Comparison of detached and radiative divertor operation in Alcator C-Mod

Abstract

The divertor of the Alcator C-Mod tokamak routinely radiates a large fraction of the power entering the scrape-off layer. This dissipative divertor operation occurs whether the divertor is detached or not, and large volumetric radiative emissivities, up to 60 MW-m-3 in ICRF-heated discharges, have been measured using bolometer arrays. An analysis of both ohmic and ICRF-heated discharges has demonstrated some of the relative merits of detached divertor operation versus high-recycling divertor operation.An advantage of detached divertor operation is that the power flux to the divertor plates is decreased even further than its already low value. Some disadvantages are that volumetric losses outside the separatrix in the divertor region are decreased, the neutral compression ratio is decreased, and the penetration efficiency of impurities increases. a) Present address, University of California at San Diego, San Diego, CA b) Present address,

Divertor Physics Research on Alcator C-Mod

“…What is least understood about the PFZ is why there is much plasma there at all given the short connection lengths from x-point to divertor plate and typical diffusive transport. Using spectroscopic measurements, it was observed that there is a significant recombination rate in the PFZ 17 ; Up to 10 22 recombinations/s occur there as compared to the plate ion current which can reach ~ 5x10 22 /s for these plasma currents (see Section IX). A drift across the outer divertor leg (between x-point and divertor plate) was postulated as the mechanism to supply the corresponding ion current to the PFZ.…”

“…For these studies, the X point had been shifted to larger major radius and up vertically to be in the field of view of the XUV spectrograph. These studies 22 show that the total radiation emissivity is dominated by XUV radiation. For plasmas with low central density (sheath-limited, n e ~ 1x10 20 m -3…”

“…They were quickly followed by more viewing chords (e.g. see Figure 2b) and tomographic inversions of the bolometer brightnesses to obtain local emissivities 22 . (The number of bolometer arrays focused just on the divertor was later increased to 5 19 ).…”

RF plasma edge interactions and their impact on ICRF antenna performance in Alcator C-Mod

“…Bolometry is used to monitor plasma radiation [30,31,32] and spectroscopic measurements are utilized to monitor specific impurity species, for example Mo and Cu. [33] The Mo and Cu densities are inferred using the measured line brightnesses, Thomson scattering electron density and temperature profiles, the MIST impurity transport code [34], transport coefficients [35] and cooling curves [36,37,38].…”

Mapping the intrinsic stochasticity of tokamak divertor configuration

“…In particular, the challenge posed by the heat flux impact on the vessel target inner surfaces has been considerably averted by means of divertor dissipative techniques [4]. In an MHD ideal tokamak the plasma is confined to a region of nested toroidal magnetic surfaces limited by one separatrix surface from the outer surfaces that divert the plasma exhaust to the target.…”