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In a tokamak, the toroidal magnetic field exhibits a ripple due to the finite coils number. In this work, the effects of this non-axisymmetric perturbation on suprathermal ions (fast ions generated by additional heating or charged fusion products) are investigated. If the ripple is high enough to create local magnetic wells along trajectories, a non diffusive transport process due to trapping in these wells occurs and leads to direct particle losses. Such a situation may be encountered during future neutral beam injection experiments in the TORE-SUPRA tokamak. Using a Fokker-Planck transport code, numerical results indicate high fast ion losses. In order to minimise these losses, various methods are proposed and analysed. When the ripple is weak (no local wells) an Hamiltonian description of the particle interaction with the perturbation leads to a stochastic instability due to resonance overlapping. Two distinct stochasticity criteria are given which are associated with two diffusion regimes. For alpha particles in a reactor sized tokamak, stochastic thresholds and diffusion coefficients are evaluated.

Additional information

Authors: GRUA P, CEA, Département de Recherches sur la Fusion Contrôlée, Saint-Paul-lez-Durance (FR)
Bibliographic Reference: Report: EUR-CEA-FC-1385 FR (1990)
Availability: Available from CEA, Département de Recherches sur la Fusion Contrôlée, Saint-Paul-lez-Durance (FR)
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