Imaging of molybdenum erosion and thermography at visible wavelengths in Alcator C-Mod ICRH and LHCD discharges

Abstract

We describe results from imaging observations of atomic line and continuum emission in the 550.6nm region on Alcator C-Mod. Both the 550.6nm neutral molybdenum emission and the adjacent 549nm continuum emission are imaged separately to isolate line emission. A few complications of using imaging to infer erosion in this wavelength region are discussed including subtraction of continuum emission and determination of an appropriate S/XB coefficient. Diagnostics of surface erosion and thermography using these emissions are briefly reviewed, and used to study phenomenology during ohmic operation, ion cyclotron range of frequencies heating (ICRH), and lower hybrid current drive (LHCD). In addition to broadening of Mo I emission regions in the outer divertor and main limiter during ICRH compared to ohmic operation, mid-plane localized heating of the main limiter associated with fast ion impact is observed which exceeds the divertor heat flux. During LHCD operation, several localized regions of increased brightness associated with hot-spots are interpreted as heating due to localized density peaking, which re-iterates the importance of imaging continuum emission for subtraction. These sources of surface heating exacerbate plasma-material interactions at the device wall and may require additional mitigation if they cannot be avoided in future machines.

Improved confinement in high-density H-modes via modification of the plasma boundary with lower hybrid wavesa)

“…Thus some LHRF power may be deposited directly into the SOL on those SOL flux-tubes just in front of the launcher, and this power may not be detected by the thermography. Such a local LHRF power deposition has been seen previously in low-density L-mode plasmas 29 .…”