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*/*L*_{T})^{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 *C*_{i}[2].
In any case, Ottaviani, *et al* do not provide a method for
calculating the critical *T*_{i} gradient and they indicate that the critical
gradient should be very small in realistic conditions.

Thu Nov 12 11:54:44 EST 1998