Prospects for edge current density determination using LIBEAM on DIII-D

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.