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The SPIDER H-/D- ion source is currently in operation in the Neutral BeamTest facility (NBTF) at Consorzio RFX (Padova, Italy) to prove the possibilityof generating up to 40 A of negative ions, with a maximum extracted currentdensity of 350 A/m2 (H)/285 A/m2 (D) and a fraction of co-extracted electronsnot greater than 0.5 (H)/1 (D). These performances are required for therealization of the ITER Neutral Beam Injector (NBI), which should deliver 16.7MW to the plasma by means of negative ions accelerated up to 1 MeV andneutralized before being injected into the ITER tokamak. In order to obtainsuch high extracted current densities and low co-extracted electron fractionsit is necessary to lower the work function of the surface of the accelerationsystem grid facing the source; this will be accomplished by coating thesurfaces with Cs, routinely evaporated by three ovens. The functionality of theovens has been tested at the CAesium oven Test Stand (CATS), hosted at NBTF.The test stand is equipped with several diagnostics, among which a LaserAbsorption Spectroscopy (LAS) diagnostic. Using a tunable laser diode, the LASdiagnostic gets the high resolution absorption spectrum of the Cs 852 nm D2line along a line of sight to measure Cs density at ground state. The paperdescribes the test stand and the LAS diagnostic, together with thecharacterization of the ovens to be installed in SPIDER. The paper will alsostudy the systematic density underestimation effect caused by Cs ground statedepopulation, as a function of laser intensity and of Cs density, in theperspective of correcting the density evaluation.

Laser absorption spectroscopy studies to characterize Cs oven performances for the negative ion source SPIDER