shear stabilization
The original GLF23 model described in the 1996 Waltz
paper [1] uses the flux-surface-averaged
shear rate 
in circular geometry with a multiplier of 
.
Here, vE is the shear velocity equal to
.
Since then it was discovered that the rate
can often exceed the maximum linear growth rate in the
outer half of the plasma leading to the formation of
an unphysical transport barrier. This was remedied
by using a real geometry extension of 
where the toroidal field in vE is replaced by
[2].
Here, the factor 
typically rises
to 2-3 in the outer half of elongated plasmas, thus
reducing the rate in that region significantly.
The corresponding multiplier for the original (retuned)
models is 
which has
been found to be consistent with recent gyro-kinetic
simulations by Waltz. To use the real geometry
shear, set bt_flag=1 in the namelist input.
The effective B-field is computed
internally in subroutine callglf2d.f.