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 , where is defined as the difference:
Note that 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 , giving:
Note, is set equal to 0 when is negative. Once again, the electron thermal diffusivity is found by multiplying by the trapped particle fraction :
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. 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.