The impact of reduced recycling on edge transport in NSTX discharges with lithium conditioning, and plans for lithium use in NSTX-U
Presenter: Rajesh Maingi
Abstract: Lithium wall conditioning via toroidally-separated overhead evaporators was extensively used between discharges in NSTX to reduce wall recycling1. Somewhat surprisingly at the time, ELMs were also completely suppressed, even with stored energy up to the global stability limit2. Interpretive analysis with the SOLPS suite of codes confirmed that the wall recycling was indeed reduced, with a global recycling coefficient dropping from > 0.99 to ~ 0.903. In the near-separatrix region where recycling dominated the fueling profile, the density gradient was reduced at nearly constant edge particle transport, but the steep gradient region was extended in to yN of 0.7-0.8, as compared to yN ~ 0.9-0.95 in the reference boronized discharges prior to lithium introduction. Inside of yN < 0.95, the Te and Ti gradients both increased, leading to substantially reduced cross-field thermal diffusivity in SOLPS4. Micro-stability analysis showed that this change in the density profile resulted in stabilization of edge micro-tearing modes that may have been responsible for electron thermal transport5. This initial set of experiments in modestly-shaped plasmas was extended to and confirmed in highly-shaped plasmas6. In all of these analyses, the change of the density gradient via reduced recycling was the key ingredient. Recent experiments on LTX-beta with global recycling coefficient dropped to < 0.5 with liquid lithium plasma-facing components confirm improved confinement and broad temperature profiles with peaked density profiles and neutral beam heating7. To follow up on these promising results, NSTX-U is planning to use four toroidally-separated lithium evaporators, including two new upward-facing evaporators to condition the upper divertor. Future plans call for the introduction of flowing lithium components and/or hardware to enable testing of the lithium vapor box concept. The NSTX results and NSTX-U plans will be summarized, including implications for pilot plant designs that deploy liquid lithium PFCs.
Bio: Rajesh Maingi is Head of the Tokamak Experimental Sciences Department at Princeton Plasma Physics Laboratory (PPPL), and the lead investigator on a domestic liquid metal plasma-facing component development program. He has published research on the boundary plasma in many fusion devices, including Alcator C-Mod (Boston), ASDEX-Upgrade (Germany), DIII-D (San Diego), EAST (China), KSTAR (S. Korea), MAST (England), NSTX (Princeton), and TdeV (Canada), in 35 first author journal articles and more than 900 total publications. He was elected a Fellow of the American Nuclear Society in 2019, a Distinguished Research Fellow at PPPL in 2014, and a Fellow of the American Physical Society in 2009. In 2018 he received the Princeton University/PPPL Kaul Foundation Prize for Excellence in Plasma Physics Research and Technology Development. He received his Ph. D. in Nuclear Engineering from North Carolina State University in 1992. Following postdoctoral fellowships at the DIII-D device, he served on the research staff at Oak Ridge National Laboratory from 1997-2012, and joined PPPL as a Principal Research Physicist in 2012
Liquid Lithium Plasma Facing Components Development and Modeling
Presenter: Andrei Khodak
Application of liquid lithium Plasma Facing Components (PFC) in fusion devices provides several advantages, such as, a renewable protective cover, enhancement of power exhaust and improvement of the confinement via particle pumping. However, there are also many technological challenges associated with the controlled delivery and extraction of liquid metal in in the fusion device environment, characterized by high heat fluxes and strong magnetic fields. Several concepts of liquid metal PFC were proposed including a new variant of plasma facing components was recently introduced at PPPL [1] where a porous wall is used to stabilize the liquid metal surface, while using magnetohydrodynamic (MHD) drive to push the liquid metal flow inside the component. This arrangement allows efficient heat exhaust, and enhanced control of the liquid metal surface temperature, leading to spatial control of evaporation of liquid lithium on the plasma interface. This feature is particularly attractive when vapor shielding is introduced to allow heat flux redistribution [2].
Numerical analysis allows to make a design choice of preferable liquid metal PFC concept and optimize the design performance. To achieve this goal coupled analysis of plasma together with the surrounding boundary structures is required. PPPL is developing a comprehensive multi-physics model for modeling of PFCs in fusion devices. The model includes the fluid-kinetic code SOLPS-ITER and the flow and heat transfer code CFX from ANSYS. SOLPS-ITER code was augmented with the liquid metal boundary condition algorithm, allowing direct two-way coupling of the plasma analysis with the two-dimensional analytical slab flow model[3] which includes heat convection by liquid metal. Results of this coupled analysis are used as a boundary condition for detailed 3D Computational Fluid Dynamics (CFD) MHD analysis using CFX code, customized at PPPL for fusion related flows. If necessary, results of the 3D analysis can be used as a boundary.
Bio: Andrei Khodak received the M.Sc. degree in engineering physics and the Ph.D. degree in physics and mathematics from the St. Petersburg State Polytechnical University, Russia, in 1988 and 1991, respectively. Since than held various research and engineering positions, all related to fluid mechanics and heat transfer. Dr. Khodak is with Princeton Plasma Physics Laboratory since 2010 where he performs multi-physics modeling including plasma simulation, magneto-hydrodynamics, computational fluid dynamics, turbulence modelling, heat and mass transfer. He worked on analysis and engineering of many existing and future fusion devices, including ITER, DIII-D (San Diego), CFETR (China), NSTX (Princeton). He is a lead author of 25 scientific publications
