Explanation for MARFE formation and subsequent evolution into a detached symmetric plasma edge

Conference · Tue Dec 31 04:00:00 EST 1996

OSTI ID:489564

Stacey, W M ^[1]

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1. Georgia Institute of Technology, Atlanta, GA (United States)

A linear thermal stability analysis leads to expressions for the maximum impurity densities for which edge plasma modes with different poloidal mode number, m, are stable against thermal, edge localized instabilities. The density limits are higher for the m > 0 modes than for the m = 0 mode because of the stabilizing effect of parallel thermal conduction on the former, and the density limits are successively higher for the successive in-modes because of the higher associated parallel temperature gradients. When atomic physics effects are taken into account, it is found that the strong positive temperature dependence of the ionization cross section in the temperature range around 10 eV provides a strong stabilization mechanism which acts to reduce the impurity density limits for all modes. These results provide an explanation for the experimentally observed tokamak phenomenon of a symmetric radiating edge evolving with increasing density into a MARFE and then cooling into a detached plasma with a symmetric edge that radiatively exhausts essentially all the heating power. As the edge impurity density increases, first the the m = 0 mode density limit is exceeded, causing the edge plasma to evolve into a mixture of m > 0 modes which becomes more highly asymmetric as the density increases further and exceeds the density limits of successively higher modes, forcing the plasma into the remaining stable higher-m modes--the stable MARFE. If the radiative cooling of the edge is sufficient to reduce the temperature to T{sub e} = 10 eV, the various thermal modes are stabilized by the strong positive temperature dependence of the ionization cooling in the presence of neutral atoms, the impurity density limits are thereby reduced, and the plasma edge evolves back into a stable m = 0 mode. On the other hand, if the plasma becomes highly enough asymmetric before sufficient cooling takes place, nonlinear effects drive a disruptive collapse of the radial temperature distribution.

• OSTI ID: 489564
• Report Number(s): CONF-960354--
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
EDGE LOCALIZED MODES
PLASMA INSTABILITY
THERMAL ANALYSIS
TOKAMAK DEVICES