The specific size and structure of the edge current profile has important e
ffects on the magnetohydrodynamic stability and ultimate performance of man
y advanced tokamak (AT) operating modes. This is true for both bootstrap an
d externally driven currents that may be used to tailor the edge shear. Abs
ent a direct local measurement of j(r), the best alternative is a determina
tion of the poloidal field. Measurements of the precision (0.1 degrees -0.0
1 degrees in magnetic pitch angle and 1-10 ms) necessary to address issues
of stability and control and provide constraints for EFIT are difficult to
do in the region of interest (rho = 0.9-1.1). Using Zeeman polarization spe
ctroscopy of the 2S-2P lithium resonance line emission from the DIII-D LIBE
AM [D. M. Thomas, Rev. Sci. Instrum. 66, 806 (1995); D. M. Thomas, A. W. Hy
att, and M. P. Thomas, Rev. Sci. Instrum. 61, 340 (1990)] measurements of t
he various field components may be made to the necessary precision in exact
ly the region of interest to these studies. Because of the negligible Stark
mixing of the relevant atomic levels, this method of determining j(r) is i
nsensitive to the large local electric fields typically found in enhanced c
onfinement (H mode) edges, and thus avoids an ambiguity common to motional
Stark effect measurements of B. Key issues for utilizing this technique inc
lude good beam quality, an optimum viewing geometry, and a suitable optical
prefilter to isolate the polarized emission line. A prospective diagnostic
system for the DIII-D AT program will be described. (C) 2001 American Inst
itute of Physics.