Scientists have worked for decades to control nuclear fusion, wherein isotopes of hydrogen fuse to form helium, thereby releasing enormous amounts of energy. The nuclear fusion reaction is the same process that produces the vast energy of the Sun. If achieved, it would provide a near unlimited form of clean energy.
The fusion reactions require the isotopes to be confined at temperatures of hundreds of millions of degrees. At these temperatures, the atoms in the fuel are ripped apart, forming a plasma. Very hot plasma is thought to be best confined by magnetic fields.
Now, after many years of gradual progress, we are entering an exciting period in attaining fusion by magnetic confinement.
The international ITER project in France that will demonstrate the feasibility of using fusion as a large scale power source is now 60 percent complete. ITER is known as the “world’s biggest fusion experiment” and, at present, the first plasma is scheduled for the middle of the next decade. A key remaining problem, however, is instability of the plasma, which can lead to events called disruptions, during which the plasma is lost very rapidly.
A recent study on the largest operating magnetic confinement device found that at least 95 percent of the disruptions on the device are preceded by the appearance of large-scale imperfections of the magnetic field called magnetic islands.
We recently predicted a physical effect, which we call “RF current condensation,” that can greatly improve a process that stabilizes and suppresses these magnetic islands. The process involves injecting RF (radio frequency) waves to drive electrical current aimed at the island center. It is, in effect, a kind of “radiation therapy” for magnetic islands.
The largest contemporary magnetic confinement devices are of a kind called a “tokamak.” In these doughnut-shaped, or toroidal devices, the magnetic field forms a set of concentric toroidal barriers, called “magnetic surfaces,” that confine the plasma. The left side of the figure shows several such magnetic surfaces in a segment of a tokamak.
Magnetic surfaces in a segment of a tokamak. A magnetic island is shown in red on the right side of the figure. Allan Reiman/Nat Fisch
An instability called a “tearing instability” can rip these surfaces apart and reform them to create magnetic islands that twist around the internal magnetic surfaces—like a malignant growth wrapped around an internal organ. An island is shown in red on the right side of the figure.
The islands short-circuit the plasma confinement, and multiple islands can overlap. If left uncontrolled, the growing magnetic islands can cause a disruption of the plasma.
The recent breakthrough builds on two papers that we published decades ago. One of those papers, published in 1978, predicted that radio frequency waves that are appropriately launched can be used to drive electrical currents efficiently in a plasma. The intention was to use that process to allow tokamaks to operate in the steady state. Indeed, this prediction led to experimentation on tokamaks worldwide.
The second paper, published in 1983, showed that the current driven by the radio frequency radiation can also be manipulated to stabilize and suppress magnetic islands. This finding also led to experimental campaigns worldwide, this time to stabilize magnetic islands, and the approach will be applied in ITER. Our recent breakthrough finds that RF current condensation can facilitate the stabilization and allow the stabilization of larger islands than previously thought possible.
RF Current Condensation
The new discovery arose from taking a fresh look at a phenomenon discussed in the 1983 paper—the sensitivity of the RF-driven current to the temperature in the island. The waves make the center of the island hotter than its periphery, leading to an increase in the RF-driven current at the center of the island relative to that at the periphery, which creates a stabilizing effect.
In revisiting this, we realized that we also needed to consider that the increased temperature at the island center increases the deposition of power there. The increased power deposition leads to a further increase in the temperature, giving a positive feedback loop.
We found a threshold power density above which the mathematical solution experiences what is called a “bifurcation,” with an associated discontinuous jump in the temperature. In the terminology of mathematical “catastrophe theory,” there is a “fold catastrophe.” The word “catastrophe” in catastrophe theory indicates a dramatic change in a mathematical solution.
Here, it also marks a “catastrophe” for the magnetic island, which is dramatically stabilized. The enhanced temperature at the island center, produced by the positive feedback loop on the power deposition combined with the sensitivity of the RF-driven current to the local temperature, produces a strong concentration of the RF-driven current at the island center, where it is most effective at stabilizing the island.
This RF current condensation then allows the stabilization of larger islands than previously thought possible. It also means that the RF waves need not be aimed so precisely to achieve stabilization, which opens the door to using waves that may not be localized very precisely.
We believe that application of this effect can be an important step towards preventing disruptions in magnetically confined plasmas.
Allan Reiman is a Distinguished Research Fellow at the Princeton Plasma Physics Laboratory and a Lecturer with Rank of Professor in the Department of Astrophysical Sciences at Princeton University.
Nat Fisch is Professor of Astrophysical Sciences at Princeton University and Associate Director for Academic Affairs at the Princeton Plasma Physics Laboratory.
The views expressed in this article are the authors' own.