Development and first operation of a Cavity Ring Down Spectroscopy diagnostic in the negative ion source SPIDER

The Neutral Beam Injectors (NBIs) of the ITER experiment will rely on negative ion sources to produce 16.7 MW beams of H/D particles accelerated at 1 MeV. The prototype of these sources was built and is currently operated in the SPIDER experiment, part of the Neutral Beam Test Facility (NBTF) of Consorzio RFX, Padua. In SPIDER, the H-/D -ion source is coupled to a three grids 100 kV acceleration system. One of the main targets of the experimentation in SPIDER is to uniformly maximize the extracted current density, firstly with pure volume H-/D production in the source and then with surface production, enhanced by Cs evaporation onto the surface of the grid facing the source plasma (i.e. the Plasma Grid, PG). To reach this aim it is important to study the density of negative ions available in proximity of the PG apertures. Firstly, monitoring the negative ion density will be useful to check the effectiveness of Cs evaporation and improve it. Moreover, in good beam optics conditions H-/D density is linearly correlated to the extracted current density; verifying this correlation can indicate whether the beam is correctly extracted or if part of it is lost inside the acceleration system. In SPIDER, line-integrated measurements of negative ion density are available by means of a Cavity Ring Down Spectroscopy (CRDS) diagnostic. Its principle of operation is based on the absorption of the photons of a laser beam pulse by H-/D -photodetachment; the absorption detection is enhanced by trapping the laser pulse in an optical cavity, containing the absorbing medium (i.e. negative ions). The paper presents and discusses the realization of the CRDS diagnostic in SPIDER; moreover, the first measurements on negative ion production are here presented, and correlated to the main source and acceleration system operative parameters.