7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Role of cross-scale coupling in energetic particle transport
Zhihong Lin
University of California, Irvine, CA 92697, USAe-mail (speaker): zhihongl@uci.edu

Alfven eigenmodes (AE) can be excited by energetic
particle (EP) pressure gradients and can drive a large EP
transport that degrades tokamak plasma confinement.
Despite the separation in the spatial and temporal scales,
there can be strong coupling between AE andmicroturbulence. Zonal flows can be nonlinearly
generated by, and in turn, suppress both the AE andmicroturbulence. Microturbulence can damp the zonal
flows and zonal structures generated by the AE. EP
scattering by the microturbulence can affect phase space
dynamics in nonlinear AE-EP interactions. There canalso be direct mode-mode coupling between AE anddriftwave eigenmode. Understanding these cross-scaleinteractions requires global integrated simulations
incorporating multiple physical processes in a complex
toroidal geometry and treating all particle species on anequal footing.
In this work, the cross-scale coupling between AE andmicroturbulence is studied in global integrated
simulations by the gyrokinetic toroidal code (GTC) withcomprehensive
electromagnetic
physicsusing
gyrokinetic thermal and fast ions and drift kinetic
electrons. Global gyrokinetic simulations of recent
critical gradient DIII-D tokamak experiments find thatreversed shear Alfven eigenmodes (RSAE) excited byenergetic ions from neutral beam injection can saturate
by self-generated zonal flows. However, saturated RSAE
amplitude and EP transport are an order of magnitude
higher than the experimental observations when themicroturbulence is artificially suppressed. In contrast, inthe realistic simulations coupling micro-meso scales, thesaturated RSAE amplitude and EP transport are greatly
reduced to the experimental level due to both zonal flow
shearing and EP scattering by the ion temperature
gradient (ITG) turbulence. The dominant RSAE mode
structure and amplitude from the cross-scale gyrokinetic
simulations agree well with experimental measurements
using electron cyclotron emission (ECE) [Liu et al, Phys.
Rev. Lett. 128, 185001 (2022)]. The EP radial scatteringby the microturbulence is much more important thanCoulomb collisions in destroying coherent structures inthe EP phase space to maintain a steady state RSAEturbulence.
Extrapolations from DIII-D to the ITER experiments
will be discussed. In this project, EP confinement
properties of the ITER operation scenarios have been
comprehensively assessed using global gyrokinetic codes(GTC, CGYRO, ORB5), kinetic-MHD codes (FAR3D,GAM-solver, M3D-C1, MEGA, NOVA-C, XTOR-K),
and reduced EP transport models (Kick, RBQ,TGLF-EP). These codes have been first verified andvalidated for simulations of EP transport in the DIII-Dexperiments. This collaborative research has been

selected for US DOE FY2022 Theory Performance
Target and adapted as the ITPA energetic particle joint
activity B.11.12 project.
These large scale simulations find that macroscopic
fishbone can be unstable in the ITER baseline scenario,
but saturate at a low amplitude with insignificant
distortion of flux surface and EP re-distribution. Various
meso-scale Alfven eigenmodes (AE) saturate at highamplitudes and drive a large EP transport in the ITER
steady state scenario, which results in a modest flatteningof the EP profiles during the short simulation time.
Strong microturbulence drives directly little EP transport
but large thermal transport in both scenarios, which
could affect EP transport driven by the AE and fishbone.
These studies point to a modestly optimistic assessmentof the EP confinement in the ITER, but also a significant
relaxation of the alpha particle profile in the steady state
scenario. Finally, integrated simulation of cross-scale
coupling is needed to reliably predict EP confinement inthe ITER, as demonstrated in the simulations of theDIII-D experiments.