Damage assessment of the Faraday Shield in the RF driver of the negative ion source test stand SPIDER

The SPIDER experiment, hosted in the Neutral Beam Test Facility of Padua, Italy, is the full-scale prototype of the negative ion source for ITER neutral beam injectors. Inside the ion source, eight radio frequency (RF) drivers are responsible for producing plasma via electromagnetic induction. The Faraday Shield lateral wall (FSLW) is a cup-shaped component of SPIDER RF drivers made out of electrodeposited copper, protecting the surrounding components from direct contact with plasma. Electromagnetic coupling with plasma is not perfect inside the driver. Indeed, RF power in the order of tens of kW is deposited on the driver components during nominal operation, thus requiring them to be actively cooled. During SPIDER 2018-2021 experimental campaign, an unexpected issue with the cooling systems resulted in the structural failure of the FSLW of four drivers. Numerical analyses initially suggested the hypothesis of a localized damage, implying the possibility of repairing the damaged components rather than replacing them with new ones. Measurements of hardness and grain size collected on the surface of the FSLWs have confirmed this hypothesis and served as a basis for the development of the refurbishment strategy. Eventually, the damaged FSLWs were successfully repaired and reinstalled in SPIDER.