NEWS & VIEWS nature physics | VOL 2 | MAY 2006 | www.nature.com/naturephysics 299 PLASMA PHYSICS Thermonuclear ringtones The ability to confine alpha particles within a burning deuterium–tritium plasma is likely to be crucial to the future of fusion power generation. Resonant interactions between alpha particles and magnetohydrodynamic vibrations could threaten their confinement. STEVEN C. COWLEY is in the Department of Physics and Astronomy, University of California, Los Angeles, California 90077, USA and Department of Physics, Imperial College, London SW7 2BZ, UK. e-mail: cowley@physics.ucla.edu T he plasmas in stars are sufciently hot that colliding light nuclei occasionally overcome the electrostatic repulsion and fuse: energy released in these fusion reactions keeps the star hot. Te ITER experiment that is being constructed in Cadarache (France) will, like the stars, have a burning plasma, which is heated by the energy released in fusion reactions 1,2 . Te fusion in ITER will take place between deuterium and tritium in a magnetically confned plasma with a temperature of over 100 million degrees. Te reaction produces 14 MeV neutrons and 3.5 MeV alpha particles, the latter of which must be confned by the magnetic feld to continually heat the plasma so that its temperature can be maintained. In 1997, the Joint European Torus generated an impressive 16 megawatts of fusion power 3 but achieved negligible alpha heating. Consequently, a central goal of ITER is to investigate the physics of alpha-heated plasmas, and it is clear that it will be possible to maintain alpha confnement in many of ITER’s plasma modes. However, in a recent issue of Physical Review Letters, Nazikian et al. show that some of the most promising fusion plasma modes ring with vibrational eigenmodes that could interact strongly with the alpha particles and limit their confnement 4 . The effectiveness of a magnetic field in confining a plasma is limited by small-scale turbulent fluctuations that transport particles and heat across its field lines. Alpha particles produced in fusion reactions move very quickly along and across the magnetic field lines and interact weakly with most fluctuations. However, by coincidence, 3.5-MeV alpha particles move just a little faster than the Alfvén speed in many fusion devices, including ITER. Alfvén waves (first discovered by Hannes Alfvén 5 in 1942) are transverse magnetic waves that propagate along magnetic field lines almost like waves on a string. They propagate at the Alfvén speed, V A = √(B 2 /(μ 0 ρ)), where B is the magnetic field strength, μ 0 is the vacuum permeability, and ρ is the plasma mass density. Alpha particles can move with the Alfvén waves (essentially surfing on the wave) and pick up strong radial perturbations. If these waves have a large amplitude the alphas are expelled rapidly Frequency (MHz) Frequency (MHz) Time (ms) Log 10 (A) Time (ms) 1.2 0.8 0.4 0.0 1.2 0.8 0.4 0.0 3,000 3,500 4,000 4,500 3,000 3,500 4,000 4,500 n = 40 n = 30 n = 20 n = 10 a b 0.0 –1.5 Log 10 (A ) Figure 1 Frequency spectrum of Alfvén-wave eigenmodes in the DIII-D tokamak experiment versus time. a, Density fluctuations measured by scattering far-infrared waves. A is a measure of scattered power. b, Theoretically predicted spectrum. Reprinted with permission from ref. 4. Copyright (2006) by the American Physical Society. Nature Publishing Group ©2006