Tokamaks use powerful magnetic fields to confine 100-million-degree plasmas and produce fusion reactions. Magnetic field fluctuations due to turbulence inherent in these plasmas are thought to reduce fusion energy production by causing particle and heat losses from the plasma to the reactor walls. These losses can degrade reactor performance, so understanding and controlling magnetic fluctuations is important for future fusion reactors. However, accurately measuring magnetic turbulence in plasma at a small-scale has been a significant challenge. Up to now, researchers used magnetic probes located outside of the plasma to measure waves that exit the plasma region. This new research developed a novel light probe that uses polarization properties that change when transmitted through the plasma. This probe reveals the presence of small-scale magnetic turbulence in detail.
Fusion scientists need a better understanding of physics to design future fusion reactors. By measuring magnetic fluctuations inside the high-temperature plasmas, scientists can validate the physics-based models they use to design reactors and predict their performance. For the first time, this work demonstrates the light probe as a way to internally detect magnetic fluctuations in fusion plasmas. The experimental observations have identified the fluctuations as originating from theoretically predicted micro-tearing mode (MTM) instability, which is a small-scale magnetic field perturbation that can modify the flow of heat and particles in the plasma, leading to enhanced energy loss.
The ability to locally measure magnetic turbulence in the internal plasma of tokomaks has been a goal of plasma researchers for decades. Scientists at the DIII-D National Fusion Facility have demonstrated a new technique that achieves this goal and, for the first time, provides detailed measurements of the theoretically predicted MTM instability. Fluctuating, or turbulent, small-scale magnetic fields can interact with other fluctuating fields, such as temperature or density. This can lead to enhanced particle and heat losses that can adversely affect fusion performance. However, these same fluctuations also change the properties of light waves passing through them. This makes the light waves useful tools for directly observing and measuring magnetic turbulence without perturbing the plasma. This method allows scientists to directly probe plasmas, which can reach temperatures up to 100 million degrees Celsius, and to detect very small changes in the magnetic fields. Scientists are using these results to validate physics-based simulations that are being developed to better understand tokamak confinement. This work will also serve to guide researchers to improve reactor designs supporting the development of fusion energy.
This research was supported by the Department of Energy (DOE) Office of Science, Office of Fusion Energy Sciences. It used the DIII-D National Fusion Facility, a DOE Office of Science user facility.