I have been writing about nuclear fusion research lately. A lot of work is going into understanding in the detail the behavior of clouds of charged particles known as plasmas. One particular area of study is the how the plasma at the edge of the donut shaped chamber inside tokamak fusion reactors behaves. Researchers at the U.S. Department of Energy (DoE) Princeton Plasma Physics Laboratory (PPPL) have made breakthroughs in understanding very complex plasma edge phenomena. 
     One recent discovery at the PPPL is that accounting for the turbulent fluctuations in the magnetic fields that confine the plasma that serves as fuel for fusion reactions in a tokamak can seriously reduce the turbulent particle flux near the edge of the plasma.
   Computer modelling has shown that the net particle flux can be reduced by as much as thirty percent in spite of the fact that the average magnitude of turbulent particle density fluctuation rises by up to sixty percent. This indicates that even if the turbulent density fluctuation are more violent, they are moving particles out of the central chamber of the tokamak less effectively.
     PPPL researchers have developed a special mathematical code called “Gkeyll” that permits sophisticated computer models. This code is a form of modeling called “gyrokinetics” which simulates the plasma particles orbiting around the magnetic field lines at the edge of the ring of fusion plasma.
    PPPL physicist Ammar Hakim is the lead author of a Physics of Plasmas paper presented to the American Physical Society’s Division of Plasma Physics (APS-DPP) conference last Fall in which he provided an overview of his team’s work. Researchers from six different institutions adapted a state-of-the-art algorithm to the gyrokinetics system to develop key numerical breakthroughs needed to provide accurate simulations according to Hakim.
    The work at PPPL is part of a worldwide effort to comprehend the science needed to create sustainable fusion reactions on Earth. Plasmas of light atoms constitute over ninety nine percent of the matter in our universe. If we can harness the fusion process which fuses lighter atoms to heavier atoms and powers the stars themselves, we would have limitless energy for the needs of our future civilization.
     Noah Mandell is a graduate student in the Princeton University Program in Plasma Physics. He built upon the PPPL’s teams work to produce the first gyrokinetic code able to deal with magnetic fluctuations in what is referred to as the plasma scrape-off layer (SOL) at the edge of plasmas in tokamaks. His work was published in the British Journal of Plasma Physics.
     Mandell studies how blobs of plasma turbulence bend the magnetic field lines in the tokamak to produce the dynamics of what are called “dancing field lines.” He said, “We know there are more physical effects that need to be added to the code for detailed comparisons with experiments, but already the simulations are showing interesting properties near the plasma edge. The ability to handle bending of the magnetic field lines will also be essential for future simulations of edge localized modes (ELMs), which we would like to do better to understand the bursts of heat they cause that must be controlled to prevent tokamak damage.”
     His findings are unique because earlier gyrokinetic codes have simulated SOL blobs but assumed that the field lines were rigid. Extending a gyrokinetic code to calculate the movement of magnetic field lines is very computationally challenging. It requires special algorithms to ensure that two big terms balance each other to an accuracy of more than one part in one million.
    While previous codes that were used to model the turbulence in tokamak cores can include magnetic fluctuations, these older codes cannot simulate the SOL region. Mandall said, “The SOL requires specialized codes like Gkeyll that can handle much larger plasma fluctuations and interactions with the walls of the reactor.”
    Future work for the Gkeyll team will include the investigation of the exact physical mechanism that affects the dynamics of the edge of the plasma. This effect will probably be connected to the bending lines of the magnetic field. Hakim said, “This work provides stepping stones that I think are very important. Without the algorithms that we made, these findings would be very difficult to apply to ITER and other machines.”