Chapter 5: Physics of energetic ions

Abstract

This chapter reviews the progress accomplished since the redaction of the first ITER Physics Basis Nucl. Fusion 39 2137 in the field of energetic ion physics and its possible impact on burning plasma regimes. New schemes to create energetic ions simulating the fusion-produced alphas are introduced, accessing experimental conditions of direct relevance for burning plasmas, in terms of the Alfvénic Mach number and of the normalised pressure gradient of the energetic ions, though orbit characteristics and size cannot always match those of ITER. Based on the experimental and theoretical knowledge of the effects of the toroidal magnetic field ripple on direct fast ion losses, ferritic inserts in ITER are expected to provide a significant reduction of ripple alpha losses in reversed shear configurations. The nonlinear fast ion interaction with kink and tearing modes is qualitatively understood, but quantitative predictions are missing, particularly for the stabilisation of sawteeth by fast particles that can trigger neoclassical tearing modes. A large database on the linear stability properties of the modes interacting with energetic ions, such as the Alfvén eigenmode has been constructed. Comparisons between theoretical predictions and experimental measurements of mode structures and drive/damping rates approach a satisfactory degree of consistency, though systematic measurements and theory comparisons of damping and drive of intermediate and high mode numbers, the most relevant for ITER, still need to be performed. The nonlinear behaviour of Alfvén eigenmodes close to marginal stability is well characterized theoretically and experimentally, which gives the opportunity to extract some information on the particle phase space distribution from the measured instability spectral features. Much less data exists for strongly unstable scenarios, characterised by nonlinear dynamical processes leading to energetic ion redistribution and losses, and identified in nonlinear numerical simulations of Alfvén eigenmodes and energetic particle modes. Comparisons with theoretical and numerical analyses are needed to assess the potential implications of these regimes on burning plasma scenarios, including in the presence of a large number of modes simultaneously driven unstable by the fast ions.

Current drive at plasma densities required for thermonuclear reactors

“…Th ese profi les, in turn, dictate the character of turbulence and turbulent transport and, eventually, the thermal plasma and fusion product profi les. Th is circumstance would occur on the characteristic spatio-temporal scales of burning plasmas, which represent complex self-organized systems 6 . Th us, for a successful development of reactor-graded plasma operations, it would be very important to have diff erent and fl exible tools capable of producing and controlling steady-state profi les with a high fraction of non-inductive plasma current driven in the outer half of the plasma column.…”

Effect of dynamical friction on nonlinear energetic particle modes

“…1 Observations of these waves provide important information about the plasma. 2 However, Alfvén waves can potentially enhance energetic particle losses from the plasma, which is undesirable.…”

“…2 However, Alfvén waves can potentially enhance energetic particle losses from the plasma, which is undesirable. 1 It is therefore important, for burning plasmas, to predict and control the consequences of the wave-particle interaction. The nonlinear behavior of individual waves is an essential element of such research.…”

Radial Transport Characteristics of Fast Ions Due to Energetic-Particle Modes inside the Last Closed-Flux Surface in the Compact Helical System

“…The dependence of growth rate is consistent with theoretical prediction. An unstable EPM is predicted in the reversed shear case of ITER [21]. Once EPMs are excited in burning plasmas, the modification of the fast-ion profile is considered to be inevitable due to the strong link between anomalous transport of fast ions and the drive of the EPMs.…”