Development of a set of movable electrostatic probes to characterize the plasma in the ITER neutral beam negative-ion source prototype

Giant negative-ion sources for fusion plasma heating are optimized to provide, by a peculiar magnetic topology in the expansion region, a separation between a relatively high electron-temperature region for plasma formation and hydrogen dissociation, and a beam extraction region in which low electron-temperature is required to avoid negative-ion destruction. The full-scale ITER neutral beam ion-source prototype SPIDER is the largest RF-driven negative ion source in operation, and it exhibits non-uniformities and drifts in the large expansion region from the eight RF drivers. A direct measurement of the plasma density and electron temperature in the expansion and extraction region as a function of the operational parameters is therefore required to correlate negative-ion beam uniformity to the operational parameters. The main operational parameters affecting plasma expansion and uniformity are the gas pressure, the perpendicular magnetic field strength, and the electric bias of ion source surfaces. This paper describes the development of a set of movable electrostatic probes that are used to measure axial profiles of plasma parameters, entering the ion source from the electrode apertures of the ion accelerator towards the RF drivers. The set of electrostatic probes encompasses eight RF-compensated Langmuir probes, one double probe, one triple probe for relatively high-frequency measurements, one Mach probe for the assessment of plasma drift velocities, and two gridded retarding field energy analyzers to measure the positive-ion energy distribution function. The mechanical constraints given by the experimental setup, based on a large in-vacuum movable structure with multiple probes entering from small apertures and required to scan about 400 mm, the thermal requirements of these relatively heat-flux components, and the electrical aspects, made the design challenging for the limited allowable dimensions. Prototyping and commissioning of the measurement system is also discussed. Examples of measured axial profiles with the various probes used in SPIDER will be presented.