Speaker
Andreas Bierwage
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/I5.119.pdf
Simulation and analysis of fast ion dynamics in a JT-60SA tokamak
plasma subject to pressure- and current-driven instabilities
A. Bierwage, N. Aiba, A. Matsuyama, K. Shinohara, M. Yagi
National Institutes for Quantum and Radiological Science and Technology (QST),
Rokkasho Fusion Institute and Naka Fusion Institute, Japan
The resistive MHD-PIC hybrid code MEGA has recently been used to study the ramp-up
phase of a high-β plasma in the JT-60SA tokamak as predicted by integrated simulations [1].
Here, we analyze the effects of MHD and fast-ion-driven instabilities observed in the MEGA
simulations. The fast ions originate from powerful negative-ion-based neutral beams (N-NB),
which are deposited off-axis and have energies up to 500 keV. Effects of E × B drifts and
magnetic perturbations with long wavelengths (low toroidal mode numbers n < 5) are examined.
We report new results on the behavior of fast ions in the presence of reconnecting instabilities,
whose magnetic perturbations evolve slowly compared to the fast ion motion. This was moti-
vated by the possibility that off-axis N-NB injection in JT-60SA may produce nonmonotonic
current and pressure profiles, whose gradients may destabilize multiple kink-tearing modes and
cause minor internal disruptions. When such reconnecting instabilities develop in our simula-
tions, we observe a flattening in the density profiles across the inner half of the plasma, both in
the bulk and the fast ion components. When the underlying dynamics are examined using orbit-
based resonance analysis combined with Poincarè maps, one can see that the flattening of the
fast ion density profile cannot be explained with the formation and growth of magnetic islands,
which influence only particles that closely follow magnetic lines of force. In contrast, in the case
of fast particles that are subject to significant magnetic drifts — such as our energetic beam ions
during current ramp-up [1], or relativistic electrons [2] — the radial redistribution is connected
with the formation of resonant islands in canonical toroidal angular momentum space. These
orbit islands have different existence conditions and, when projected into real space, they can
be located in a different region of the plasma than the magnetic islands.
The insights won in this study may be used for the development of reduced models that will
allow to incorporate the effects of MHD activity into integrated codes. The ability to simulate
the complex interplay of processes such as MHD activity, fast ion transport, current drive, torque
and heating using integrated codes is necessary for shedding light on feedback loops that may
exist on multiple spatio-temporal scales, and for reliably predicting their effects in experiments.
[1] Bierwage et al., Plasma Phys. Control. Fusion 59 (2017) 125008.
[2] de Rover et al., Phys. Plasmas 3 (1996) 4468; Matsuyama et al., Nucl. Fusion 54 (2014) 123007.
