Publications (2)0 Total impact
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ABSTRACT: The results of particle simulations of a typical NNB-heated JT-60U discharge (# E036378), performed by the HMGC particle-in-cell code, are presented. We observe that the linear-growth phase is dominated by a fast-growing mode, localized at half radius, near the maximum of the energetic-ion pressure gradient and with a significant coupling with the Alfvén continuum, showing the energetic-particle-driven nature of the mode. As the nonlinear effects become important, a macroscopic outward displacement of the energetic ions is observed, producing a significant reduction of their density in the central region. These results seem to match quite well the dynamics of the abrupt large-amplitude event (ALE) experimentally observed in the JT-60U discharge. The time scale and frequency spread of the ALE are well reproduced too. Then, the issue of the almost quiescent phase following the ALEs, characterized by weaker fast frequency sweeping (fast FS) modes, is investigated. We show that it is not possible to explain such a phase as the consequence of the mere relaxation of the energetic-ion density profile produced by the ALE. The decrease of the instability level of the system can however be fully accounted for by properly taking into account the nonlinear modification of the energetic-ion distribution function in the velocity space. An interpretation of the full JT-60U bursting-mode phenomenology is finally presented.
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A. Fasoli,
C. Gormenzano,
H. L. Berk,
B. Breizman,
S. Briguglio,
D.S. Darrow,
N. Gorelenkov,
W. W. Heidbrink,
A. Jaun,
S. V. Konovalov,
R. Nazikian,
J.-M. Noterdaeme,
S. Sharapov, K Shinohara,
D. Testa,
K. Tobita,
Y. Todo,
G. Vlad,
F. Zonca
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ABSTRACT: This chapter reviews the progress accomplished since the redaction of the first ITER Physics Basis (1999 Nucl. Fusion 39 2137-664) 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 Alfvenic 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 Alfven 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 Alfven 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 Alfven 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.
