Mapping of magnetic field of SPIDER by a three-axis automatic positioning system

SPIDER is the prototype of the ion source of the Neutral Beam Injector (NBI) for ITER, and is currently in operation at the Neutral Beam Test Facility (NBTF), Consorzio RFX, Padova, Italy. In SPIDER, plasma is generated by a radio frequency inductively-driven plasma source; negative ions are produced on a cesium catalyzed surface and then extracted and accelerated by mean of three grids biased at increasing potential and having 1280 apertures each. SPIDER target parameters are a beam current of 40 A of D- accelerated at 100 keV for 3600 s stable operation. Magnetic field is very important in SPIDER and in general in negative ion sources. It is required for filtering the fast electrons in the plasma source (magnetic filter field), thus enhancing the negative ion production rate, and for suppressing the co-extracted electrons in the accelerator (electron suppression field). The filter field in SPIDER is generated by a current flowing through the plasma grid (the first grid), while the electron-suppression field is generated by permanent magnets embedded in the extraction grid (the second grid). Moreover, in SPIDER, a system of additional permanent magnets and a ferromagnetic plate are installed at the level of the grounded grid (the third grid) with the purpose of adjusting the negative ion trajectory. In order to measure and map the magnetic field in SPIDER, a robotic system has been designed and realized, then applied with success during 2018. A three-axis positioning system holds a Hall probe, and can be controlled manually or remotely by graphic user interface. Once the three-axis system is installed on SPIDER, it can be programmed to automatically move through the 1280 accelerator apertures and acquire magnetic field profiles. This paper describes the design and the realization of the magnetic field mapping system, along with the results of the measuring campaigns carried out in 2018.