Plasma Processing of Large Curved Surfaces for SRF Cavity Modification (original) (raw)
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Etching of Niobium Sample Placed on Superconducting Radio Frequency Cavity Surface in Ar/CL2 Plasma
Plasma based surface modification is a promising alternative to wet etching of superconducting radio frequency (SRF) cavities. It has been proven with flat samples that the bulk Niobium (Nb) removal rate and the surface roughness after the plasma etchings are equal to or better than wet etching processes. To optimize the plasma parameters, we are using a single cell cavity with 20 sample holders symmetrically distributed over the cell. These holders serve the purpose of diagnostic ports for the measurement of the plasma parameters and for the holding of the Nb sample to be etched. The plasma properties at RF (100 MHz) and MW (2.45 GHz) frequencies are being measured with the help of electrical and optical probes at different pressures and RF power levels inside of this cavity. The niobium coupons placed on several holders around the cell are being etched simultaneously. The etching results will be presented at this conference.
Plasma treatment of bulk niobium surface for SRF cavities
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2006
Accelerator performance, in particular the average accelerating field and the cavity quality factor, depends on the physical and chemical characteristics of the superconducting radio-frequency (SRF) cavity surface. Plasma based surface modification provides an excellent opportunity to eliminate non-superconductive pollutants in the penetration depth region and to remove the mechanically damaged surface layer, which improves the surface roughness. Here we show that the plasma treatment of bulk Nb presents an alternative surface preparation method to the commonly used BCP and EP methods. We have optimized the experimental conditions in the microwave glow discharge system and their influence on the Nb removal rate on the flat samples. We have achieved etching rate of 1.7 μm/min using only 3% Cl 2 in the reactive mixture. Combining a fast etching step with a moderate one, we have improved the surface roughness without exposing the fresh sample surface to the environment. We will apply the optimized experimental conditions to the preparation of some single cell cavities, in pursuing improvement of their RF performance.
Physical Review Special Topics - Accelerators and Beams, 2010
Accelerator performance, in particular the average accelerating field and the cavity quality factor, depends on the physical and chemical characteristics of the superconducting radio-frequency (SRF) cavity surface. Plasma based surface modification provides an excellent opportunity to eliminate nonsuperconductive pollutants in the penetration depth region and to remove the mechanically damaged surface layer, which improves the surface roughness. Here we show that the plasma treatment of bulk niobium (Nb) presents an alternative surface preparation method to the commonly used buffered chemical polishing and electropolishing methods. We have optimized the experimental conditions in the microwave glow discharge system and their influence on the Nb removal rate on flat samples. We have achieved an etching rate of 1:7 m= min using only 3% chlorine in the reactive mixture. Combining a fast etching step with a moderate one, we have improved the surface roughness without exposing the sample surface to the environment. We intend to apply the optimized experimental conditions to the preparation of single cell cavities, pursuing the improvement of their rf performance.
We are pursuing the development of low cost environmentally friendly dry etching of superconducting radio frequency (SRF) cavities in Ar/Cl 2 discharges. It has been proven with flat samples that the bulk Niobium (Nb) removal rate and the surface roughness after plasma etchings are equal to or better than wet etching processes. The plasma properties inside the single cell SRF cavity depend on frequency, pressure and power. To understand the plasma properties and chemical kinetics of the plasma etching process inside the single cell cavity, we are using a single cell cavity with 20 sample holders symmetrically distributed over the cell. These holders are being used for niobium coupon etching as well as diagnostic ports for optical measurements. Multiple optical probes with optical fibers have been utilized for optical emission spectroscopy measurements. A power supply in the radio frequency regime (100 MHz) and another power supply in the microwave frequency regime (2.45 GHz) are use...
Etching mechanism of niobium in coaxial Ar/Cl2 radio frequency plasma
Journal of Applied Physics, 2015
The understanding of the Ar/Cl 2 plasma etching mechanism is crucial for the desired modification of inner surface of the three dimensional niobium (Nb) superconductive radio frequency cavities. Uniform mass removal in cylindrical shaped structures is a challenging task, because the etch rate varies along the direction of gas flow. The study is performed in the asymmetric coaxial RF discharge with two identical Nb rings acting as a part of the outer electrode. The dependence of etch rate uniformity on pressure, RF power, DC bias, Cl 2 concentration, diameter of the inner electrode, temperature of the outer cylinder and position of the samples in the structure is determined. To understand the plasma etching mechanisms, we have studied several factors that have important influence on the etch rate and uniformity, which include the plasma sheath potential, Nb surface temperature, and the gas flow rate.
Etching Mechanism of Niobium in Coaxial Ar/Cl2 RF Plasma
The understanding of the Ar/Cl2 plasma etching mechanism is crucial for the desired modification of inner surface of the three dimensional niobium (Nb) superconductive radio frequency cavities. Uniform mass removal in cylindrical shaped structures is a challenging task, because the etch rate varies along the direction of gas flow. The study is performed in the asymmetric coaxial RF discharge with two identical Nb rings acting as a part of the outer electrode. The dependence of etch rate uniformity on pressure, RF power, DC bias, Cl2 concentration, diameter of the inner electrode, temperature of the outer cylinder and position of the samples in the structure is determined. To understand the plasma etching mechanisms, we have studied several factors that have important influence on the etch rate and uniformity, which include the plasma sheath potential, Nb surface temperature, and the gas flow rate.
Experiment and Results on Plasma Etching of SRF Cavities
6th Int. Particle Accelerator Conf. (IPAC'15), Richmond, VA, USA, May 3-8, 2015, 2015
The inner surfaces of SRF cavities are currently chemically treated (etched or electro polished) to achieve the state of the art RF performance. We designed an apparatus and developed a method for plasma etching of the inner surface for SRF cavities. The process parameters (pressure, power, gas concentration, diameter and shape of the inner electrode, temperature and positive dc bias at inner electrode) are optimized for cylindrical geometry. To study the etching of the inner surface of the varied diameter cylindrical structure, a stainless steel pill box cavity has been made. The niobium samples placed inside this cavity has been studied for etch affects purposes. The inner electrode has been moved and plasma response to the movement of the powered electrode has been seen. Plasma characterization is done with the help of optical emission spectroscopy.
PLASMA ETCHING RATES AND SURFACE COMPOSITION OF BULK NIOBIUM TREATED IN Ar/Cl2 MICROWAVE DISCHARGE
To achieve theoretically predicted values of the accelerating fields in superconducting radiofrequency (SRF) cavities, their inside surface should be fairly smooth and free of impurities. Thus, surface preparation is the critical step in production of SRF cavities. Plasma etching process is a dry chemistry technique that can be used to achieve these requirements. It is based on interaction between reactive halogen species produced in the glow discharge and the surface. During this process, volatile Nb halides are evaporated from the surface of Nb, removing the mechanically damaged and contaminated layer. We present a treatment of bulk Nb samples in the Ar/Cl2 microwave discharge. We have shown that etching rates of bulk Nb as high as 1.5 mm/min can be achieved without introducing impurities in Nb. The rate dependence on various discharge parameters and reactive gas composition is presented. Surface composition and topology measurements were carried out before and after plasma treatment
The Effect of Process Parameters on the Surface Properties of Niobium During Plasma Etching
2018
We have shown that plasma etching using an electronegative Ar/Cl 2 discharge can effectively remove surface oxide layers on Nb samples as well as bulk Nb from single cell SRF cavities [1]. With accelerating fields on the order of wet etching processes and a decrease in field emission the use of plasma assisted etching for bulk Nb processing is a worthwhile endeavor. We are presenting work on the surface properties of plasma etched Nb. Cavity grade Nb coupons were made by water jet cutting, and then polished to achieve surface properties equivalent to electropolishing (<1 micron). The coupons were plasma etched while process parameters (rf power, gas pressure, temperature and dc bias voltage) are varied. These samples are placed on the inner surface of the cylindrical cavity to be etched. The experimental setup is similar to the single cell cavity plasma etching setup [2]. Each sample is weighed and scanned before and after plasma processing with an AFM, SEM, and digital optical microscope that provide both atomic composition and surface roughness profiles. Comparing the scans allows us to make conclusions about the effect of each experimental parameter on the surface properties.
Applied Surface Science, 2006
The performance of niobium superconducting radiofrequency (SRF) accelerator cavities is strongly impacted by the topmost several nanometers of the active (interior) surface, especially as influenced by the final surface conditioning treatments. We examined the effect of the most commonly employed treatment, buffered chemical polishing (BCP), on polycrystalline niobium sheet over a range of realistic solution flow rates using electron back scatter diffraction (EBSD), stylus profilometry, atomic force microscopy, laboratory XPS and synchrotron (variable photon energy) XPS, seeking to collect statistically significant datasets. We found that the predominant general surface orientation is (1 0 0), but others are also present and at the atomic-level details of surface plane orientation are more complex. The post-etch surface exhibits micron-scale roughness, whose extent does not change with treatment conditions. The outermost surface consists of a few-nm thick layer of niobium pentoxide, whose thickness increases with solution flow rate to a maximum of 1.3-1.4 times that resulting from static solution. The standard deviation of the roughness measurements is AE30% and that of the surface composition is AE5%. #