Alfvén eigenmodes — unstable, wave-like disturbances produced by fusion reactions that move through plasma — pose a significant challenge for the doughnut-shaped, next-generation fusion reactors called “tokamaks.” When heated to temperatures approaching 100 million degrees Celsius, tritium and deuterium fuels react, producing high-energy helium ions called alpha particles. These particles in turn heat the plasma, which sustains fusion reactions.
Scientists are still trying to better understand these Alfvén eigenmodes — specifically, how they can drive alpha particles to escape from the reaction chamber even before the plasma is heated. If enough of these waves are excited — and they throw out enough alpha particles — the walls of the reactor chamber are endangered. So, then, is its capacity to heat fuel efficiently—a theoretical prediction confirmed by these recent experiments.
In these trials, researchers used deuterium beam ions to simulate alpha particles and other higher-energy beam ions that occur in fusion reactors. They found that when the ions excited many Alfvén waves, losses of high-energy particles of up to 40 percent were observed.
After the research was conducted, physicists produced a quantitatively accurate model of the effects of these Alfvén waves had on high-energy deuterium beams in the DIII-D tokamak. The modeling predicted that up to 40 percent of the energetic particles would be lost — in quantitative agreement with the experimental results. The modeling demonstrated that it’s possible to make quantitatively accurate predictions of the effect of multiple Alfvén waves on energetic particles in the DIII-D tokamak — a world first for this kind of high-performance plasma.
The joint findings represent a major advance in our understanding of the fusion process, and made it possible for researchers to suggest specific plasma conditions that could dramatically lower the rate of energetic-particle loss by reducing the number of Alfvén waves.