A LIDAR Thomson Scattering System for the Divertor Tokamak Test (DTT) facility

In spite of its very high reliability, LIDAR Thomson Scattering (LIDAR-TS) has so far only been installed on JET. The new Divertor Tokamak Test (DTT) facility at Frascati will be similar in size to JET and thus is clearly suited for a LIDAR-TS system. The proposal here follows the concept of a no vignetting region, which circumvents the need for relative density calibrations and provides much better signals than those on the JET system, particularly at the outer boundary. The scattered spectrum reaches the spectrometer through a train of relay lenses. The spectrometer uses fast MCP-PMTs for detection of the scattered signal. The fast detectors are generally limited to a spectral range below 900 nm. Hence, a laser at a single wavelength slightly below 900 nm would be ideal. Here we investigate using an Nd:YAG laser, emitting simultaneously both the fundamental and the second harmonic wavelength. The expected performance is analyzed in a simulation program, confirming the ability of this choice to cover the full temperature range. The spatial resolution of the system on JET was about 7 cm. For DTT it is possible to improve this value both by using more modern hardware and by deconvoluting the measured signals. Using a commercially available Nd:YAG two wavelength system with 100 ps pulses in combination with 180 ps MCP-PMTs a resolution of less than 4 cm is achieved. Deconvolution works well at the outer boundary of an H-mode plasma. With deconvolution of the spectral channel signals, the effective spatial resolution in this region can be reduced to ~2 cm.