One of the major problems with achieving commercial nuclear fusion is the fact that when magnetic confinement is used to squeeze a plasma, instabilities can develop which interfere with the production of fusion. This is a problem is universal in all tokamaks which are donut-shaped experimental fusion reactors. The Princeton Plasma Physics Laboratory (PPPL) of the U.S. Department of Energy (DoE) is working on a way to control the worst of these plasma instabilities.
The PPPL is working on what are called “tearing modes.” These are instabilities in plasma that create magnetic islands. These islands are like bubbles in a fluid. They can grow and cause disruptions that halt the fusion reaction and can actually damage the tokamaks where they occur.
Fusion researchers in the 1980s discovered that they could inject radio-frequency (RF) waves to cause a current that would stabilize tearing modes and reduce the chance of fusion disrupting events. This is referred to as “RF current drive.” They did not know then was that small changes in the temperature of the plasma could enhance the stabilization of the plasma beyond a specific threshold of power. The PPPL is exploiting this process to improve the stability of plasmas.
Tiny fluctuations in temperature influences the intensity of the current and how much of that current enters the magnetic islands. The interaction of the fluctuations and the amount of energy that winds up in the magnetic bubbles interact in a complex non-linear fashion. The interaction between the fluctuations and the energy deposited in the islands can stabilize the plasma. This stabilization is less sensitive to misalignments of the injection current. The result of this process is called “RF current condensation” which refers to the increase in RF energy inside the magnetic islands that keeps the islands from growing and disrupting the plasma reaction.
Allan Reiman is a theoretical physicist at PPPL and lead author of the paper reporting their work. He said, “The power deposition is greatly increased. When the power deposition in the island exceeds a threshold level, there is a jump in the temperature that greatly strengthens the stabilizing effect. This allows the stabilization of larger islands than previously thought possible.”
Nat Fisch is associate director for academic affairs at PPPL and coauthor of the report. He published a paper in the 1970s which revealed how RF waves could be used to drive currents into tokamak plasmas. Reiman published a paper in 1983 that predicted that RF current drive could be utilized to stabilize tearing modes. He said, “The use of RF current drive for stabilization of tearing modes was perhaps even more crucial to the tokamak program than using these currents to confine the plasma. Hence Reiman's 1983 paper essentially launched experimental campaigns on tokamaks worldwide to stabilize tearing modes. Significantly, in addition to predicting the stabilization of tearing modes by RF, the 1983 paper also pointed out the importance of the temperature perturbation in magnetic islands.”
He went on to say, “We basically went back 35 years to carry that thought just a bit further by exploring the fascinating physics and larger implications of positive feedback. It turned out that these implications might now be very important to the tokamak program today.”