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.
Highlights
In the late 1990s, fusion scientists at the Japanese tokamak JT-60U discovered abrupt largeamplitude events during beam-driven deuterium plasma experiments
Since both JT60SA and ITER will heavily rely on negativeion-based neutral beams (N-NB) drive, it is important to understand amplitude events (ALE)-like relaxation events and develop the ability to predict their occurrence in future experiments
The results reported above show that, using presently available supercomputers, it is possible to reproduce intermittent massive migrations of energetic N-NB ions during ALEs as observed in JT-60U experiments and gain insights concerning the underlying physical processes
Summary
In the late 1990s, fusion scientists at the Japanese tokamak JT-60U discovered abrupt largeamplitude events during beam-driven deuterium plasma experiments. 1234567890():,; In the 2030s, large-scale fusion experiments with magnetically confined deuterium-tritium plasmas will be performed at the ITER1 tokamak, which is currently under construction in France. Following the first observations of ALEs in the late 1990s5, intensive studies during subsequent experimental campaigns revealed that ALEs are usually accompanied by a massive migration of energetic deuterons from the plasma core into the periphery[6,7]. Since both JT60SA and ITER will heavily rely on N-NB drive, it is important to understand ALE-like relaxation events and develop the ability to predict their occurrence in future experiments. The particular phenomenology depends on the magnetic geometry, plasma parameters and the distribution of energetic ions, so that numerical simulations are usually needed to determine the response of a particular system
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