Effect of self-bias on cylindrical capacitive discharge for processing of inner walls of tubular structures—Case of SRF cavities

J Upadhyay,L Vušković,J Peshl,S Popović,A.-M Valente-Feliciano

doi:10.1063/1.5045692

Abstract

Cylindrical capacitive discharge is a convenient medium for generating reactive ions to process inner walls superconductive radio-frequency (SRF) cavities. These cavities, used in particle accelerators, presents a three-dimensional structure made of bulk Niobium, with axial cylindrical symmetry. Manufactured cavity walls are covered with Niobium oxides and scattered particulates, which must be removed for desired SRF performance. Cylindrical capacitive discharge in a mixture of Ar and Cl2 is a sole and natural non-wet acid choice to purify the inner surfaces of SRF cavities by reactive ion etching. Coaxial cylindrical discharge is generated between a powered inner electrode and the grounded outer electrode, which is the cavity wall to be etched. Plasma sheath voltages were tailored to process the outer wall by providing an additional dc current to the inner electrode with the help of an external compensating dc power supply and corrugated design of the inner electrode. The dc bias potential difference is established between two electrodes to make the set-up favorable for SRF wall processing. To establish guidelines for reversing the asymmetry and establishing the optimal sheath voltage at the cavity wall, the dc self-bias potential and dc current dependence on process parameters, such as gas pressure, rf power and chlorine content in the Ar/Cl2 gas mixture was measured. The process is potentially applicable to all concave metallic surfaces.

Highlights

Hollow non-planar surfaces, found in superconductive RF (SRF) cavities, supersonic nozzles or other large-volume tubular devices, often contain particulates or chemical impurities that lead to electric losses or unfavorable hydrodynamic turbulences
Cylindrical coaxial capacitive discharges are the natural choice for discharge-plasma etching of hollow surfaces, where a discharge is generated between a cylindrical convex powered electrodes, which is centered inside the grounded coaxial tube representing second, outer electrode with the concave surface to be processed
085008-3 Upadhyay et al We have developed a method to modify the inner surface of superconducting radio frequency (SRF) cavities made of Niobium (Nb) by using coaxial cylindrical capacitive radio-frequency discharge in Argon/Chlorine (Ar/Cl2) gas mixture.[17]

Summary

Introduction

Hollow non-planar surfaces, found in superconductive RF (SRF) cavities, supersonic nozzles or other large-volume tubular devices, often contain particulates or chemical impurities that lead to electric losses or unfavorable hydrodynamic turbulences. These problems can be solved by purification and reduction of surface roughness by critical material removal from the surface either by various acid or discharge-plasma etching techniques. The processes for removal of impurities and particulates are reactive ion etching and field emission This arrangement is geometrically asymmetric since the two cylindrical smooth surfaces have substantially different areas, leading to distinctively smaller sheath voltage at the outer electrode, which provides unfavorable reactive ion etching rates for processing the concave surface. There are geometrical and electric techniques to reverse the asymmetry and provide

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Conclusion