A key requirement for future facilities aiming to capture and control the fusion energy that drives the sun and the stars on Earth is accurate predictions of the pressure of the plasma ̵
Researchers at the US Department of Energy's Princeton Plasma Physics Laboratory (PPPL) have gained new insights into the physics of pressure balance in the stripping layer. This balance must ensure that the pressure of the plasma throughout the tokamak is high enough to cause a largely self-heating fusion reaction. The balance must also limit the potentially harmful influence of heat and plasma particles striking the divertor and other plasma-facing components of the tokamak.
"Previous simple assumptions about the pressure balance in the release liner are incomplete." said the PPPL physicist Michael Churchill, lead author of a publication of Nuclear Fusion which describes the new findings. "The codes that simulate the peel-off layer have often discarded important aspects of physics, and the field is beginning to recognize this."
Fusion, the power that drives the sun and stars, is the fusion of light elements in the form of plasma – the hot, charged state of matter of free electrons and atomic nuclei – that generates enormous amounts of energy. Scientists are trying to replicate the fusion on earth for a virtually inexhaustible power supply for power generation.
Churchill and PPPL colleagues determined the key factors for pressure equalization by using the state-of-the-art XGCa computer code on the Cori and Edison supercomputers of the National Energy Research Scientific Computing Center (NERSC ), a user facility of the DOE Office of Science. The code treats plasma at a detailed kinetic level – or particle motion level – rather than as a fluid.
The main features included the influence of mass drifting of ions, largely ignored by earlier codes. Such drifts "can play an integral role," the authors wrote, and "are very important to consider".
Also important for momentum or pressure equalization were the kinetic particle effects due to ions with different temperatures in their direction. As the temperature of ions in the stripping layer is difficult to measure, the publication states: "Increased diagnostic efforts should be made to accurately measure ion temperature and flux, thus enabling a better understanding of the role of ions in SOL . "
The new findings may improve the understanding of the peel-off layer pressure at the divertor and lead to accurate predictions for the international ITER experiment under construction in France and other next-generation tokamaks.
Clearly seen: Revised computer code accurately models instability in fusion plasmas
R.M. Churchill et al., Pressure Compensation in a Lower Collision, Attached Tokamak Peelcoat, Nuclear Fusion (2019). DOI: 10.1088 / 1741-4326 / ab2af9
Forecast for today: How to predict the key plasma pressure in future fusion systems? (2019, 20th of September)
21 september 2019 retrieved
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