One dimensional multipoint Thomson scattering system using a high repetition rate Nd: YAG laser and its extension to two or three dimensional multipoint measuring system

We present the design consideration and the performance of the 28 points 100 Hz repetition rate YAG laser Thomson scattering system constructed for the JIPPT-IIU tokamak. An emphasis was placed on achieving a high temporal resolution while keeping the spatial resolution and accuracy of measurement at a reasonable level. The essence of realizing high repetition rate operation (100 Hz) is to use a bright optical system (f3 = 3) and Si-avalanche photo diode (APD) detectors with high quantum efficiency (η = 0.8 at 900 nm), combination of which provides high quality data (accuracy better than 10% at the electron density 1.0 x 1013 cm-3) even with a small laser energy (0.5 J) at which the laser can operate at a high repetition rate. A small spatial resolution of 1.5 cm was realized by stacking 28 polichromators which were made as thin as possible with their slits aligned along the 3.7 times magnified image of the laser beam. The extension of this system to a two or three dimensional measuring system is, in principle, quite easily realized by arranging laser beam path in two or three dimensional configuration and preparing a large number of detection system. A practical method of obtaining a multi dimensional laser beam is to use a set of high power mirrors which steer the laser beam. Development of high quality mirrors and appropriate stop apertures is a key issue for solving the stray light problem. Use of optical fibers is indispensable for transporting the scattered lights from the image points to the entrance slits of the polychromators. Fabrication of arrayed APD, which seems quite feasible with the present technology, is also requisite for detecting a large number of light signals. (author). 4 refs, 9 figs