The fluid theory of electrostatic perturbations in a scrape-off layer (SOL) plasma is analysed. The main difficulty is that the edge is theoretically found to be stable, while experimentally it is unstable. A possible explanation relies on the fact that the commonly used ballooning representation is not correct in the SOL. An alternative representation is proposed which reproduces the instability of the edge in several simple configurations and explains many experimental features.
Fast ion ripple losses during neutral beam injection in TFR are measured near-quantitatively with an array of Faraday collectors mounted between two toroidal field coils. The collected currents show multiple evidence for fast ion ripple losses and correspond to global losses of 30-40% of the injected current. The measurements are complemented by a simple Fokker-Planck model, itself checked against Monte Carlo simulations on several cases. These calculations clarify many experimental features but predict only about half of the observed ripple lost current. This discrepancy could be due in part to various experimental and theoretical uncertainties although there are indications of some enhanced ripple losses in the experiments. Such losses could arise from other processes than toroidal field ripple not included in the calculations and may suggest, along with other diagnostics, an enhanced depletion of central fast ions, the origin of which remains unclear. Fast ion ripple losses are calculated and observed to be fairly insensitive to factors of two variations in most plasma parameters. Substantial losses are predicted only when injecting near perpendicularly into a tokamak plasma of edge ripple ≥1%. Fast ion ripple losses in several devices are considered and calculations for TORE SUPRA are presented.
The collisionless dynamics of trapped particles in a non-axisymmetric tokamak configuration with toroidal magnetic field ripple is investigated. The resonances between the particle motion and the perturbation are studied with a Hamiltonian description based on action-angle variables. When the ripple exceeds a critical value the particle motion is shown to become stochastic through resonance overlapping mechanisms, and collisionless diffusion occurs. Two regimes are clearly identified: in low ripple regions, the stochastic threshold is given by a standard Chirikov criterion, whereas in higher ripple zones the motion is dominated by the perturbation structure around the resonances. Critical ripple values and diffusion coefficients are analytically determined in both regimes, and the case of 3.5 MeV alpha particles in a reactor size tokamak such as ITER is discussed.
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