The Thomson Scattering System for DTT (Divertor Test Tokamak) edge-pedestal

Abstract The preliminary design of the Thomson Scattering (TS) System devoted to the measurement of the electron density and temperature in the edge-pedestal region of DTT (Divertor Tokamak Test) is described. The evaluation of the DTT pedestal width based on the presently elaborated models tested on JET and DIIID (EPED1 model) leads to a pedestal width Δ = 10 mm along a radial line of sight on the equatorial plane, assuming a scenario with maximum core temperature Te = 20 keV and density ne = 5 10 20 m -3 . The plasma region to be measured could be evaluated in 150 mm (15 times the pedestal width). The required spatial resolution is δr ≈ 3 mm to determine the pedestal profile from top to bottom with a minimum of three measurements. The number of the spatial points object of measurement is 50. The installation of the TS is in the DTT equatorial port #17: both the laser focusing optics and the collection optics are installed inside the port, thus realizing a compact design with θ s = 116° scattering angle. The minimum laser energy per pulse is 3 J at a 100 Hz repetition rate. The evaluation of numerical aperture of the collection optics is F#/5 leading to a measurement accuracy of 10% in the electron temperature and 5% in density. These numbers are consistent with the measurements technical specifications of the ITER EDGE TS. Given the neutron flux evaluated for DTT scenarios, a collection optics made by rhodium coated molibdenum mirrors can be considered, together with collimating lenses while the collecting fibres are placed outside the port exit window.