Design of Thomson scattering diagnostics for the Divertor Tokamak Test (DTT) Facility

In DTT two TS systems at are under design for the measurements of Te and ne in the core plasma region and in the divertor respectively. The divertor TS system under study is a conventional TS system based on a Nd:YAG laser source, a fiber optic based light collection system and a set of filter polychromators equipped with Si APD detectors. The laser beam and the collection optics share an aperture between adjiacent cassettes of the lower divertor and the scattering signal is collected from a set of scattering volumes located into one of the divertor leg by in vacuum collection optics and sent to the spectrometers by fiber optic bundles located outside the vacuum vessel. The filters spectrometers are designed to cover a Te range down to 1 eV. Measurements with spatial resolution of ~ 5-10 mm are possible, with an accuracy limited by the plasma ne and the background light. For the core TS system, two options are under consideration: a conventional system, similar to that desiged for the ITER core TS, in which Te and ne are measured along a large fraction of a laser beam crossing the plasma in the equatorial plane and the detection system is again based on fiber optic coupled filter polychromators. The spatial resolution is ~ 3 cm in the central region and ~ 1 cm in the pedestal region. In alternative a TS system based on the LIDAR concept, previously implemented in JET, is under consideration in which, recent advancements in laser and detector technology allow to achieve a spatial resolution similar to that of the conventional system, but with a simpler and reliable experiemtal set-up and at a lower cost. In this paper the two approaches are discussed and their performances compared with reference to specific DTT plasma scenarios.