Physics of Radiation-driven Islands Near the Tokamak Density Limit
Authors: D.A. Gates, L. Delgado-Apricio and R.B. White
Abstract:
In previous work [1], the onset criterion for radiation driven islands [2] in combination
with a simple cylindrical model of tokamak current channel behavior was shown to
be consistent with the empirical scaling of the tokamak density limit [3]. A number of the
unexplained phenomena at the density limit are consistent with this novel physics mechanism.
In this work, a more formal theoretical underpinning, consistent with cylindrical
tearing mode theory, is developed for the onset criteria of these modes. The appropriate
derivation of the radiation-driven addition to the modified Rutherford equation is discussed.
Additionally, the ordering of the terms in the MRE is examined in a regime near the density
limit. It is hoped that given the apparent success of this simple model in explaining
the observed global scalings will lead to a more comprehensive analysis of the possibility
that radiation driven islands are the physics mechanism responsible for the density limit.
In particular, with modern diagnostic capabilities detailed measurements of current densities,
electron densities and impurity concentrations at rational surfaces should be possible,
enabling verification of the concepts described above.
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Submitted to: Nuclear Fusion Proceedings (January 2013)
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Download PPPL-4841 (pdf 589 KB 20 pp)
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