How blobs affect plasma dynamics in tokamaks

Blobs – field-aligned electron and ion filaments that form at the edges of tokamaks – interact both elastically and inelastically with neutral particles. The inelastic collisions are not as well-studied as their elastic counterparts, though they play a crucial role in important phenomena such as ionization and charge-exchange collisions. To identify how different source terms affect seeded blob dynamics, Thrysøe et al. conducted simulations of the inelastic interactions of blobs with neutral particles.

“Seeded blob simulations, as this one, are good because you have a very controlled setup, but also very far from realistic conditions,” said author Alexander Thrysøe. “Reality is way more messy.”

The group modeled seeded blobs as initial Gaussian perturbations to the plasma density and temperature. They identified the presence of density sources as the primary influencer in blob dynamics, which increases the lifetime of the blob by slowing down dispersion and increasing the coherency of the system. Regardless of the density source, however, the general tendency of the blobs is to slow down as they grow larger and more coherent.

According to the authors, the inelastic collisions may also play a role in the formation of so-called density shoulders in the outer region of the tokamak, where the time-averaged density of the blobs increases.

“The effects are very hard to disentangle from one another experimentally,” said Thrysøe. “This work is a piece in the puzzle of understanding what causes shoulder formation.”

Further investigations are underway to better understand the connections between inelastic blob interactions and shoulder formation. According to Thrysøe, the system would be more turbulent in reality, so the next step is to conduct a similar analysis for full-turbulence simulations.

Source: “Dynamics of seeded blobs under influence of inelastic neutral interactions,” by A. S. Thrysøe, V. Naulin, A. H. Nielsen, and J. Juul Rasmussen, Physics of Plasmas (2020). The article can be accessed at https://doi.org/10.1063/5.0003262 .