Lawrence Livermore National Laboratory
Friday, April 28, 2017
Abstract: One of the most critical engineering constraints for future tokamaks is the peak heat load on the plasma-facing components, 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. Plasma convection, associated with a divertor null point, dubbed “the churning mode”, is investigated in a numerical model based on toroidally symmetric reduced-MHD equations. It is found that the plasma pressure profile and poloidal magnetic flux evolve in a spiraling pattern near the divertor null-point; for a higher-order null, and for sufficiently high plasma pressure at the null point, the convective motion is strong enough to affect the distribution of thermal energy in the divertor, which is consistent with the results of recent 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 stable fully detached divertor operation. As the input power is reduced below a threshold value, the outer leg transitions to a fully detached state, which defines the upper limit on the power for detached divertor operation. Reducing the power further results in the detachment front shifting upstream but remaining stable. At low power, the detachment front eventually moves all the way to the primary X-point, which is usually associated with a MARFE and which leads to degradation of the core plasma, and this defines the lower limit on the power for detached divertor operation. For the long-legged divertors studied here, detached operation can be maintained in a window with a factor of 5-10 relative variation in the input power, while for similar parameters for a standard divertor there is no detached operation window.
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 the Fusion Energy Sciences Program at 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.