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.
Experiments have been carried out in JT-60U to verify the modelling of fast ion ripple transport. The ripple induced loss was estimated from the neutron decay following neutral beam pulse injection and the loss related heat load on the first wall. Comparison of the lost fraction and the hot spot positions between measurements and orbit following Monte Carlo calculations exhibited good agreement, indicating that the ripple transport governing fast ion losses is explained within the framework of existing theory. Neutral beam heating experiments in JT-60U also indicate that H modes free of ELMs are still obtainable for ripple amplitudes of up to 2.2%
Alpha particle loss due to the toroidal field ripple is significantly enhanced in reversed shear plasmas, especially in a high-q regime. To ensure a tolerable heat load on the wall due to the loss of alpha particle in reversed shear discharges with various q profiles, ferritic steel (FS) inserts are considered as a countermeasure for reducing the ripple amplitude at a low cost. With this being the situation, an assessment of ripple loss for various conditions in ITER is an important study that should to be conducted in the International Tokamak Physics Activities. This paper describes an assessment result, indicating that FS inserts lead to a dramatic reduction of ripple loss. When the arrangement of the FS inserts is optimized, the resulting alpha particle loss is anticipated to be reduced by an order of magnitude or more, and the power loss fraction is as low as 1% even for a reversed shear plasma with a relatively high q min of 3.
Physics knowledge (theory and experiment) in energetic particles relevant to design of a reactor scale tokamak is reviewed, and projections for ITER are provided in this Chapter of the ITER Physics Basis. The review includes single particle effects such as classical alpha particle heating and toroidal field ripple loss, as well as collective instabilities that might be generated in ITER plasmas by energetic alpha particles. The overall conclusion is that fusion alpha particles are expected to provide an efficient plasma heating for ignition and sustained burn in the next step device. The major concern is localized heat loads on the plasma facing components produced by alpha particle loss, which might affect their lifetime in a tokamak reactor. Contents 1. Introduction 2471 2. Classical physics of energetic particle confinement and plasma heating 2473 2.1. Classical interaction with the main plasma . . . . . . . . . . . . 2473 2.2. Alpha particle density and beta profiles2474 2.3. Neoclassical diffusion of alpha particles 2475 3. Toroidal field ripple loss and other single particle effects 2475 3.1. Mechanisms and loss channels of toroidal field ripple loss . . . . . . . 2475 3.2. Experimental results on fast ion ripple loss .
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