Expanding the role of impurity spectroscopy for investigating the physics of high-Z dissipative divertors

• Main Authors: M.L. Reinke, A. Meigs, E. Delabie, R. Mumgaard, F. Reimold, S. Potzel, M. Bernert, D. Brunner, J. Canik, M. Cavedon, I. Coffey, E. Edlund, J. Harrison, B. LaBombard, K. Lawson, B. Lomanowski, J. Lore, M. Stamp, J. Terry, E. Viezzer
• Format: Article
• Language: English
• Published: Elsevier 2017-08-01
• Series: Nuclear Materials and Energy
• Online Access: http://www.sciencedirect.com/science/article/pii/S2352179116302034

New techniques that attempt to more fully exploit spectroscopic diagnostics in the divertor and pedestal region during highly dissipative scenarios are demonstrated using experimental results from recent low-Z seeding experiments on Alcator C-Mod, JET and ASDEX Upgrade. To exhaust power at high parallel heat flux, q∥ > 1 GW/m2, while minimizing erosion, reactors with solid, high-Z plasma facing components (PFCs) are expected to use extrinsic impurity seeding. Due to transport and atomic physics processes which impact impurity ionization balance, so-called ‘non-coronal’ effects, we do not accurately know and have yet to demonstrate the maximum q∥ which can be mitigated in a tokamak. Radiation enhancement for nitrogen is shown to arise primarily from changes in Li- and Be-like charge states on open field lines, but also through transport-driven enhancement of H- and He-like charge states in the pedestal region. Measurements are presented from nitrogen seeded H-mode and L-mode plasmas where emission from N1+ through N6+ are observed. Active charge exchange spectroscopy of partially ionized low-Z impurities in the plasma edge is explored to measure N5+ and N6+ within the confined plasma, while passive UV and visible spectroscopy is used to measure N1+-N4+ in the boundary. Examples from recent JET and Alcator C-Mod experiments which employ nitrogen seeding highlight how improving spectroscopic coverage can be used to gain empirical insight and provide more data to validate boundary simulations.