LIDAR Thomson Scattering

JET LIDAR-Thomson Scattering Diagnostic

LIDAR-Thomson Scattering Diagnostic at JET

Normally a light ray cannot be seen unless it hits your eye, but with dust or mist in the air, one can spot it from the side too by the scattering of light. When an intense laser beam is sent into plasma, its light will get scattered on free electrons. The analysis of the scattered laser light is essential in determining local density and temperature of plasma electrons.
The Thomson Scattering Diagnostic shines an intense lase pulse into the plasma and then detects the light scattered from the electrons. The monochromatic laser input light is scattered and Doppler shifted by the fast moving plasma electrons, producing a broad spectrum of scattered light. By measuring the width of this scattered spectrum the velocity distribution and hence the electron temperature can be determined and by measuring the total intensity of the scattered light the density of the electrons can be deduced.

JET features three Thomson scattering diagnostics – the “Core LIDAR” system looks at the bulk of the plasma, the “Divertor LIDAR” system looks at the edge of the plasma and the High Resolution Thomson Scattering (HRTS) system only measures the outer half of the plasma, but at a much higher spatial resolution than the Core LIDAR system.

Temperature and Density profiles obtained by firing the laser several times during a JET plasma pulse. The changes in the temperature and density profiles due to 18MW of Neutral Beam heating are clearly seen.

To determine how the temperature and density vary across the plasma, Thomson Scattering is combined with distance measurements (LIDAR). A short laser pulse (0.3 nano-seconds duration which, at the speed of light, is only 10 cm long) is sent across the plasma diameter. By using a fast detection and recording system, one can observe its progress by capturing the changes in the back-scattered spectrum. One can then analyse these changes as the pulse passes from the relatively cool edge, through the hot core and out again through the opposite plasma edge. The time of flight, or LIDAR, principle yields information about where the laser pulse is in the plasma at each instant, and one can compute from the instantaneous scattered spectrum the local values of temperature and density in the plasma, ie. from the time of flight of one laser pulse through the plasma we can obtain the temperature and density variations across the whole diameter.