Translation. Region: Russian Federation –
Source: Peter the Great St. Petersburg Polytechnic University –
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Scientists from Peter the Great St. Petersburg Polytechnic University and the A.F. Ioffe Physicotechnical Institute have conducted the world's first study of the electric field behavior and plasma rotation velocity during edge localized modes (ELMs)—fast, short bursts of activity that inevitably occur during the operation of fusion reactors. The researchers experimentally determined the nature of the redistribution of energy, current, and electric field in the edge plasma, which, in the worst case, can damage the reactor walls. Investigating the mechanisms of these processes is essential for the development of reliable fusion energy. The results were published in the prestigious journal Physics of Plasmas, the research was supported by a grant from the Russian Science Foundation.
In a tokamak, the plasma is confined by a magnetic field and resembles a very hot medium, constantly experiencing oscillations, flows, and instabilities. One of the most important instabilities is edge localized modes, or ELMs. For efficient fusion, the plasma in a tokamak must be in the so-called H-mode (enhanced confinement mode). In this state, an invisible barrier forms at the edge of the plasma, acting like a wall to trap the heat inside. However, due to the enormous pressure difference across this barrier, disruptions—edge localized modes—occur periodically. These can be compared to a safety valve: they periodically release excess energy and impurities, preventing the plasma from escalating out of control. However, if this valve is triggered too forcefully, the impact on the chamber walls can be devastating to the entire facility.
Modern approaches to plasma physics considered only large ELMs dangerous, while small ones were considered not only safe but also almost ideal for fusion plant operation: plasma confinement was good, and there were no destructive large bursts. Scientists from St. Petersburg Polytechnic University and the Ioffe Institute conducted experiments on the Globus-M2 spherical tokamak and, for the first time in the world, determined how peripheral localized modes profoundly restructure the entire peripheral plasma region.
Experiments have shown that during an ELM, the plasma temperature and density in the near-plasma region increase sharply, currents outside the plasma change noticeably, fast ions are transported and accelerated, suprathermal electrons are lost, and plasma filaments are formed. It has been shown that the plasma rotation velocity increases during an ELM, with the effect extending several centimeters into the plasma rather than being limited to a narrow layer where the ELM develops, as previously thought. And all of this occurs in microseconds, meaning it's extremely fast. Individually, small ELMs appear harmless, but together they create intense and complex dynamics at the plasma periphery. "Small ELMs are not simply weakened bursts, but an independent dynamic regime in which the plasma periphery operates according to its own rapid and complex laws," noted Arseny Tokarev, a research assistant at the Scientific Laboratory of Advanced Methods for Studying Spherical Tokamak Plasma at the Institute of Physics and Mechanics at St. Petersburg Polytechnic University.
The results of the measurements showed that the electron temperature in the boundary region during ELM increases up to 5 times, the concentration increases approximately 2 times, the plasma rotation speed increases by approximately 50% at a depth of up to several centimeters, fast ions are registered with an energy 6 keV higher than the injection energy, and filaments move at a speed of 3–10 km/s.
The practical significance of these results for the future of global fusion energy is enormous, as they provide a more realistic understanding of the stresses on the walls of fusion reactors. Future fusion reactors must operate continuously for long periods, rather than in short experimental pulses. Modes with small ELMs are considered prime candidates. Furthermore, unique data were obtained on the behavior of plasma parameters during ELMs, in particular, the rotation speed, which was measured for the first time in the world. This facilitates the transition from empirical selection of operating modes to deliberate control and reduces the risk of unexpected effects when scaling from experimental setups to reactors. The results will help make future fusion reactors not just operational, but reliable, predictable, and economically feasible, noted Alexander Yashin, head of the High-Temperature Plasma Diagnostics research laboratory at the Institute of Physics and Mechanics at St. Petersburg Polytechnic University.
The research was supported by grant No. 23-72-00024 from the Russian Science Foundation using the Federal Center for Collective Use "Materials Science and Diagnostics in Advanced Technologies" at the A.F. Ioffe Physical-Technical Institute, which includes the unique scientific facility "Spherical Tokamak Globus-M."
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