I have blogged before about the problems with magnetic confinement in nuclear fusion reactors such as stellarators and tokamaks. Now scientists from Princeton University and the Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) may have found a way to deal with a problems that have been known for decades.
   Tokamaks are shaped like a donut. They use magnetic fields to trap a twisted donut of hydrogen plasma that is as hot as the sun. As the plasma moves around the reaction chamber, it is compressed by magnetic fields. The plasma must be kept from touching any part of the physical structure of the reaction chamber. The goal is to achieve a fusion reaction in the plasma that will generate a great deal more energy that the energy required to confine and heat the plasma.
    Unfortunately, there is a serious problem in stabilizing the plasma. As the plasma travels around the tokamak, “bubble-like” structures referred to as “magnetic islands” form. The Oak Ridge National Laboratory released a report back in 1974 that said, “These may be produced from helical current perturbations in the plasma. Physically the islands are a result of having a harmonic component of the radial component of the perturbed magnetic field in resonance with the unperturbed rational surface field lines, in the presence of shear.”
     When one of these islands disrupts the flow of the plasma, the tokamak can lose temperature to the point where it can take days or even months to restore. The plasma can also escape the safe zone and actually hit and damage the physical structure of the tokamak. Repairing such damage in expensive, difficult and time-consuming. For decades, fusion researchers have been working on ways of dealing with these islands. Such problems often end plasma streams that have been sustained for lengthy periods.
    It has been forty-six years since the Oak Ridge report and magnetic islands are still one of the major problems with the development of tokomaks. Stellarators were developed before tokamaks but fell out of favor with researchers even though it is claimed that they do a better job of controlling the configuration of the plasma than tokamaks. While the exterior of a tokamak is smooth, the surface of a stellarator is covered with hundreds of cylinders and rectangular compartments to hold the magnets that confine the plasma.
     The work at the U of P and the PPPL is a continuation of research that has been ongoing since the 1980s. Researchers found that injecting radio waves helped to stabilize the plasma stream and reduced the factors that led to islands disrupting the geometry of the plasma. In 2019, a new group of researchers identified a way to improve just how much help these radio waves could be when they were boosted by exploiting tiny fluctuations of temperature called perturbations.
    The researchers found that when they “deposit” radio frequency waves inside the magnetic islands, the islands can be suspended and prevented from growing. In the new research paper, researchers suggested that by the careful control and variation of the temperature of the plasma, they can magnify the controlling effects of radio waves.
    This nonlinear treatment of plasma temperature does introduce new variables that will need to be dealt with. The researchers are excited about the possibility of new plasma control schemes and other subsequent research.