Scientists have found that reversing a standard approach to combating a key hurdle in generating fusion energy on Earth has dramatic effects. Theorists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have suggested taking the exact opposite of the prescribed procedure to drastically improve future results.
tear hole in plasma
The problem, known as “lock-tear mode,” is present in all of today’s tokamak, doughnut-shaped magnetic facilities designed to create and control the nearly limitless fusion energy that drives the sun and stars. Instability-induced patterns spin as hot, charged plasma (the fourth state of matter made up of free electrons and atomic nuclei that fuel fusion reactions) and tear holes called islands (confining gases) in the magnetic field, allowing Critical heat leakage.
When the pattern stops spinning and locks into place, the islands grow larger, and this growth rate increases heat loss, degrades plasma performance, and can cause disruptions that allow energy stored in the plasma to strike and damage the interior of the tokamak wall. To avoid such risks, the researchers now fire microwaves into the plasma to stabilize the modes before mode locks.
However, the PPPL findings strongly suggest that researchers can stabilize the mode after locking in a large, next-generation tokamak.In today’s tokamak, “the patterns lock on faster than one might think, and it becomes more difficult to stabilize them while they’re still spinning,” said Richard Nies, a doctoral student in the Princeton Plasma Physics Program and lead author of a nuclear fusion Papers that set out surprising findings.
Another disadvantage, he added, is that “these microwaves increase their width by refracting the plasma, making it less efficient to stabilize the mode while spinning, and this problem has become more severe in recent years.”
Along with these problems, in future large tokamak installations, such as the international facility ITER under construction in southern France, “the plasma is so huge that it rotates much more slowly, and the modes are still in them. It gets locked up pretty quickly at a very young age,” Ness said. “So it would be more efficient to switch stabilization packs in future large tokamaks and have them lock first and then stabilize them.”
This reversal can facilitate a fusion process that scientists around the world are seeking to replicate. The process combines light elements in plasma form to release large amounts of energy. “This provides a different way of looking at things, and may be a more efficient way to solve problems,” said Alan Lehman, distinguished researcher and co-author of the paper. “People should take the possibility of allowing islands to lock down more seriously,” Lehman said.
close to subversion
The recommended technique is unlikely to work in today’s tokamak, because the tear-mode island grows so fast and so large when locking down these facilities that once locked, the plasma is close to destruction. That’s why researchers must now use large amounts of energy to stabilize the mode at the expense of limiting fusion output. In contrast, the slow growth of islands in the next-generation tokamak “has a long way to go before causing disruption, so there’s a lot of time to stabilize the pattern,” Nies said.
Once the future tokamak’s modes are locked in place, the microwaves can be aimed directly at them, rather than only stabilizing them as they spin past the microwave beams in the current facility. “These theoretical calculations show the efficiency of our proposed scheme,” Nies noted.
What is needed now, he said, are experiments to test the proposed course of action. “We don’t want to open up ITER and then find out which strategies work. There’s a real opportunity to explore the physics that we’re solving in current devices.”
State-of-the-art computer code could advance efforts to harness fusion energy
Richard Nies et al, on stabilization of locked tear mode in ITER and other large tokamak, nuclear fusion (2022). DOI: 10.1088/1741-4326/ac79bd
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