The JET LIDAR system now

The time of flight or LIDAR approach to the measurement of Te and ne profiles by Thomson scattering was first demonstrated on the JET tokamak in preliminary form in 1986. These first single shot, 4 channel measurements were followed in 1987 with results from the full 1/2 Hz repetition rate, 6 channel temperature and density profile system. The diagnostic uses a 300 ps ruby laser and MCP photomultipliers detectors coupled to fast transient recorders. The spatial resolution achieved initially was 12-13 cm. The advantages of the LIDAR technique over conventional Thomson scattering were apparent from the beginning. Short detection integration times gave low plasma light levels and therefore good signal to noise ratios. This also allowed comparatively large scattering volumes to be used resulting in insensitivity to misalignment. These factors, combined with the simplicity of the optical system close to the tokamak and the relatively undemanding access requirements, have made it possible to consider Thomson scattering as a viable diagnostic technique for the next generation of fusion devices. Several enhancements to the performance of the original LIDAR system have been considered both in terms of the spatial resolution (now < 10 cm), the repetition rate and the reliability of the measurements. The key features of the JET LIDAR system and progress on these improvements will be reviewed. (author). 22 refs, 11 figs, 4 tabs