Numerical and analytical work is used in tandem to address the problem of turbulent transport of energetic ions in magnetized plasmas. It is shown that orbit-averaging is not valid under rather generic conditions, and that perpendicular decorrelation effects lead to a slow 1/E decay of the electrostatic particle diffusivity of beam ions, while the respective magnetic quantity is even independent of the particle energy E.While over the last several years, there has been a lot of progress in our understanding of turbulent transport of particles, momentum, and energy in magnetized plasmas (see, e.g., Ref.[1]), many open questions remain, e.g.: Is there a significant interaction of turbulent fluctuations with energetic ions? Despite previous experimental investigations along these lines (see, e.g., Refs. [2,3] and references therein), this issue was raised again, in particular, by recent experimental investigations at AS-DEX Upgrade which showed a fast radial broadening of the current profile driven by off-axis neutral beam injection in the absence of any measurable magnetohydrodynamic activity.[4] It is also important in light of the fact that future D-T based plasma experiments like ITER [5] will have a significant population of fast ions. Moreover, many related astrophysical problems depend on a solid understanding of fast ion dynamics in a turbulent medium. [6] As is well known, fast particle trajectories in toroidal, axisymmetric magnetic fields deviate from the field lines in two ways. First, they perform a gyration about the field lines, and second, grad-B drifts and curvature drifts also induce an oscillation of a fast particle gyrocenter about a magnetic field line in a nearly circular (actually slightly elliptical) fashion, [7] but on a much slower time scale. Past theoretical studies often addressed this topic by alluding to a presumed analogy between orbit averging and gyroaveraging (see, e.g., Refs. [8,9]). According to this point of view, concerning its long-time-scale dynamics, a fast particle only feels reduced, orbit-averaged (and gyroaveraged) potentials. Consequently, even for only moderately energetic particles, one would expect practically no cross-field transport. However, the validity of such an approach is usually not discussed, and as will be shown below, fast ions generally do not fulfill the required conditions.Inspired by these experimental and theoretical findings, the present work represents a systematic study of the interaction between fast (passive) particles and a turbulent background based on first principles. Revisiting this problem, we find that due to a specific perpendicular decorrelation mechanism, the turbulent particle diffusivity of beam ions decreases only quite slowly in the electrostatic case, inversely proportional to the particle energy, and is even found to be independent of the particle energy in the magnetic case. To allow for a better understanding of the underlying physical effects, we adopt a two-step approach. First, we give an analytical treatment of the scalin...

Beyond a certain heating power, measured and predicted distributions of NBI driven currents deviate from each other even in the absence of MHD instabilities. The most reasonable explanation is a redistribution of fast NBI ions on a time scale smaller than the current redistribution time. The hypothesis of a redistribution of fast ions by background turbulence is discussed. Direct numerical simulation of fast test particles in a given field of electrostatic turbulence indicates that for reasonable parameters fast and thermal particle diffusion can indeed be similar. -High quality plasma edge density profiles on ASDEX Upgrade and the recent extension of the reflectometry system allow for a direct comparison of observed TAE eigenfunctions with theoretical ones as obtained with the linear, gyrokinetic, global stability code LIGKA. These comparisons support the hypothesis of TAE-frequency crossing the continuum at the plasma edge in ASDEX Upgrade H-mode discharges. -A new fast ion loss detector with 1 MHz time resolution allows frequency and phase resolved correlation between the observed losses and low frequency magnetic perturbations such as TAE modes and rotating magnetic islands.Whereas losses caused by TAE modes are known to be due to resonances in velocity space, by modelling of the particle drift orbits we were able to explain losses caused by magnetic islands as due to island formation and stochasticity in the drift orbits.

The turbulent E × B advection of energetic ions in 3D tokamak geometry is investigated both analytically and numerically. It is shown that orbit averaging (leading to a significant reduction of the diffusivity) is only valid for low magnetic shear. At moderate or high magnetic shear, a rather slow decrease of the diffusivity is found, proportional to (E/Te) −1 or (E/Te) −1.5 for particles with a large or small parallel velocity component, respectively. The decorrelation mechanisms responsible for this behavior are studied and explained in detail. Moreover, it is found that resonances between the toroidal drift of the particles and the diamagnetic drift of the turbulence can lead to an enhancement of the fast ion transport.

Abstract.The turbulent E×B advection of charged test particles with large gyroradii is investigated. To this aim, a recently developed theory -the so-called decorrelation trajectory method -is used together with direct numerical simulations and analytical calculations. It is found that for Kubo numbers larger than about unity, the particle diffusivity is almost independent of the gyroradius as long as the latter does not exceed the correlation length of the electrostatic potential. The underlying physical mechanisms leading to this surprising and initially counterintuitive behavior are identified.

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