Simulations tackle abrupt massive migrations of energetic beam ions in a tokamak plasma

Abstract

In the late 1990s, fusion scientists at the Japanese tokamak JT-60U discovered abrupt large-amplitude events during beam-driven deuterium plasma experiments. A large spike in the magnetic fluctuation signal followed by a drop in the neutron emission rate indicates that energetic ions abruptly migrate out of the plasma core during an intense burst of Alfvén waves that lasts only 0.3 ms. With continued beam injection, the energetic ion population recovers until the next event occurs 40–60 ms later. Here we present results from simulations that successfully reproduce multiple migration cycles and report numerical and experimental evidence for the multi-mode nature of these intermittent phenomena. Moreover, we elucidate the role of collisional slow-down and show that the large-amplitude Alfvénic fluctuations can drive magnetic reconnection and induce macroscopic magnetic islands. In this way, our simulations allow us to gradually unravel the underlying physical processes and develop predictive capabilities.

Introduction to the interaction between energetic particles and Alfven eigenmodes in toroidal plasmas

“…The reader is referred to a review article that discusses EPM in detail . Comprehensive kinetic MHD hybrid simulations have been performed for EPMs in JT-60U plasmas (Bierwage et al 2014(Bierwage et al , 2018. For the evolution of EPM, the interaction with the shear Alfvén continuous spectra is also an essential issue.…”

On the Effect of Beating during Nonlinear Frequency Chirping

“…Although transient, such large spikes may have an influence on the rate at which the hole and clump vortices advance radially and accumulate (or lose) material. Similar processes may be relevant for triggering Abrupt Large-amplitude Events (ALE) [36] whose precise timing, according to our current understanding, depends sensitively on the trajectories of multiple players, just like the events of spontaneous phase alignment in Fig. 26.…”

“…MHD-PIC hybrid simulations often compare well with experiments, at least for EP-driven Alfvén modes with long wavelengths (n < 10) [12,[34][35][36]. However, such simulations require high-performance supercomputers, primarily because of the expensive field solver that has to evolve the electromagnetic fields, plasma density and pressure on a dense three-dimensional (3D) mesh consisting of tens of millions of grid points.…”

Hybrid simulation of NBI fast-ion losses due to the Alfvén eigenmode bursts in the Large Helical Device and the comparison with the fast-ion loss detector measurements

“…2015; Todo 2016; Todo, Van Zeeland & Heidbrink 2016; Bierwage et al. 2018; Todo 2019). The MEGA code has been applied also to the LHD plasmas.…”

“…The multiphase MHD hybrid simulation, which is a combination of classical simulation and MHD hybrid simulation, has been developed and implemented in the MEGA code to investigate the fast-ion distribution formation process with the interaction of the MHD instabilities in the collisional time scale (Todo et al 2014). The multiphase hybrid simulations were successfully validated with the tokamak experiments on the significantly flattened fast-ion pressure profile, the electron temperature fluctuations brought about by the AEs and the AE bursts (Todo et al 2015;Todo 2016;Todo, Van Zeeland & Heidbrink 2016;Bierwage et al 2018;Todo 2019). The MEGA code has been applied also to the LHD plasmas.…”