Two color multichannel heterodyne interferometer set up for high spatial resolution electron density profile measurements in TJ-II.

A high spatial resolution two color [CO(2), λ=10.6 μm/Nd:YAG (Nd:YAG denotes neodymium-doped yttrium aluminum garnet), and λ=1.064 μm] expanded-beam multichannel heterodyne interferometer has been installed on the TJ-II stellarator. Careful design of the optical system has allowed complete control on the evolution of both Gaussian beams along the interferometer, as well as the evaluation and optimization of the spatial resolution to be expected in the measurements. Five CO(2) (measurement) channels and three Nd:YAG (vibration compensation) channels have been used to illuminate the plasma with a probe beam of 100 mm size. An optimum interpolation method has been applied to recover both interferometric phasefronts prior to mechanical vibration subtraction. The first results of the installed diagnostic are presented in this paper.