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

The SPIDER H-/D- ion source is currently in operation at the Neutral Beam Test facility (NBTF) in Consorzio RFX (Padova, Italy) to prove the possibility of generating up to 40 A of negative ions, with a maximum extracted current density of 350 A/m2 (H)/285 A/m2 (D) and a fraction of coextracted electrons not greater than 0.5 (H)/1 (D). These performances are required for the realization of the ITER Heating Neutral Beam (HNB), which should deliver 16.7 MW to the plasma by means of negative ions accelerated up to 1 MeV and neutralized before being injected into the ITER tokamak. In order to obtain such high extracted current densities and low co-extracted electron fractions it is necessary to lower the work function of the surface of the acceleration system grid facing the source; this will be accomplished by coating the surfaces with Cs, routinely evaporated by three ovens. The functionality of the ovens has been tested at the CAesium oven Test Stand (CATS), hosted at NBTF. The test stand is equipped with several diagnostics, among which a Laser Absorption Spectroscopy (LAS) diagnostic. Using a tunable laser diode, the LAS diagnostic gets the high resolution absorption spectrum of the Cs 852 nm D2 line along a line of sight to measure Cs density at ground state. The paper describes the test stand and the LAS diagnostic, together with the characterization of the three ovens to be installed in SPIDER. The paper will also study the systematic underestimation effect on measurements caused by Cs ground state depopulation, as function of laser intensity and Cs density, in the perspective of correcting the deviation of measurements.