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Neutral Beam Injectors (NBIs) based on negative ions are the workhorses offuture fusion reactors, such as ITER, which they are expected to provide withup to \SI{33}{MW} of power to heat the fusion plasma. The negative hydrogenions are extracted from a RF plasma, in which a magnetic filter field coolsdown the electrons reaching the so-called expansion region and allows theformation of negative ions near the apertures in the plasma grid. To furtherimprove the production of negative ions, cesium is usually evaporated insidethe source and deposited onto the plasma walls, reducing the work function ofthe surfaces. This dramatically increases the density of negative hydrogen ionsnear the surfaces, causing the transition to an electronegative plasma in thevicinity of the plasma grid. This condition can be observed with Langmuirprobes, which can then be used to provide a local meaurement of negative iondensity in the ion source.  In this paper we use the measurements provided by the Langmuir probe sensorsembedded in the plasma grid of SPIDER, the prototype ion source of ITER NBIs,to determine the density of negative ions. A fitting method based on thedetermination of the collection area of the different plasma species isproposed and adapted to SPIDER experimental condition, taking into account theshape of the probes and the local topology of the magnetic field. The method isthen applied to the experimental data, determining the densities of thepositive and negative ions and of the electrons during a plasma pulse. Finally,a vertical array of four probes in the plasma grid is used to assess thevertical profile of plasma parameters.

Measure of negative ion density in a large negative ion source using Langmuir probes