In the quest for sustainable and efficient nuclear fusion energy, scientists are continually exploring ways to improve plasma confinement in tokamaks. A recent study led by B.S. Patel from the UK Atomic Energy Authority (UKAEA) at the Culham Campus, sheds new light on the complex interplay between electromagnetic microtearing modes (MTMs) and equilibrium E × B sheared flows in spherical tokamaks. The findings, published in Nuclear Fusion, could significantly impact the design and operation of future fusion reactors, potentially accelerating the path to commercially viable fusion energy.
The study delves into the behavior of MTMs, which are known to degrade plasma confinement and thus reduce the efficiency of fusion reactions. Patel and his team investigated how E × B sheared flows, which are flows of plasma driven by electric and magnetic fields, influence the saturation of these disruptive modes. Their work builds on previous studies that have shown mixed results regarding the impact of E × B shear on MTM suppression.
“Our research indicates that the response of MTMs to E × B shear is highly dependent on the magnetic shear, which is a measure of how the magnetic field changes within the plasma,” Patel explains. “At low magnetic shear, the growth rate of MTMs is relatively insensitive to the ballooning angle, but at higher magnetic shear, the growth rate peaks at specific angles, providing a window for suppression by E × B shear.”
The team analyzed data from two experimental regimes: the Mega Ampere Spherical Tokamak (MAST) at Culham and the National Spherical Torus Experiment (NSTX) in the United States. They found that in MAST, MTM-driven transport was more resilient to suppression via E × B shear compared to NSTX, where the transport was significantly suppressed.
This difference is crucial for the design of future spherical tokamaks, as it suggests that the safety factor profile—the ratio of the toroidal magnetic field to the poloidal magnetic field—plays a pivotal role in determining the impact of E × B shear on MTM saturation. “Understanding these dynamics could lead to more effective control strategies for MTMs, ultimately improving plasma confinement and the overall efficiency of fusion reactions,” Patel notes.
The implications of this research extend beyond academic curiosity. For the energy sector, particularly for companies and governments investing in fusion technology, these findings offer valuable insights into optimizing plasma performance. By fine-tuning the safety factor profile and leveraging E × B shear, future spherical tokamaks could achieve higher beta values—a key parameter for fusion efficiency—leading to more stable and efficient reactors.
The study also highlights the importance of bicoherence analysis, a technique used to examine the coupling between different plasma modes. The researchers found that E × B shear enhances the coupling between linearly driven drift-waves and zonal modes, which in turn enhances damping and suppresses MTM-driven transport. This discovery could pave the way for new control mechanisms that harness these interactions to stabilize plasma and improve confinement.
The research published in Nuclear Fusion, the English translation of the journal title, is part of a broader effort to understand and mitigate the challenges posed by MTMs in spherical tokamaks. As the field moves closer to practical fusion energy, studies like Patel’s will be instrumental in shaping the next generation of fusion reactors, potentially revolutionizing the energy sector with a clean, abundant, and sustainable power source.
