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This contribution describes the commissioning and first operation of a fixed triple Langmuir probe in source for the production of ions of deuterium extracted from a radio frequency plasma (SPIDER), the full-scale prototype of the negative-ion source for ITER neutral beam injectors. This diagnostic is capable of measuring the time evolution of the main plasma features for the entire duration of the plasma discharge, allowing to correlate local plasma properties, such as density, electron temperature, and plasma potential, to control parameters, i.e., filter field intensity, radio frequency (RF) power, and bias voltages. The probe was installed on the rear wall of the expansion chamber, and this being a region of specific interest for the operation with cesium since it may act as a reservoir, contributing to the cesium recycling toward the extraction region. Furthermore, the capability of the triple probe arrangement of measuring the instantaneous plasma density and electron temperature (also at high frequency) has been exploited to investigate the modulation of plasma parameters.
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