How to predict crucial plasma pressure in future fusion plants

The accurate prediction of plasma pressure, a hot, charged gas that fuels the fusion reactions inside donut-shaped tokamaks, providing optimal conditions for future installations to capture and control Earth's energy. fusion that animates the sun and stars reactions. At the heart of these predictions is the pressure exerted by the scraping layer, the thin gas band at the edge of the plasma, on the divertor – the device that removes heat lost during fusion reactions.

Researchers at Princeton's Laboratory of Plasma Physics (PPPL) at the Department of Energy (DOE) have developed new insights into the physics governing the pressure balance in the scraping layer. This balance must ensure that the plasma pressure in the tokamak is high enough to produce a largely self-heating fusion reaction. The scale should also limit the potentially damaging impact of heat and plasma particles that strike the divertor and other components of the plasma-facing tokamak.

"The simple previous assumptions regarding the pressure balance in the scratch layer are incomplete," said PPPL physicist Michael Churchill, lead author of a Nuclear fusion paper that describes the new discoveries. "The codes that simulate the scratch layer have often thrown important aspects of physics, and the field is beginning to recognize it."

Fusion, the force that animates the sun and the stars, is the fusion of light elements in the form of plasma – the hot and charged state of matter composed of free electrons and atomic nuclei – generating enormous quantities of 'energy. Scientists seek to replicate the fusion on Earth to obtain a virtually inexhaustible source of energy for the production of electricity.

Key factors

Churchill and his colleagues at PPPL have determined the key factors of pressure balance by running the state-of-the-art XGCa computer code on the Cori and Edison supercomputers of the National Center for Scientific Computing of Energy Research (NERSC) , a user tool from the DOE Office of Science. The code processes the plasma at a detailed kinetic level – or particle motion – rather than as a fluid.

Among the main characteristics observed, there was the impact of mass drift of ions, impact that previous codes had largely ignored. Such excesses "can play a vital role" wrote the authors and "are very important to take into account".

The kinetic effects of particles due to ions having different temperatures depending on their direction were also considered important for the equilibrium of momentum or pressure. Since the temperature of the ions is difficult to measure in the scraping layer, the document states that "it would be necessary to intensify the diagnostic efforts to accurately measure the temperature and the flow rates of the ions and allow thus a better understanding of the role of ions in the SOL. "

The new findings could improve the understanding of the pressure of the scraper layer at the level of the entertainer, said Churchill, and could lead to accurate predictions for the international ITER experiment under construction in France and for others tokamaks of the next generation.