For example, an analysis of core transport barriers reveals that even when the ion energy transport is suppressed, the electron energy transport is not and may remain large. Surprisingly, electron thermal transport in the core of the TFTR tokamak remained large even though the electron temperature gradient was essentially zero and the density gradient was nonzero, a configuration that is apparently stable to all linear modes.
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From page 42... ...
The complexity of the plasma medium is reflected in the scientific richness of the field. · The fusion program has been the fundamental driver for the development of the field of plasma science, including energy principles for exploring plasma stability, wave dynamics, resonant waveparticle interactions, chaos, magnetic reconnection, plasma turbulence, and transport.
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From page 43... ...
fusion energy program on a par with the goal of developing fusion energy technology, and decision-making should reflect these dual and related goals. Since the redirection of the fusion program in 1996, a greater emphasis has been placed on understanding the basic plasma dynamics underlying the operation of the various confinement configurations.
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From page 44... ...
Improvements in scientific understanding and progress towards fusion energy are coupled, and both should serve as measures of program success and be given equal weight. The program planning and budgetary justification carried out by DOE must be organized around answering key scientific questions as well as around progress toward the eventual energy goal.
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