Results from a Prototype Second Harmonic Tangential Array Interferometer

A tabletop tangential array interferometer operating at 1.06 and 0.53 mm has been built to test its sensitivity for use on C-Mod. This type of interferometer is insensitive to mechanical vibrations. The prototype uses a pulsed, 35 mJ, 10 Hz multimode, Nd:YAG laser, LiB3O5 non-critically phase matched doublers, a fan beam created by a cylindrical lens, several retro-reflector elements to define the array, and a CCD camera detector. This interferometer has beam diameters of a few millimeters and spatial resolution of a few centimeters as needed for C-Mod. Commercial lasers and CCD arrays are available which can scale this design to ~20kHz. Tokamak applications typically require a limiting sensitivity of 10^-3 fringes and maximum changes of ~1 fringe. The limiting sensitivity of the prototype system has been investigated along with technical limitations on elements of the optical system relevant to its use on tokamaks. In particular methods of normalizing shot-to-shot and spatial mode variations in the laser intensity are examined. The use of visible and near visible components allows a compact optical design and efficient use of port space. Supported by US DoE Contract Nos. DE-AC02-76-CH0-3073 and DE-AC02-78ET51013.