Results from a Prototype Second Harmonic Interferometer

A tabletop second harmonic interferometer operating at 1.06 and 0.53 μm has been built to test its sensitivity for use on tokamaks. This type of interferometer is insensitive to vibrations. The prototype uses a pulsed, 35 mJ, 10 Hz multimode, Nd:YAG laser, LiB_3O₅ non-critically phase matched doublers, a retro-reflector, and a CCD camera detector. This interferometer has beam diameters of a few millimeters and spatial resolution of a few centimeters. Commercial lasers and CCD arrays are available which can scale this design to 20kHz. Tokamak applications typically require a sensitivity of 10^-3 fringes and maximum changes of 1 fringe. The 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. A gas cell whose pressure can be varied has been used as a dispersive medium for calibrating and testing the instrumental sensitivity. The use of visible and near visible components allows a compact optical design and efficient use of port space.