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

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.

Excitation of flow-stabilized resistive wall mode by coupling with stable eigenmodes in tokamaks.

In a rotating toroidal plasma surrounded by a resistive wall, it is shown that linear MHD instabilities can be excited by couplings between the resistive wall mode (RWM) and stable ideal MHD modes. In particular, it is shown that the RWM can couple not only with stable external kink modes but also with Alfvén eigenmodes that are ordinarily in the stable continuum of a toroidal plasma. The RWM growth rate is shown to peak whenever the Doppler shift caused by the plasma rotation cancels the frequency of an ideal MHD mode, so that the mode appears to have zero frequency in the laboratory frame. At these values of the rotation frequency, the RWM can overcome the stabilizing effects of plasma rotation, continuum damping, and ion Landau damping.

Alfvén acoustic channel for ion energy in high-beta tokamak plasmas.

When the plasma beta (ratio of thermal to magnetic pressure) in the core of a tokamak is raised to values of several percent, as required for a thermonuclear fusion reactor, continuous spectra of long-wavelength slow magnetosonic waves enter the frequency band occupied by continuous spectra of shear Alfvén waves. It is found that these two branches can couple strongly, so that Alfvén modes that are resonantly driven by suprathermal ions transfer some of their energy to sound waves. Since sound waves are heavily damped by thermal ion Landau resonances, these results reveal a new energy channel that contributes to the damping of Alfvénic instabilities and the noncollisional heating of bulk ions, with potentially important consequences for confinement and fusion performance.