High Resolution Thomson Scattering for JET
A new Thomson scattering system is being developed for JET, with 15 mm spatial resolution and a foreseen accuracy for temperature better than 15% at a density of 1019 m-3. This resolution is required at the internal transport barrier and edge pedestal and it can not be fully achieved with the present LIDAR systems. The laser for this system is Nd:YAG, 5 Joule, 20 Hz. Scattering volumes from R= 2.9 m to R= 3.9 m are imaged onto 1 mm diameter fibres, with F/25 collection aperture. Two fibres are used per scattering volume. Using optical delay lines, three scattering volumes are combined in each of the 21 filter polychromators. The signals are recorded with transient digitisers, which allow the combined time delayed signals to be resolved. Knowledge of the time delay between signals allows the use of correlation techniques in determining signal levels. The AC output of the amplifier is used, which tolerates a higher level of background signal without affecting dynamic range. The noise resulting from plasma light is determined directly.