Two-color interferometer system for Alcator C-MOD

High densities with sharp, fast rising profiles are expected during pellet injection in Alcator C-MOD. Consideration of interferometer systems other than far infrared will be needed to avoid refractive effects and make profile measurements during pellet injection possible. CO2 wavelengths are proving to be the most interesting from the standpoint of density measurements, with the necessary vibration subtraction provided by a coaxial He:Ne system. We will describe the two-color interferometer as it is presently designed and present results from simulations made of the system to determine noise levels, minimum measurable density, and optimal number and location of chords. Ray tracing results both with and without pellet injection will be compared. Possible fringe counting schemes will also be discussed