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Thresholded OHE Model

The second version of the OHE model (called the ``modified OHE'' or OHEm model), a version that explicitly makes use of the marginal stability criterion.

If the plasma is near marginal stability, it is well-known that the ITG-driven turbulent diffusivity is linearly proportional to the parameter tex2html_wrap_inline192, where tex2html_wrap_inline192 is defined as the difference:
Note that tex2html_wrap_inline196 denotes the critical ion temperature scale length. The only modification to the OHE model is in the scaling with normalized temperature gradient; the (R/LT)3/2 term is replaced with tex2html_wrap_inline192, giving:
Note, tex2html_wrap_inline204 is set equal to 0 when tex2html_wrap_inline206 is negative. Once again, the electron thermal diffusivity tex2html_wrap_inline174 is found by multiplying tex2html_wrap_inline178 by the trapped particle fraction tex2html_wrap_inline180:
It is not clear that this version of the OHE model was ever calibrated; we simply use the calibration
that was found for the thresholdless model. This deficiency is not significant, however; work with this OHEm model has indicated that it has very little predictive power in tokamak plasma simulations - and its deficiencies are unlikely to be corrected with a new value for Ci[2]. In any case, Ottaviani, et al do not provide a method for calculating the critical Ti gradient and they indicate that the critical gradient should be very small in realistic conditions.

Thu Nov 12 11:54:44 EST 1998