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Magnetic field plays an essential role in negative ion-driven neutral beam injectors (NBIs), being necessary to counteract the inevitable interaction between electrons and negative ions, both to maximize the ion production and to minimize the electron extraction and acceleration. Magnetic field influences also the generation and confinement of the plasma from where the ions are extracted, and this is particularly significant for radio frequency (RF) inductively generated plasmas, such as the case of SPIDER. SPIDER is the full-scale prototype of the ITER NBI ion source, in operation since 2018. Its magnetic field configuration was modified in 2020 to solve issues related to plasma generation. The new configuration has significantly improved the performances, but some limitations and lack of symmetries emerge at high field. The present article describes the numerical and experimental assessment recently carried out in order to verify the actual distribution of the new magnetic field configuration inside the SPIDER ion source and accelerator, and to investigate on possible reasons of asymmetries.
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