Upgrade of the far-infrared interferometer and polarimeter on TEXTOR

A combined HCN laser interferometer/polarimeter comprising nine vertical probing beams has been in routine use on TEXTOR for simultaneous measurements of the electron density and poloidal magnetic field distributions and for feedback control of the gas injection and horizontal plasma positioning systems. Motivated by a growing interest in rapid changes of the internal magnetic field structure as well as by the need for reliable control diagnostics of the plasma shape and vertical position under long pulse conditions, the instrument has been complemented recently with a horizontal channel. The upgrading has been achieved by minor modifications of the existing optical arrangement and mechanical structure without impairing the function of the original apparatus. In this contribution, the constraints and technical solutions for the alterations are outlined and the improved performance is illustrated by practical examples.