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Alpha and Energetic Particle Physics
Energetic particles — ions with energies appreciably above the background thermal plasma ion energy — arising from fusion-generated 3.9 MeV alpha particles and from the effects of certain types of rf heating will be present in ITER. Such energetic particles are predicted to have detectable effects on the plasma MHD stability and/or the plasma thermal energy confinement, and in certain cases localized losses of fast ions to the plasma-facing torus wall may give rise to appreciable sputtering and/or thermal heating of the impacted surfaces. These considerations plus the unique (relative to present fusion experience) presence of a population of energetic alphas with an isotropic momentum distribution make study of energetic particle effects one of unique science study opportunities that ITER offers.

Much is already known about the behavior of energetic ions in present tokamak plasmas, and single-particle effects are well understood and can be extrapolated to ITER with confidence. One well-known and important single-particle effect — direct loss of superthermal ions to the torus first wall owing to toroidal field ripple— sets stringent limits on the allowable magnetic field ripple in ITER and has implications for the thermal design of at-risk plasma facing surfaces.

Collective effects — those involving a population of energetic ions — are less well understood, particularly in the plasma regime ITER and future reactor tokamaks will operate in. Collective MHD modes of concern include toroidicity-induced and ellipticity-induced Alfvén eigenmodes (TAE and EAE  modes), kinetic ballooning modes and internal kink modes. While the underlying basis for these MHD modes is nominally understood on the basis of both theory and present experiment, completely reliable prediction of the full range of collective instability effects expected in ITER and similar large-normalized scale reactor plasmas is not yet possible. ITER will therefore present a first-of-kind opportunity to study such collective energetic particle instability and transport effects and thus validate the understanding needed for any future toroidal fusion reactor


Toroidal magnetic field ripple contours
and fast-alpha loss regions in ITER



AlfvÈn and energetic particle modes
in an ITER-like plasma

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