## More gyrofluid R/L_T_crit issues

Oct 14, 1997

I don't see the point of doing the R/L_T=2 and 3 points. The real
question is: Do the GF errors near threshold affect the GF results
above threshold? If the R/L_T=2 and 3 points fall on the line (which
seems likely), we have not answered this question. We know these
points will give finite chi, while the real answer is zero. But this
does not show whether or not these errors affect the results well
above threshold.

Instead, I've been looking for a place where the GF threshold is
right on, to see if the GF/GK chi discrepancy remains. I just found
such a point: the NTP parameters with epsilon=0. The GF thesis
closure gets the threshold very close to what I found with the Vlasov
code, as shown below.

At the NTP point (R/L_T=10), the GF linear growth rates are in very
good agreement with kinetic theory, as shown below.

We already have nonlinear runs for this case: the chi's are the
epsilon=0 points on this page. Even
though the GF thesis R/L_Tcrit is right on, the GK chi is 2x lower
than the GF chi. Thus in this case, GF linear errors near
threshold are not causing the discrepancy in chi. The agreement here
(2x) is better than the DIII-D base case (3x), so if other factors account
for this 2x difference, perhaps the GF linear errors are at most
accounting for 1.5x of the difference for the DIII-D base case.

It might be useful for someone to run the epsilon=0 NTP case with
Andris's parallel code. The results I'm comparing are from IAEA '94.

--Mike.

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