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 i n plasmas with core transport barriers and centrally peaked profiles, and s econd, 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 cor e profiles an existing DIII-D broadband frequency swept (FMCW) reflectomete r system was converted to use O-mode propagation. This test system covers 5 0-75 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. Res ults demonstrate that adequate signals, and hence, accurate profiles, can i n fact be obtained to the plasma center in discharges with centrally peaked profiles. (C) 1999 American Institute of Physics. [S0034-6748(99)75301-3].