Beam-Ion Acceleration during Edge Localized Modes in the ASDEX Upgrade Tokamak

The acceleration of beam ions during edge localized modes (ELMs) in a tokamak is observed for the first time through direct measurements of fast-ion losses in low collisionality plasmas. The accelerated beam-ion population exhibits well-localized velocity-space structures which are revealed by means of tomographic inversion of the measurement, showing energy gains of the order of tens of keV. This suggests that the ion acceleration results from a resonant interaction between the beam ions and parallel electric fields arising during the ELM. Orbit simulations are carried out to identify the mode-particle resonances responsible for the energy gain in the particle phase space. The observation motivates the incorporation of a kinetic description of fast particles in ELM models and may contribute to a better understanding of the mechanisms responsible for particle acceleration, ubiquitous in astrophysical and space plasmas.

Figure 1

(a) Time traces of the edge electron density (in blue), the loop voltage (in red), and the divertor current (in black). (b) Time traces of two different FILD signals. (c) and (d) show an enlargement of an individual ELM.

Figure 2

(a) Velocity space of fast-ion losses measured by a FILD during an ELM. (b) Gyroradius profile of the FILD signal. The blue crosses indicate the experimental FILD signal. In red, the undistorted gyroradius profile obtained after the tomographic inversion is plotted. The black curve is the expected gyroradius profile for the inverted distribution.

Figure 3

Gyroradius profile evolution of the FILD signal in two AUG discharges. The divertor current is plotted in white. (a) Discharge no. 33127 without MPs. The NBI time trace is plotted in red. (b) Discharge no. 34540 with MPs (gray box). The time window in which ELM suppression is achieved is indicated by two vertical dashed lines.

Figure 4

(a) Pitch angle profile of the FILD signal. In black (dashed line), the profile at the main NBI injection energy. In color, the profile at the high-energy component for different time points indicated in (b), which shows the temporal evolution of $q_{95}$.

Figure 5

(a) Poloidal contour of the magnetic perturbation of an ELM calculated with JOREK. Typical orbits measured by FILDs are overlaid. (b) Time trace of simulated fast-ion losses in a full ELM crash. The level of fast-ion losses without the perturbation is indicated for reference by a dashed red line. (c) Simulated energy gain of the markers in the presence of a parallel electric field given by Eq. (1). The separatrix position is indicated in white. The approximated deposition of beams $Q7$ and $Q8$ is indicated by the dotted white lines.