Scientists have discovered the remarkable impact of reversing a standard method for combatting a key obstacle to create sustained nuclear fusion on Earth. Theorists at the U.S. Department of Energy’s (DoE) Princeton Plasma Physics Laboratory (PPPL) have put forth a proposal to do exactly the opposite of the prescribed procedure to sharply improve future results.
The problem is referred to a “locked tearing modes”. This occurs in all of today’s tokamaks which are doughnut-shaped magnetic chambers designed to create and control the same nuclear fusion that powers the Sun and stars. These modes cause instability in the plasma and tears holes in islands in the magnetic field that confines and heats the plasma. This results in the leakage of heat that is needed to trigger the fusion.
These magnetic islands grow larger when the modes stop rotating and lock into place. This growth rate increases the heat loss, reduces the plasma performance and can cause disruptions that allow the energy stored in the plasma to strike and damage the inner walls of the tokamaks. In order to avoid such risks, researchers now beam microwaves into the plasma to stabilize modes before they can lock.
The PPPL findings suggest that the researchers stabilize the modes in large, next-generation tokamaks after they have locked. Richard Nies is a doctoral student in the Princeton Program in Plasma Physics. He is the lead author of a paper in the journal Nuclear Fusion that reveals the surprising findings. He said that in today’s tokamaks “these modes lock more quickly than people had thought, and it becomes much harder to stabilize them while they're still rotating.”
He added that another drawback is that “these microwaves increase their width by refracting off the plasma, making the stabilization of the mode while it's rotating even less efficient today, and this problem has become more exacerbated in recent years.”
In addition to these issues, in large future tokamaks like the ITER under construction in France, “the plasma is so huge that the rotation is much slower and these modes lock pretty quickly when they're still pretty small," Nies said. "So, it will be much more efficient to switch up the stabilization package in big future tokamaks and let them first lock and then stabilize them.”
That reversal could facilitate the fusion process which researchers around the world are seeking to reproduce. The fusion process combines light elements in the form of plasma to release huge amounts of energy. Allan Reiman is a distinguished research fellow and co-author of the paper. He said, "This provides a different way of looking at things and could be a much more effective way to deal with the problem. People should take more seriously the possibility of allowing the islands to lock.”
The recommended technique is not likely to work in the current tokamaks because tearing mode islands grow so fast and are so large when they lock in these devices that the plasma is close to disrupting once it has locked. That is why researchers must now use large amounts of power to stabilize the modes at the cost of limiting the energy released by fusion. In contrast, the slow growth of islands in the next generation tokamaks “leaves a long way to go before you have a disruption so there's a lot of time to stabilize the mode.”
Once the modes in future tokamaks are locked in place, microwaves can target them directly instead of stabilizing them only when they rotate past the microwave beam in current tokamaks. Nies pointed out that “These theoretical calculations show the efficiency of what we are proposing.”
Nies said that what is needed now are experiments to test the proposed course of action. “We would not want to turn on ITER and only then find out which strategy works. There is real opportunity to explore the physics that we address in current devices.”