Plasma-assisted Preparation And Modification Of Superconducting Radio Frequency Materials

Superconducting Radio Frequency（SRF）cavities are widely used in various particle accelerator facilities due to their advantages of large beam aperture,high energy efficiency,and stable operation with high gradient and continuous wave mode.At present,most of the SRF cavities are made of high purity niobium（Nb）,but pure Nb SRF cavities have gradually exposed a series of problems about surface treatment and stable operation of accelerators.On the one hand,due to the stable chemical properties of pure Nb,the surface treatment is very complicated.So,buffer chemical polishing（BCP）,electropolishing（EP）are often used for the treatment of pure Nb SRF cavity.These processes need to use hazardous and corrosive solutions,such as HF and HNO₃,which are very harmful to environment and human.In addition,owning to the low thermal conductivity of Nb,the cavities’wall thickness is limited to a few millimeters,which is very sensitive to environmental vibration.In order to solve the problems mentioned above,this paper introduces the RF plasma technology to study the surface modification of SRF cavity.First,this paper utilizes the plasma etching method to process SRF cavity for its low cost,reliability and pollution-free.It is expected to replace the traditional chemical method.In addition,Nb thin film was prepared on copper substrate by plasma-enhanced chemical vapor deposition method.Using the energy of the plasma to reduce the deposition temperature,the film can be formed under low temperature,which could solve the problems faced with Nb SRF cavity.Firstly,the characteristics of capacitively coupled plasma sources for surfacetreatment of SRF cavities were studied by simulation.The study found that uniform plasma distribution can be obtained inside SRF cavity by optimizing the shape of electrodes.The electron density is sensitive to the change of the electrode structure,the which increases with the increasing power or driving RF frequency.Meanwhile,the electron temperature remains inert to the changing power and frequency.The shape and range of the ion energy distribution function（IEDF）at different positions of the cavity were studies as well.Due to the sheath superposition effect,the position of the beam pipeline is dominated by low-energy ions.With the same frequency and power,the IEDF ranges at the iris,the equator and transition position are almost the same.But,when the frequency and power change,the IEDF range increases with power and decreases with frequency.Then,the etching and polishing based on physical sputtering of Nb SRF cavity was studied.A sputtering yield probability distribution function model was introduced by taking the IEDF and the sputtering yield dependence on ion energy into consideration.The model quantitatively estimated the etching rate was by combining the sheath model and the sputtering yield probability distribution function.For the simplest case,the etching rate of energetic argon ions hit on the Nb surface,both theoretical calculation and the experimental measurement using a biased ICP source were performed.It was found that the experimental results were consistent with the theoretical calculation.Meanwhile,the plasma physical etching was found effective in reducing the surface roughness.Therefore,the plasma physical etching technique can be applied to Nb SRF cavity,which provides a new route for plasma surface treatment.Finally,Nb thin films were deposited on copper substrate using PECVD technique with ICP Ar/H₂plasma.The study found that maximum electron density,temperature and particle densities of Ars,Ar⁺,Ar H⁺,H⁺,and H₂⁺all exist near the RF coil.While,active H atoms are uniformly distributed throughout the reaction area.The electron density increases with the increasing gas pressure and power,but the electron temperature shows an opposite trend.As the increase of H₂ concentration,the electron density and Ar⁺density decrease monotonically.While the electron temperature and the density of H and H⁺always increase.The PECVD experiment was carried out using a five-zone tube furnace.The characterization of the samples found that the PECVD method successfully prepared Nb thin films at 500°C,indicating that the deposition temperature can be effectively reduced with this method.But the content of Nb element is low,and the film thickness is too thin,so the process needs to be further optimized to improve the film quality.