Introduction
The UCD basic laboratory reflectometry experiment is intended to simulate microwave/millimeter wave reflectometry diagnostics which are currently being employed on devices such as TFTR, DIII-D and JET and are planned for future installation on TPX and ITER and to understand and elucidate the physics and measurement limitations.
The study utilizes an unmagnetized, pulsed discharge plasma generated in a cylindrical chamber. Grid-launched Ion Acoustic Waves (IAWs) perturb the density gradient, where the launch frequency is controlled and where the waves are simultaneously measured with other techniques. Typical density and temperature of the plasma are 1-5 x 10^11 cm-3 and 1-2 eV, respectively. The IAW frequency and wavelength lie in the range 20-100 kHz, and 3-11 cm respectively. A monostatic, homodyne, quadrature reflectometry system is employed, which includes a horn as a microwave transmitting and receiving antenna and a quadrature mixer to separate phase and amplitude information. The microwave frequency, which corresponds to the cutoff frequency at the critical density, lies in the range of 3-6 GHz with a free space wavelength of 5-10 cm.
Physics Issues
The physics issues, which are essential to the interpretation of the reflectometry data, include:
While our early experiments demonstrate highly localized reflectometry responses when 
(Ref. S. Baang, et. al., Rev. Sci. Instrum., 61, 3013, (1991)), they imply the existence of multi-dimensional effects (Ref. T. Rhodes, S. Baang, et. al., Rev. Sci. Instrum., 63, 4599, (1992)). Therefore, there has been considerable effort devoted to making the experiment 1-dimensional, and to increase the scale ratio by a factor of 2 or more. As a result, recent experiments with a mesh horn show that there is interference between the responses from near the critical layer and from the further time-delayed scattered region.
UCD/LLNL
The UCD basic laboratory reflectometry studies are being conducted at the Livermore site which is co-located with the Lawrence Livermore National Laboratory (LLNL). As a consequence, extensive facilities are available, and the experiments are complimented through a comparison with numerical and theoretical investigations in collaboration with LLNL researchers.
Tokamak Plasma Reflectometry Study
In an effort to extend the basic reflectometry studies into its use as a tokamak plasma microturbulence diagnostic, a direct comparison of reflectometry with probe measurement has been made on the UCLA CCT tokamak. The experiment includes FM radar reflectometry for density profile measurement and a coherence study of probe and reflectometer signals. A strong coherence has been observed for H-mode plasmas, while ohmic- and L-mode plasmas exhibit relatively low coherence levels.
For more information, contact Ross at geck@faust.ece.ucdavis.edu.
