Part 2 of 2 Parts (Please read Part 1 first)
     One complication with the design and operations of divertors in tokamaks is impurity contamination in the plasma caused by edge-localized modes or ELMs. Some of these rapid, high-energy events similar to solar flares can damage or destroy vessel components such as divertor plates. The frequency of the ELMs is an indicator of the amount energy released from the plasma to the wall. High-frequency ELMs can release low amounts of plasma on each eruption. However, if the ELMs are less frequent, the release of plasma and energy during each eruption is high with a much greater probability for serious damage. Recent research has been studying ways to control and increase the frequency of ELMs such as injection of pellets or using additional magnetic fields at very small magnitudes.
     Unterberg’s team found that when they placed tungsten far from the high-flux strike-point, it significantly increased the probability of contamination when exposed to low-frequency ELMs that have higher energy content and surface contact per event. The team also found that this divertor far-target region was more prone to contaminate the SOL even though it generally has lower fluxes than the strike-point. These results are counter-intuitive, but they are being confirmed by divertor modeling efforts in relation to this project and future experiments on DIII-D.
     This project involved a team of experts from across North America, including collaborators from Princeton Plasma Physics Laboratory, Lawrence Livermore National Laboratory, Sandia National Laboratories, ORNL, General Atomics, Auburn University, the University of California at San Diego, the University of Toronto, the University of Tennessee—Knoxville, and the University of Wisconsin-Madison, as it provided a significant tool for plasma-material interaction research. DOE’s Office of Science (Fusion Energy Sciences) provided support for the study.
     The research of Unterberg and his team was published online earlier this year in the journal Nuclear Fusion. Their research could be of immediate benefit to the Joint European Torus, or JET, and ITER, now under construction in Cadarache, France, both of which use tungsten armor for the divertor. Unterberg said, “Where is it best to put tungsten, and where should you not put tungsten? Our ultimate goal is to armor our fusion reactors, when they come, in a smart way.”
     Unterberg says that the ORNL’s unique Stable Isotopes Group which developed and tested the enriched isotope coating before putting it in a form that was useful for the experiment made the research possible. That isotope would not have been available anywhere but from the National Isotope Development Center at ORNL which maintains a stockpile of almost every element separated into different isotopes. Unterberg said, “ORNL has unique expertise and particular desires for this type of research. We have a long legacy of developing isotopes and using those in all kinds of research in different applications around the world.”
      The next project for Unterberg’s team will be to investigate how putting tungsten into differently shaped divertors might affect contamination of the core. They theorize that different divertor geometries could minimize the effects of plasma-material interactions on the core plasma. Divertors are a necessary component for a magnetically-confined plasma device. Knowing the optimal shape for a divertor would bring scientists one step closer to a viable commercial nuclear fusion reactor. Unterber said, “If we, as a society, say we want nuclear energy to happen, and we want to move to the next stage, fusion would be the holy grail.”