Electrostatic and magnetic transport of energetic ions in turbulent plasmas

Analytical and numerical 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 $E$.

Figure 1

The dashed line denotes a circle over which the potential is (orbit-)averaged for a particle starting at the origin, while the solid line denotes a real particle trajectory with a large drift velocity $V^{\mathrm{eff}}$. After one period, the particle is displaced by $T_{\mathrm{orbit}}{V}^{\mathrm{eff}}$ from the origin as well as from the corresponding point on the dashed curve. Therefore, if the particle does not return into the correlated zone [in the background, the autocorrelation function $⟨\varphi (0)\varphi (\mathbf{x})⟩$ of an isotropic stochastic potential with correlation length ${\lambda }_c$ is plotted], orbit averaging is not valid.

Figure 2

Electrostatic (solid lines) and magnetic (dashed lines) particle diffusivities of fast ions for large (black lines) and small (red lines) pitch angles as obtained from Gene simulations. The results agree well with the theoretical expectations (see text) which are shown for comparison.