Janardan Upadhyay

Etching mechanism of niobium in coaxial Ar/Cl2 radio frequency 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 radio-frequency (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.

Plasma processing of large curved surfaces for superconducting rf cavity modification

In this study, plasma based surface modification of niobium is a promising alternative to wet etching of superconducting radio frequency (SRF) cavities. The development of the technology based on Cl2/Ar plasma etching has to address several crucial parameters which influence the etching rate and surface roughness, and eventually, determine cavity performance. This includes dependence of the process on the frequency of the RF generator, gas pressure, power level, the driven (inner) electrode configuration, and the chlorine concentration in the gas mixture during plasma processing. To demonstrate surface layer removal in the asymmetric non-planar geometry, we are using a simple cylindrical cavity with 8 ports symmetrically distributed over the cylinder. The ports are used for diagnosing the plasma parameters and as holders for the samples to be etched. The etching rate is highly correlated with the shape of the inner electrode, radio-frequency (RF) circuit elements, chlorine concentration in the Cl2/Ar gas mixtures, residence time of reactive species and temperature of the cavity. Using cylindrical electrodes with variable radius, large-surface ring-shaped samples and d.c. bias implementation in the external circuit we have demonstrated substantial average etching rates and outlined the possibility to optimize plasma properties with respect to maximum surface processing effect.

Reversal of the asymmetry in a cylindrical coaxial capacitively coupled Ar/Cl2 plasma

The reduction of the asymmetry in the plasma sheath voltages of a cylindrical coaxial capacitively coupled plasma is crucial for efficient surface modification of the inner surfaces of concave three-dimensional structures, including superconducting radio frequency cavities. One critical asymmetry effect is the negative dc self-bias, formed across the inner electrode plasma sheath due to its lower surface area compared to the outer electrode. The effect on the self-bias potential with the surface enhancement by geometric modification on the inner electrode structure is studied. The shapes of the inner electrodes are chosen as cylindrical tube, large and small pitch bellows, and disc-loaded corrugated structure (DLCS). The dc self-bias measurements for all these shapes were taken at different process parameters in Ar/Cl2 discharge. The reversal of the negative dc self-bias potential to become positive for a DLCS inner electrode was observed and the best etch rate is achieved due to the reduction in plasma asymmetry.

On a dielectric-barrier and a microwave-cavity discharge in synchronized operation?the case of a helium/oxygen mixture

We have investigated the performance of a microwave-cavity discharge (MWD) operating in tandem with a fast rise-time pulsed dielectric-barrier discharge (DBD). The tandem discharge operated in a helium/oxygen mixture, where metastable molecular oxygen could be produced efficiently using MWD in proportionally large quantities (order of 20% of total oxygen number density). In this new arrangement, a DBD operating at high E/N provided a metastables-rich mixture, thereby modifying the discharge kinetics of the MWD, which operated in the E/N range centered around 10?Td. Both discharges operated in synchronized pulse-repetitive mode, which was tailored to maximize the oxygen metastable production efficiency. The system operated at pressures up to 350?Torr with an average power between 3 and 20?W.

Cryogenic rf test of the first SRF cavity etched in an rf Ar/Cl2 plasma

An apparatus and a method for etching of the inner surfaces of superconducting radio frequency (SRF) accelerator cavities are described. The apparatus is based on the reactive ion etching performed in an Ar/Cl2 cylindrical capacitive discharge with reversed asymmetry. To test the effect of the plasma etching on the cavity rf performance, a 1497 MHz single cell SRF cavity was used. The single cell cavity was mechanically polished and buffer chemically etched and then rf tested at cryogenic temperatures to provide a baseline characterization. The cavity’s inner wall was then exposed to the capacitive discharge in a mixture of Argon and Chlorine. The inner wall acted as the grounded electrode, while kept at elevated temperature. The processing was accomplished by axially moving the dc-biased, corrugated inner electrode and the gas flow inlet in a step-wise manner to establish a sequence of longitudinally segmented discharges. The cavity was then tested in a standard vertical test stand at cryogenic temperatu...

Apparatus and method for plasma processing of SRF cavities

Abstract An apparatus and a method are described for plasma etching of the inner surface of superconducting radio frequency (SRF) cavities. Accelerator SRF cavities are formed into a variable-diameter cylindrical structure made of bulk niobium, for resonant generation of the particle accelerating field. The etch rate non-uniformity due to depletion of the radicals has been overcome by the simultaneous movement of the gas flow inlet and the inner electrode. An effective shape of the inner electrode to reduce the plasma asymmetry for the coaxial cylindrical rf plasma reactor is determined and implemented in the cavity processing method. The processing was accomplished by moving axially the inner electrode and the gas flow inlet in a step-wise way to establish segmented plasma columns. The test structure was a pillbox cavity made of steel of similar dimension to the standard SRF cavity. This was adopted to experimentally verify the plasma surface reaction on cylindrical structures with variable diameter using the segmented plasma generation approach. The pill box cavity is filled with niobium ring- and disk-type samples and the etch rate of these samples was measured.

High etching rates of bulk Nb in Ar/Cl2 microwave discharge

Plasma-based Nb surface treatment provides an excellent opportunity to eliminate surface imperfections and increase the cavity quality factor in important applications such as particle accelerators and cavity quantum electrodynamics, as well as Josephson junctions. In this study, plasma etching of bulk Nb is performed on the surface of disk-shaped samples with the goal of eliminating nonsuperconductive pollutants in the penetration depth region and the mechanically damaged surface layer. The authors have demonstrated that in the microwave glow discharge, an etching rate of 1.5 μm/min can be achieved using Cl2 as a reactive gas. The influence of plasma parameters such as input power, pressure, and concentration of the reactive gas on the etching rate is determined. Simultaneously, plasma emission spectroscopy was used to estimate the densities of Cl, Cl+, and Cl2 under various plasma conditions.

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

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.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 ...

Resonant-frequency discharge in a multi-cell radio frequency cavity

We are reporting experimental results on a microwave discharge operating at resonant frequency in a multi-cell radio frequency (RF) accelerator cavity. Although the discharge operated at room temperature, the setup was constructed so that it could be used for plasma generation and processing in fully assembled active superconducting radio-frequency cryo-module. This discharge offers a mechanism for removal of a variety of contaminants, organic or oxide layers, and residual particulates from the interior surface of RF cavities through the interaction of plasma-generated radicals with the cavity walls. We describe resonant RF breakdown conditions and address the issues related to resonant detuning due to sustained multi-cell cavity plasma. We have determined breakdown conditions in the cavity, which was acting as a plasma vessel with distorted cylindrical geometry. We discuss the spectroscopic data taken during plasma removal of contaminants and use them to evaluate plasma parameters, characterize the process, and estimate the volatile contaminant product removal.

Plasma modification of bulk niobium surface for SRF cavities

Summary form only given. Particle 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 non-equilibrium plasma treatment of bulk polycrystalline Nb presents a viable surface preparation method due to possibility to use plasma-generated radicals and due to the inherent anisotropy of the etching action. We have optimized the experimental conditions in the microwave glow discharge system and their influence on the Nb removal rate on the flat samples and achieved etching rate of 1.7 μm/min using up to 3% Cl2 in the reactive Ar/Cl2 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.The geometry of SRF cavities made of bulk polycrystalline Nb requires the use of asymmetric RF discharge configuration for plasma etching. The asymmetry in the surface area of a driven and grounded electrode creates a difference in the voltage drop over the plasma sheath attached to the driven electrode and the sheath attached to the cavity surface. The driven electrode geometry is optimized to achieve homogeneous sheath conditions at the surface. Specially designed single cell cavity is used to study these asymmetric discharges which contain 20 sample holder holes symmetrically placed over the cell. These sample holder holes can be used for both diagnostics and sample etching purposes. The approach is to combine radially and spectrally resolved profiles of optical intensity of the discharge with direct etched surface diagnostics to obtain an optimum combination of etching rates, roughness and homogeneity in a variety of discharge types, conditions and sequences.