While solar power harnesses energy produced by the Sun, fusion power seeks to harness the very process used by the Sun to generate a practically limitless supply of clean electricity. Despite decades of research and numerous breakthroughs, “net-gain” nuclear fusion is yet to appear. One of the hurdles is the so-called density, or Greenwald, limit that sees the plasmas within experimental fusion reactors (called tokamaks) spiraling apart and disrupting the fusion process. Now scientists have come up with a new theory as to why this occurs that, if proven, could provide a way to clear the density limit hurdle.
In fusion power, energy is generated through the combining of atomic nuclei in plasmas of sufficient heat and density. However, in experiments, minute, bubble-like islands appear in the plasma and collect impurities kicked up from the tokamak wall that cool the plasma. Since tokamak plasma densities are much lower than the Sun’s, tokamak plasma temperatures need to be even hotter than those in the Sun, so any cooling of the plasma can disrupt the process. But surprisingly, simply pumping up the heat doesn’t solve the problem.
“The big mystery is why adding more heating power to the plasma doesn’t get you to higher density,” said David A. Gates, a principal research physicist at the U.S. Department of Energy’s (DoE) Princeton Plasma Physics Laboratory (PPPL) and co-author of a proposed solution with Luis Delgado-Aparicio, a post-doctoral fellow at PPPL and a visiting scientist at MIT’s Plasma Science Fusion Center. “This is critical because density is the key parameter in reaching fusion and people have been puzzling about this for 30 or 40 years.”
The scientists believe that the islands’ detrimental effect on the fusion process is actually twofold. While the impurities cool the plasma, they believe that they also act as shields that block out added power. When the islands grow large enough, more power escapes from the islands than can be pumped into the plasma using ohmic - or joule - heating, causing the electric current that helps to heat and confine the plasma to collapse. This allows the plasma to fly apart in a flash of light.
Using a tokamak called Alcator C-Mod located at MIT, and the DIII-D tokamak at General Atomics in San Diego, Gates and Delgado-Aparicio hope to put their theory to the test in a number of experiments. One of these will involve injecting power directly into the islands to see if that leads to higher density. If it does, it could allow future tokamaks to attain the extreme density and temperatures that fusion requires.