C. J. Boswell, J. L. Terry, B. LaBombard, B. Lipschultz, J. A. Goetz
M.I.T. Plasma Science and Fusion Center, 175 Albany St., Cambridge, MA 02139 USA
Cold, high density plasma (Te ≤ 1 eV,ne ≈ 8 x 1020 m-3) has been observed in the private flux zone (PFZ) of the Alcator C-Mod divertor during attached, Ohmic, moderate density (<ne> ∼ 1.2 → 1.7 x 1020 m-3) discharges with strong impurity puffing. Plasma in the PFZ has been seen previously on the DIII-D tokamak during partial detached divertor operation [1]. Current modeling of the divertor does not reproduce this plasma in the PFZ, implying that the modelling is incomplete. Since the only source of plasma or energy to the PFZ is from the common flux zone or the core plasma there must exist a mechanism for large cross field transport into this region which has short connection lengths along field lines to divertor surfaces.
The existence of this plasma is shown using CCD camera images filtered for Dg light. Intense Dg emission is seen to fill almost the entire private flux region indicating strong volume recombination. High densities and low temperatures measured spectroscopically support this observation. Using a collisional-radiative model to generate recombination per Dg photon curves as functions of electron density and temperature [2], 2-D volume recombination profiles in the divertor were calculated. A total volumetric recombination rate of 1021 recombinations per second is found in the PFZ. This rate is approximately 20% the total ion flux to the divertor plates.
When this cold, high density plasma is seen to exist in the PFZ, Langmuir probes mounted on the outer divertor plates show a shift in the peak of the density from on the separatrix to just inside the private flux region. The effective scrape-off width of the density in the private flux region also increases when the enhanced emission is seen. These two effects imply an increased effective perpendicular diffusion coefficient and a net drift across the separatrix.
[1] M.\ Fenstermacher, et al., Plasma Phys.\ Control.\ Fusion 41 (1999) A345.
[2] J.\ Terry, et al., Physics of Plasmas 5 (1998) 1759.
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