Improvements to SPIDER beam source after two years of operation

SPIDER experiment is operating at NBTF site in Padova since June 2018, with the aim of testing and optimizing the negative ion source prototype for ITER Heating Neutral Beam Injectors. SPIDER is the first experiment in which the beam source is fully immersed in vacuum, at a gas pressure level that shall assure the voltage hold off. The first campaign highlighted the occurrence of electrical discharges on the rear side of the source when operating the Radio Frequency (RF) circuit, above a pressure threshold. In order to continue the operation, a temporary mask was installed on the beam source plasma grid, leaving open only 80 out of 1280 apertures, in order to reduce the gas conductance between the inside of the source and the surrounding volume. The final solution to avoid the discharges is under finalization and includes both an optimization of the RF circuit and a significant upgrade of the vacuum pumping system. SPIDER operation showed also a reduction of the plasma density inside some RF drivers due to the interaction between the magnetic filter field and the plasma particles. Therefore, the layout of the filter field circuit was modified introducing new busbars aiming to improve the magnetic field configuration. A major revision applied to the layout of the RF circuit allowed to achieve a reduction of the interactions among the four RF oscillators, and also a strong reduction of the current flowing through the thermocouple shields onboard the RF source, that compromised the quality of measurements and even damaged the thermocouples themselves. The paper gives details about the main issues discovered in the first two years of SPIDER operation and the improvements introduced during short shutdowns. An overview of the modifications to be implemented during the major shut-down planned between end 2020 and mid 2021, is also given.