[BUG]Alphaj becomes negative, giving error in Wilson bootstrap routine

[mkovari]

This was observed by Jon in #213, and leads to error 76 in bootstrap_fraction_wilson. This is because iprofile=1 and qstar has dropped below q0 in routine culcur; so alphaj becomes negative, eventually leading to aj being NaN. I suggest that alphaj is prevented from becoming negative at the point at which it is set in routine culcur:

        if iprofile == 1:
            # Ensure current profile consistency, if required
            # This is as described in Hartmann and Zohm only if icurr = 4 as well...

            # Tokamaks 4th Edition, Wesson, page 116
            alphaj = qstar / q0 - 1.0

Note that the Wilson bootstrap routine is always run, even if it is not selected in the input file:

PROCESS/process/physics.py

Line 921 in 7e15d39

# Calculate bootstrap current fraction using various models

James claimed to have fixed this problem but it seems to have come back.

Steps to reproduce

Pirizad IN.DAT.txt

Expected behaviour

alphaj should remain positive somehow.

Evidence

[chris-ashe]

This was something I cam across doing #3263. The definition for alphaj above is from Wesson and assumes a circular cross-section plasma using $q_{cyl}$ or qstar in this case. Which is hardly a case used for normal PROCESS runs.