JOINT MAE-CER SEMINAR

Plasma Flows and Detachment in Innovative Divertor Configurations

Maxim V. Umansky, Lawrence Livermore National Laboratory
May 10, 2017, 11:00am - 12:00pm, EBU-II 479

ABSTRACT:

The peak heat load on the plasma-facing components is one of the most critical engineering constraints for future tokamaks, which prompts a search for innovative divertor configurations using non-standard magnetic geometry and additional X-points. The present computational investigation reveals profound effects that innovative divertor geometry can have on plasma flows and detachment in the divertor. In the vicinity of the tokamak divertor null-point, the poloidal magnetic field is near zero, which can lead to localized loss of plasma MHD equilibrium near the null point and ensuing plasma interchange motion localized at the null-point. This plasma convective motion associated with a divertor null point is investigated in a numerical model based on toroidally symmetric reduced-MHD equations. It is found that plasma pressure and poloidal magnetic flux evolve in a spiraling pattern near the divertor null-point, and for sufficiently high plasma pressure at the null point, in particular for a higher-order null, the calculations show that the convective motion can be strong enough to affect the distribution of thermal energy in the divertor, which is consistent with results of snowflake divertor experiments on TCV. On the other hand, for divertor configurations with radially or vertically extended, tightly baffled divertor legs, a study with the tokamak edge transport code UEDGE demonstrates existence of passively-stable fully detached divertor regimes in a broad range of input power from the core, for several divertor configurations, in particular for those with radially or vertically extended, tightly baffled, outer divertor legs, with or without a secondary X-point in the divertor leg volume. As the power from the core is varied, the detachment front merely shifts up or down in the leg but remains stable. The simulations show that long-legged divertors have a large increase of the peak power handling ability, by up to a factor of 10, compared to conventional short-legged divertors. These modeling results suggest a possibility of stable fully detached divertor operation for a high- power tokamak with tightly baffled extended divertor legs.

BIO:

Dr. Maxim Umansky earned his PhD degree in plasma physics in 2000 at MIT where he worked on data analysis and numerical modeling of edge plasma in the Alcator C-Mod tokamak; his thesis work resulted in significant findings in the area of tokamak boundary- plasma transport. Beyond MIT, he had a post-doctoral appointment at the University of Rochester, where he worked on ICF hydrodynamics and tokamak MHD theory and modeling. He joined LLNL in 2001, and since then has been working there, primarily on tokamak edge-plasma modeling and the development of large-scale simulation tools for boundary plasmas. His recent work has focused on plasma physics in innovative divertors.