The world’s largest stellarator uses external magnetic coils to generate fields that can contain the extreme conditions required to fuse nuclei. The Large Helical Device (LHD) operating in Japan creates a magnetic bottle that can hold hot plasma with the intention of re-creating conditions for nuclear fusion akin to those on the sun.
Shaping and controlling these fields is somewhat like analyzing a soap bubble for its weakest spot and creating a solution to keep it afloat.
Engineering Physics Professor Chris Hegna analyzes magnetic fields to understand the conditions that cause them to fail. “We generate these magnetic fields externally and what we would like to do is precisely control what they are doing in the confinement region where the plasma exists,” Hegna says. “Since the fields are still in the three-dimensional phase, they are sensitive to small perturbations that give magnetic irregularities in the confinement region and produce things we don’t want.”
The irregularities are small magnetic “islands” that can overlap, creating a path for the hot plasma to escape.
Generally, these islands are something to avoid. However, researchers at LHD intentionally have directed fields to create islands in an effort to study their properties. They found the plasma was able to heal itself. The magnetic islands completely disappeared under the proper conditions, a result that was in sharp contrast to existing theoretical models, which were unable to predict magnetic island healing.
Hegna developed a new theory to explain the phenomena based upon the properties of the plasma flow. His theory was tested experimentally and validated. “If the flow physics is of the right amplitude and the right structure, it can produce enough of a plasma response that it can actually completely offset the magnetic field perturbation generated externally,” Hegna says. “So what happens is the plasma spontaneously cures itself.”