In the quest for sustainable and clean energy, scientists are continually pushing the boundaries of fusion research. A recent study published in the journal “Fusion Energy” (formerly known as Nuclear Fusion) has shed new light on the behavior of runaway electrons in tokamak plasmas, a critical area of research for future fusion reactors like ITER. The study, led by Yueqiang Liu from General Atomics in San Diego, California, focuses on the excitation of Alfvén eigenmodes (AEs) by runaway electrons, a phenomenon that could significantly impact the stability and efficiency of fusion reactions.
Tokamaks, the doughnut-shaped devices designed to harness the power of fusion, rely on the confinement of high-energy plasmas. However, disruptions in these plasmas can lead to the generation of runaway electrons, which can pose a substantial risk to the integrity of the reactor. Understanding and controlling these runaway electrons is therefore a top priority for fusion researchers.
Liu and his team have updated the MARS-K code, a non-perturbative MHD-kinetic hybrid code, to include relativistic effects for kinetic fast particles. This update enables the code to model the excitation of Alfvén eigenmodes by runaway electrons in post-disruption tokamak plasmas. “By incorporating relativistic effects, we can more accurately simulate the behavior of runaway electrons and their interaction with the plasma,” Liu explained.
The researchers applied the updated code to runaway electron beams in both the DIII-D tokamak, a major experimental facility in San Diego, and the ITER tokamak, which is under construction in France and represents the next step in fusion energy research. Their simulations revealed a complex “zoo” of AE modes triggered by trapped runaway electrons due to precessional drift-kinetic resonances. These modes possess radially different eigenmode structures, ranging from global modes to core-localized ones.
One of the most significant findings of the study is the quantitative match between the computed mode frequency and experimental measurements in the DIII-D tokamak. “This agreement gives us confidence that our simulations are accurately capturing the underlying physics,” Liu noted. The study also identified the nature of the modeled eigenmodes, which were found to be compressional Alfvén eigenmodes (CAEs) in DIII-D and a mixture of CAE and shear Alfvén waves in ITER.
The implications of this research are far-reaching for the energy sector. By improving our understanding of runaway electrons and their interaction with tokamak plasmas, scientists can develop more effective strategies for controlling these potentially damaging particles. This, in turn, could enhance the stability and efficiency of future fusion reactors, bringing us one step closer to realizing the dream of clean, limitless energy.
As we look to the future, the work of Liu and his team underscores the importance of continued investment in fusion research. “Every new discovery brings us closer to overcoming the challenges of fusion energy,” Liu said. With each breakthrough, we edge closer to a future powered by sustainable, clean energy sources.