Abstract
We present a detailed numerical study of the interaction between fast particles and large-scale magnetic perturbations and toroidal field ripple. In particular we focus our study on the losses of fast ions created by neutral beam injection (NBI) for an ASDEX Upgrade discharge with neoclassical tearing mode (NTM) activity. For these investigations, we use as input an equilibrium carefully reconstructed from experimental data. The magnetic field ripple is self-consistently included by a three-dimensional, free-boundary equilibrium computation. The magnetic islands caused by a (2,1)-NTM are introduced by a field perturbation superimposed on the equilibrium magnetic field. The experimental data are used to reproduce size and location of those islands numerically. Starting from a realistic seed distribution, the guiding centres of about 100 000 fast ions are traced up to a given time limit, or until they hit plasma-facing structures. A detailed analysis of the particle trajectories provides important information on the underlying loss mechanisms such as: (i) losses of passing particles caused by drift island formation, and (ii) losses of trapped particles due to stochastic diffusion.
Highlights
Numerical methodIn agreement with the motional Stark effect (MSE) data the q = 2 surface is located at approximately 60% of the plasma radius
The plasma region, we investigate a real ASDEX Upgrade equilibrium including the magnetic field ripple produced by the 16 toroidal field coils of ASDEX Upgrade
ASDEX Upgrade has a 20 MW neutral beam injection (NBI) system composed of two injectors, which are located at the low-field side
Summary
In agreement with the MSE data the q = 2 surface is located at approximately 60% of the plasma radius We show the separatrix, the poloidal position of the FILD, and an outer limiting surface. In order to avoid the numerical effort connected with the determination of the positions where the guiding centres of the fast ions hit the 3D wall structures, we approximate the PFCs by an axisymmetric, closed surface. In order to obtain the experimentally observed (2,1)-island width and position, and at least the same order of magnitude for the measured radial perturbation field strength (3 × 10−4 T) in the vacuum region, we use α = 0.04, β = 0.87, γ = 0.01, ρ2,1 = 12 × 10−4 and s2,1 = 0.2693. We do not consider the rotation of the islands, because the transit frequencies of fast particles are much higher than the mode frequency of the (2,1)-NTM [12]
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