Plasma Electrolytic Polishing and Vibrotumbling as innovative surface treatments for Superconducting Radiofrequency cavities

Superconducting radio frequency (SRF) cavities performances strongly depend on the surface preparation. For that reason, surface treatments represent an important aspect in the development of future accelerators. The state of art of the Nb cavity production relies on 60-years’ experience on superconducting radiofrequency (SRF) cavities field dealing with this material. Nb is a type II superconductor and high critical temperature natural material. A copper substrate may be used for deposition of a superconducting (SC) layer to reduce costs and improve mechanical stability comparing to the bulk niobium technology. The conventional protocol of substrates surface preparation includes chemical and electrochemical polishing techniques, and may include as well mechanical processing, such as chemical barrier polishing (CBP) and grinding. Harsh and corrosive solutions are typically used for chemical preparation: concentrated HF and H2SO4 acids for Nb, and H3PO4 with Butanol mixtures for Cu. Vibro-tumbling (VT) technique is a variation of the vibratory and mass-finishing process. Usually, the systems are composed of an eccentric motor that provides the vibration motion at a certain frequency. The rotation of the cavity with the abrasive media and vibration allows uniform abrasion of the internal surface given by a step motor. A VT system has been refurbished and updated. A series of experiments were conducted on both Cu and Nb prototype cavities varying frequency, abrasive, media, volume. A study was done on the two types of substrates: elliptical 6 GHz cavities and coupons, placed in the dummy 6 GHz cavities. A two-step protocol was developed, that includes erosion step with the peak rate of 24 μm/min and a finishing step with coconut shells abrasive. Surface morphology and roughness, erosion rates and other parameters were studied, and a final protocol was implemented into the continuous workflow of the National Legnaro Laboratories Cu 6 GHz cavity production. In this dissertation, an innovative treatment – Plasma Electrolytic Polishing (PEP) has been studied to substitute the conventional treatments and possibly eliminate the need of mechanical preparation. Plasma electrolytic polishing is an evolution of electropolishing. The PEP uses diluted water-based salt solutions, able to achieve low roughness values of ≤100 nm. The smoothing of the surface is superior comparing to the standard treatments at the parity of the removal thickness. This is possible due to the different working regime, that involves high voltage DC anodic polarisation, high temperature of the bath and the formation of uniform stable vapor-gas layer (VGL) over the anode surface, allowing the ignition of the plasma. The stable VGL and plasma discharges allow the levelling of the surface. There is no common theory on the mechanism of the PEP technique. In this research work, 4 unique solutions were developed for Nb and Cu PEP processing. The process parameters behaviour was characterised by parameters such as voltage, temperature, and solution composition. Surface morphology and roughness, erosion rates, current efficiency were obtained and discussed. Experiments were conducted on the planar samples, and more complex geometries like dummy 6 GHz cavities, Quadrupole resonator (QPR) samples. 3d printed coupons were studied as well, and polished with PEP. The planar samples were a key tool for preliminary study of the PEP process and later for solutions optimisation and characterisation of final surface quality (linear profilometer, optical and electron microscopy). Dummy 6 GHz cavities substrates were used to simulate the elliptical geometry of the cavities during the PEP process and to analyse the possibility of PEP application in cavity preparation. QPR sample is a test instrument for characterisation of surface resistance with accuracy in the nano-ohm range. Thus, the polishing of QPR samples by PEP were done for future comparison with conventional treatments.