Tokamak rotation may not be as important to plasma confinement as previously thought
Tokamak rotation may not be as important to plasma confinement as previously thought lead image
The rotation of tokamaks has typically been thought to play an important role in plasma confinement and stabilization, but research suggests this may not be as beneficial as previously believed. To study this, Seiferling et al. published results of simulations of the effects of applied external torque and structure formation in the E×B flow shear, or the spatial rate of change of the plasma’s rotation driven by electric and magnetic forces, on plasma confinement and turbulence suppression.
Though E×B flow shear plays a critical role on its own, the researchers found little to no change in turbulent transport with the addition of external torque to E×B shear. Mesoscale shear structures, known as staircases, are effective in regulating turbulence. However, the external torque does not contribute to these staircases in a simple additive way. While the average shear is modified, the turbulent transport remains unaffected. Even a large torque does not promote plasma confinement.
“In general, it is, of course, true that more shear leads to increased turbulence suppression,” said author Felix Seiferling. “However, we found that the situation in a likely operating regime is more complex.”
To study the effects on staircases, the group conducted simulations with parameters chosen within the likely experimental operating regime. They applied an external torque to match experimental plasma rotation values and included a temperature gradient, which placed the system slightly above the stability threshold.
The authors believe these findings may cause a paradigm shift. “There are still many open questions about E×B structure formation, but the results might require the community to rethink the conditions necessary to obtain improved confinement,” said Seiferling.
Source: “The interplay of an external torque and E×B structure formation in tokamak plasmas,” by F. Seiferling, A. G. Peeters, S. R. Grosshauser, F. Rath, and A. Weikl, Physics of Plasmas (2019). The article can be accessed at https://doi.org/10.1063/1.5120903 .
