Core reflectometer density profile measurements on DIII-D

Measurements of core (normalized flux coordinate rho less than or equal to 0.3-0.4) electron density profiles have been obtained on the DIII-D tokamak via a reflectometer system operating in the ordinary polarization (O mode). This system was developed and implemented for two primary reasons: first, there is a pressing need for core density profile measurements on DIII-D in plasmas with core transport barriers and centrally peaked profiles, and second, the measurements reported here further extend the demonstrated range of conditions in which reflectometry can provide reliable density profile measurements. As a test of the ability of reflectometry to measure such core profiles an existing DIII-D broadband frequency swept (FMCW) reflectometer system was converted to use O-mode propagation. This test system covers 50-75 GHz, corresponding to densities of 3.1-7.0 x 10(19) m(-3). Using this system, core access has been investigated in medium density discharges. Results demonstrate that adequate signals, and hence, accurate profiles, can in fact be obtained to the plasma center in discharges with centrally peaked profiles. (C) 1999 American Institute of Physics. [S0034-6748(99)75301-3].