Abstract
Niobium thin films are used at CERN (European Organization for Nuclear Research) for coatings of superconducting radio-frequency (SRF) accelerating cavities. Numerical simulations can help to better understand the physical processes involved in such coatings and provide predictions of thin film properties. In this article, particle-in-cell Monte Carlo 3D plasma simulations are validated against experimental data in a coaxial cylindrical system allowing both DC diode and DC magnetron operation. A proper choice of ion induced secondary electron emission parameters enables to match experimental and simulated discharge currents and voltages, with argon as the process gas and niobium as the target element. Langmuir probe measurements are presented to further support simulation results. The choice of argon gas with a niobium target is driven by CERN applications, but the methodology described in this paper is applicable to other discharge gases and target elements.Validation of plasma simulations is the first step towards developing an accurate methodology for predicting thin film coatings characteristics in complex objects such as SRF cavities.
Highlights
Thin film coatings used at CERN (European Organization for Nuclear Research) cover a wide range of applications including non-evaporable getters for distributed pumping [1] and amorphous carbon for electron cloud mitigation [2]
We focus here on niobium on copper (Nb/Cu) thin film deposition used in the production of superconducting radio-frequency (SRF) accelerating cavities, as an alternative to bulk niobium cavities [3]
We focus on the numerical simulation of the DC discharge itself with the validation of two physical parameters which have a strong impact on the plasma behaviour: the secondary electron emission yield induced by argon ion bombardment on the niobium target, and the initial energy distribution function of the secondary emitted electrons from the niobium cathode
Summary
Thin film coatings used at CERN (European Organization for Nuclear Research) cover a wide range of applications including non-evaporable getters for distributed pumping [1] and amorphous carbon for electron cloud mitigation [2]. We focus here on niobium on copper (Nb/Cu) thin film deposition used in the production of superconducting radio-frequency (SRF) accelerating cavities, as an alternative to bulk niobium cavities [3]. The link between thin film properties and RF performances of the coated cavity is non-trivial [6]. The use of numerical simulations could help in the design of sputtering sources and tuning of the deposition process, providing qualitative (relative thickness uniformity) and quantitative properties (absolute thickness profile, film morphology)
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