A two color multichord infrared interferometer for RFX

A multichord interferometer designed for the Reversed Field experiment (RFX) (R=2 m, a=0.5 m, I=2 MA) is presented. The high density expected and the rather limited diagnostic access to the plasma motivated the choice of a two-color CO2/CO laser (10.6/5.4 ?m) interferometer. Due to the time-varying magnetic fields >=0.1 T around the torus entailed by the operation of the machine, the system has been mounted on a 3 ton graphite C-shaped optical table supported by a stainless steel structure on pneumatic vibration isolators. This led to long term stability of alignment and a very low level of mechanical vibrations. The interferometer can be moved in and out of the torus with ease by a carriage-rail system for maintenance both of the diagnostic and of the machine. The design foresees eight vertical chords spanning the arms of the C-shaped table and includes more additional measurements using a horizontal port and mirrors mounted inside the vacuum chamber. The expected accuracy in line density units is <=1×10^18 m^-2, and the overall concept is that of a reliable system for high density and large size magnetic confinement plasmas.