From: VAXK::LAO
Date: 18-OCT-1991  15:57:37
Description: N1 Coil Compensation                                    

This is to update you on the effects of the non-axisymmetric
N1 coil on the MHD analysis using the EFITD code.  In the last two weeks,
we have implemented a simple procedure into EFITD to try
to compensate for the effects of the N1 coil on the magnetic probes
located in the 322 degree toroidal plane.  The EFITD code treats the
plasma and the surrounding coils as an axisymmetric toroidal system.
To accurately taken into account the effects of the N1 coil
will require a 3-D MHD code which we presently do not have.
So, we instead introduce an approximate procedure into EFITD to
compensate for the 322 degree magnetic probes, which are close
to the N1 coil, by correcting for the N1 coil contribution to these
probes using a look up table based on a DIII-D vacuuum shot
provided by T. Scoville and R. Lahaye.
 
From the results of a number of shots that we looked at, the correction
does significantly improve the fitting to the magnetic signals.  The
plasma shapes, stored energy, $\ell_i $ are very similar with or without
N1 coil compensation.  However, the separatrix boundary at the Thomson
viewing location may differ by about 1-2 cm and in many cases, the
separatrix is moved toward the region of steeper gradient.
 
We are still studying this problem as well as whether the N1 coil
may create some small ergodic region near the X point.
In the mean time, you can turn on this option in EFITD by setting
N1COIL=2 in the SNAP files. To use it in the FILE mode, new K
files must first be created in order to obtain the N1 coil
current. Please use it with caution and contact us if
you have questions.