B0 from eqdsk is evaluated incorrectly

[kripnerl]

Hello,

I have a suspicion that B0 is evaluated incorrectly when read from the eqdsk file.

• TORAX B0: Toroidal magnetic field on axis (Which axis (magnetic?), vac. mg. field or real tor. field?)
• eqdsk:
  • rcentr: center of the grid (?)
  • bcentr: vacuum toroidal field in rcentr

The definition in Lao 2/7/97 is quite redudant:

RCENTR: R in meters of vacuum toroidal magnetic field BCENTR
BCENTR: Vacuum toroidal magnetic field in Tesla at RCENTR

The definition in FreeQDSK is clearer:

rcentr  Reference value of R, float [meter]
rleft   R at left (inner) boundary, float [meter]

Anyway, I would evaluate on axis vacuum magnetic field as

RCENTR * BCENTR / RMAGX

However, TORAX is reading:

torax/torax/_src/geometry/eqdsk.py

Line 147 in 8b65e14

B_0 = eqfile['bcentre']

Fix code:

B_0 = eqfile['rcentre'] * eqfile['bcentre'] / eqfile['xmag']

If B_0 is supposed to be a real toroidal magnetic field on the magnetic axis. Then one needs to evaluate sth. like (and hope, that f function is correctly evaluated in the eqdsk file - may happen)

eqfile['fpol'][0]  / eqfile['xmag']

, or better integrate ff'.

[jcitrin]

Apologies for our radio silence here! Thanks for looking into this. You are correct. We will consider the options for a more definitive B0 definition and make some PRs to fix.