Integration of new sets of magnets for improved plasma confinement in the SPIDER experiment

SPIDER is the full-scale prototype of the plasma source of the negative-ion driven neutral beam injector for the heating and current drive of the ITER plasma. The uniqueness and complexity of the system requested this ad hoc test stand aiming at optimizing the performance of the RF inductively generated plasma, negative ion production and extraction, electron filtering, and robustness and controllability of all systems required to work together. After about three years of operation, presently SPIDER is in a long shutdown, in which the whole plasma source and accelerator were dismounted. In this phase, additional modifications with respect to the original design will be introduced to improve the system performance, driven by the experience acquired in the last years. These include the addition of further sets of permanent magnets in the plasma source expansion chamber and around the RF drivers, with the aim of improving the plasma confinement and consequently its density and possibly its uniformity. The present paper reports the study and the analyses behind this modification, which impacts on the original already complex magnetic configuration, made particularly difficult by the limited space available and the high voltages. The use of ferromagnetic shields, necessary to limit stray fields possibly increasing the breakdown probability, make the design particularly complex because of the greater impact on the previous configuration. An iterative process between analyses to determine the ideal configuration and CAD verifications was required. The analyses had to take into account the new magnetic configuration to be created in the particular area of interest, and the overall configuration in order to not compromise its efficacy.