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CREDIT: JEFF CANDY/GENERAL ATOMICS CORP. turns out that plasmas are not quiescent or classical at all, but are always quivering with small-scale fluctuations in all of their parameters—density, temperature, and even the local microscopic magnetic and electric fields. It was clear by the late 1970s that these turbulent fluctuations in a tokamak were driven by the high-pressure plasma in the center expanding into the lowpressure outer regions. The fluctuating, turbulent, electric and magnetic fields move the particles across the mean field in convective eddylike motions—much more quickly than classical diffusion. The transport of heat and particles across the field is effectively a random walk with an enhanced step length and short correlation time. The density fluctuations in a computer simulation of a tokamak can be seen in the figure. The eddies are characteristically elongated along the field lines and have small-scale lengths perpendicular to the field. The complexity of this turbulence is immense. For example, the shape of the magnetic bottle, the profiles of plasma density and temperature within the bottle, and the microscopic dynamics of both ions and electrons all play critical roles (2–4). Furthermore, the turbulence can exist on many different spatial scales simultaneously. Indeed, it is also possible that turbulence excited in one region of the plasma can produce transport in another. A concerted effort to measure, understand, and model the turbulence has brought significant results in recent years. One of the most important is the development of sophisticated computational models (5–8)—the figure, for example, was produced with such a model. These models make quantitatively accurate predictions in many situations. However, some of the most exciting experimental results, in which turbulence has been observed to be substantially reduced, cannot yet be accurately predicted by the codes. The first such observation occurred in 1982 at the ASDEX experiment at the Max- Planck Institute for Plasma Physics near Munich, Germany—where the so-called H- mode (or high-confinement mode) for tokamak operation was discovered (9). It was found that when the heat leaking out of the plasma reached a critical threshold, a radial electric field, a plasma velocity gradient, and a steep pressure gradient spontaneously arose at the edge of the plasma. This led to a dramatic enhancement of the confinement time and was presumed to be the result of a reduction in turbulent transport at the edge. This was consequently termed an “edge A new twist. Gyro-kinetic simulation of plasma density fluctuations in a shaped tokamak. Image shows a cut-away view of the density fluctuations (red/blue colors indicate positive/negative fluctuations). The blue halo is the last closed magnetic flux surface in the simulated tokamak. transport barrier,” which kept plasma particles and thermal energy from escaping to the material walls of the device. It was surprising and gratifying to the researchers that—almost by itself—the plasma had found a way to reduce the turbulence, if only in a small region of the plasma. It was subsequently discovered, in the 1990s, that such transport barriers do not only exist at the edge of the plasma, but can also be produced in the interior regions of the plasma (so-called internal transport barriers, ITBs) (10). These transport barriers in the core can be induced by rapid localized changes to the “twist” of the magnetic fields of the tokamak (reversed magnetic shear) or to the plasma drift velocity (velocity shear). ITBs have enabled access to enhanced performance regimes of these tokamaks and have generated much excitement throughout the research community. However, precise measurements of the reduction of turbulence in one of these ITBs near the core region has been very difficult. In fact, only recently have experimental P ERSPECTIVES techniques been developed to determine which components of the turbulence are being suppressed within a transport barrier. New measurements at the JT-60U tokamak in Japan by a team led by Nazikian from the Princeton Plasma Physics Laboratory have used microwave reflectometry to determine that the density correlation length of the plasma fluctuations is reduced substantially during the establishment of an internal transport barrier—from about 20 cm to 4 mm—and that this corresponds to a reduction in turbulence (11). The radius of the plasma is about 1 m in JT-60U. The recent discoveries about turbulence and control of turbulent transport through transport barriers and progress in both experimental measurements and improved (predictive) simulation capabilities are now leading to renewed optimism for fusion as an energy source and confidence that new designs for fusion experiments will indeed work. However, the large-scale “reactor-relevant” experiments in which many of these discoveries were made—the Joint European Torus (JET) in the United Kingdom, JT- 60U in Japan, and the Tokamak Fusion Test Reactor (TFTR) in the United States—were constructed more than 20 years ago. The next critical step in fusion research is the International Thermonuclear Experimental Reactor (ITER), a US$5 billion project, which is planned to be operational in 2015 in Cadarache, France (12–16). Edge transport barriers are key to ITER achieving a “burning” fusion plasma. Advanced operational regimes using internal transport barriers are also planned for ITER. Such experiments may enable fusion power to be economical sooner than anyone has previously thought. For the past 40 years, some have criticized the fusion community for perpetually claiming to be only 30 years away from realizing success. As of today, fusion power may be much closer than that. References and Notes 1. Original patent reproduced in M. G. Haines, Plasma Phys. Control. Fusion 38, 643 (1996). 2. W. Horton, Rev. Mod. Phys. 71, 735 (1999). 3. T. L. Rhodes et al., Phys. Plasmas 9, 2141 (2002). 4. X. Garbet et al., Plasma Phys. Control. Fusion 46,B557 (2004). 5. A. Dimits et al., Phys. Rev. Lett. 77, 71 (1996). 6. Z. Lin et al., Science 281, 1835 (1998). 7. W. Dorland et al., Phys. Rev. Lett. 85 5579 (2000). 8. J. Candy, R. E.Waltz, Phys. Rev. Lett. 91, 045001(2003). 9. F. Wagner et al., Phys. Rev. Lett. 49, 1408 (1982). 10. Y. Koide et al., Phys. Rev. Lett. 72, 3662 (1994). 11. R. Nazikian et al., Phys. Rev. Lett. 95, 135002 (2005). 12. R. Aymar, P. Barabaschi, Y. Shimomura, Plasma Phys. Control. Fusion 44, 519 (2002). 13. X. Litaudon et al., Plasma Phys. Control. Fusion 46, A19 (2004). 14. The ITER Web page is at www.iter.org. 15. D. Clery, D. Normile, Science 309, 28 (2005). 16. D. King, Nature 428, 891 (2004). 10.1126/science.1113477 www.sciencemag.org SCIENCE VOL 309 2 SEPTEMBER 2005 1503
The W. M. Keck Foundation is proud to announce its Seventh Class of Distinguished Young Scholars in Medical Research Lu Chen, Ph.D., University of California, Berkeley Brian Kuhlman, Ph.D., University of North Carolina, Chapel Hill David M. Sabatini, M.D., Ph.D., Whitehead Institute, Cambridge Kang Shen, M.D., Ph.D., Stanford University, Stanford SmartShutter TM Stepper-Motor Driven Shutter As fast as 8msec from trigger to open or close Choose between fast or “soft” speeds Programmable control of exposure time delay Free running or timed interval operation Variable aperture settings for neutral density Life tested to over 100 million cycles 25mm, 35mm or 50mm shutters available Modular repairable design USB or TTL control The W. M. Keck Foundation Distinguished Young Scholars in Medical Research Program is a ten-year, $50 million initiative designed to give up to 50 of the nation’s best young scientists the financial resources they need to pursue pioneering research into the fundamental mechanisms of human disease. Under the program, each investigator’s sponsoring institution will receive a grant of up to $1 million to fund these distinguished young scholars’ work. Selected for their ability to carry forward groundbreaking basic medical research, potential for making a significant impact on the field of biomedicine and capacity for future leadership, the W. M. Keck Foundation believes these investigators will make profound advancements in critical areas of medical research. We are proud to offer them our congratulations and ongoing support. 51 DIGITAL DRIVE, NOVATO, CA 94949 PHONE: 415.883.0128 | FAX: 415.883.0572 EMAIL: <strong>IN</strong>FO@SUTTER.COM | WWW.SUTTER.COM GetInfo science.labvelocity.com Science announces a new online life science product information system, GetInfo, powered by LabVelocity • Quickly find and request free information on products and/or services found in the pages of Science magazine • Ask vendors to contact you with information • View detailed product information • Link directly to vendors’ websites Visit GetInfo today at science.labvelocity.com The W. M. Keck Foundation www.wmkeck.org
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