Preparation of the measurement system for the spatial characterization of SPIDER negative-ion plasma source

Neutral beam injectors are fundamental auxiliary heating systems for nuclear fusion machines. The formation of the negative-ion beam, precursor of the neutral beam, occurs by extracting the ions from a plasma through a multi-aperture, multi-electrode electrostatic accelerator. In the case of the ITER neutral beam injector, to obtain sufficient current, the extraction takes place through 1280 openings distributed over a very large area, which must be illuminated by a sufficiently uniform plasma. The plasma, formed in 8 radio-frequency drivers, passes through an expansion chamber before reaching the extractor. The experimental study of the plasma parameters at the location of the ion extraction, and of its expansion from the driver region, is possible using Langmuir probes on mobile supports. In this dissertation, a preparatory study is conducted for characterizing negative ion plasma in SPIDER negative-ion plasma source. A bibliographic research is performed in order to understand how similar measures were taken in the past in similar devices, at the end of which three different kind of probes are designed and tested: a Mach probe, a double Langmuir probe and a planar Langmuir probe. The Mach probe is composed of 4 cylindrical pins and is designed in order to measure plasma flux and velocity drift. It was tested by assembling it on a rotating system inside an Argon plasma, in order to verify its symmetry and if it is suitable for an RF source like SPIDER. Next, the double probe (named ADEL, i.e. A Double Electrode Langmuir probe) consists of two parallel cylindrical electrodes, and it should give a completely floating measures of the plasma parameters. It was tested inside the CRISPy experiment (Compact RF Ion Source Prototype for emittance scanner testing) in order to give an estimate of electron temperature and plasma density. Regarding the planar Langmuir probe, it is composed of two electrodes: a main electrode and a compensation one. A model in LTSpice is used to simulate its behaviour in an RF plasma, so as to find the optimum capacity to be put in parallel between the two electrodes. Finally, a plan for probes installation and measurements in SPIDER is established in light of the results obtained.