Build an artificial sun and you will get artificial solar flares. Our own star is a roiling mass of plasma (hot, ionised gas) that sometimes shoots powerful eruptions into space. Like the Sun, the hot plasma in a fusion device doesn’t sit there quietly – it bucks against its magnetic restraints, building up pressure until all of a sudden a stream of energy hotter than the surface of the sun bursts forth headed towards the inner wall of the vacuum chamber.
Needless to say, fusion researchers are eager to learn how to dampen these flares they call Edge Localised Modes, ELMs for short, before they cause any damage. Understanding and controlling such plasma instabilities to ensure smooth, high-performance plasmas is one of the eight missions in the EUROfusion Roadmap to Realising Fusion Energy.”
Left unchecked, ELMs can eventually blast the inner wall of a fusion device with a heat pulse of up to a thousand megawatts per square meter. If you’re hazy on the units, that’s more intense than a blowtorch, hotter even than the surface of the sun, and in fact is most comparable to a lightning strike. Too many of those ELMs, and the power plant would need to shut down for costly repairs, causing delays to important fusion research.
ELMs could be good, however. Counter-intuitively, causing the plasma to produce ELMs more often than it would on its own may actually reduce the heat load on the plasma exhaust components of a tokamak fusion device. This important finding is the result of detailed computer simulations by a European research team lead by Andres Cathey at the German Max Planck Institute for Plasma Physics (IPP).
In the scientific journal Plasma Physics and Controlled Fusion, Cathey and his colleagues show how shooting the plasma edge with a tiny pellet of frozen deuterium triggers a smaller, more manageable ELM ahead of time. Their new virtual simulations match actual experiments performed at EUROfusion research tokamak devices like JET and ASDEX Upgrade, as well as at the U.S. tokamak DIII-D.
Balancing energy and area
As in all complex systems, just popping ELMs early isn’t the end of this story. A trade-off will be necessary, because the simulations show that a triggered ELM focuses its energy on a smaller wall area than an unmitigated one. Achieving the right balance between the reduced ELM size and deposition area will therefore crucial to properly use this ELM control method. Still, bursting the bubble might just turn out to be a great idea for future fusion power plants.
Comparing spontaneous and pellet-triggered ELMs via non-linear extended MHD simulations