Upgrade of the RFX CO2 interferometer using in-vessel optics for extended edge resolution

Abstract

The extension to 16 chords of the eight-chord two-color MIR interferometer of the RFX reversed field pinch experiment (R=2 m, a=0.46 m) is presented. The new measurements are done through a 100 mm equatorial port with beams which are reflected back by corner cube metal mirrors mounted inside the vacuum vessel. Moreover, for four of the new chords, the beams are also steered by copper mirrors mounted inside the access pipe, whose edges are at 20–30 mm from the plasma surface. In this way measurements along lines of sight that are at a mean distance of 67 and 21 mm from the vessel first wall are possible. This results in a significant improvement of the density profile reconstruction towards the plasma edge. All the optics inside the vacuum vessel are fixed and the beam layout and focusing have been optimised to take in the misalignments due to the vacuum vessel thermal deformations. The system proved to work well despite mechanical vibrations as large as ≈1.5 mm at frequencies of ≈200 Hz. Stability of alignment and survival of the optics in the proximity of the plasma is also satisfactory.