High resolution scrape-off layer profile measurements in limited and diverted plasmas in C-Mod -- investigation of heat flux channel width physics

Narrow scrape-off layer (SOL) heat flux channel widths (λq) are seen on many tokamaks, both in inner-wall limited (IWL) and diverted discharges. These observations have important consequences for ITER and reactors, impacting the design of inner limiter tiles for heat load during startup and pushing the limits of dissipative divertor operation and control. A dominant ∼ 1/Ip scaling for λq is seen in a wide range of cases (IWL, H-mode and L-mode diverted [at low density]), suggestive of a poloidal ion gyroradius effect. It is troubling that λq does not appear to scale with major radius -- a challenge for reactors. The latter observation contrasts with H-mode pedestal widths that increase with machine size, implying that the physics that sets the local gradient scale lengths in pedestal and SOL may be different. We have recently implemented a scanning Mirror Langmuir Probe'' diagnostic on C-Mod with the idea of exploring this critical interface with very high resolution. Narrow λq features'' in IWL discharges have been mapped out in detail, exhibiting a ∼ 1/Ip scaling with some evidence of a break-in-slope feature at the LCFS. We will report on these findings and on L and H-mode experiments in progress, in which divertor conditions are varied (low recycling, high-recycling, detached).

Presented at:
56th Annual Meeting of the APS Division of Plasma Physics, New Orleans, Louisiana, October 27–31, 2014
Year:
2014
Laboratories:
SPC
CRPP